ES2395156A2 - Refrigeration method of internal transformation centers (cts) through underground electric channels (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

The cooling of electrical cts is a pressing problem and always due to the high temperatures reached inside, especially in summer, which cause equipment aging and loss of useful life of the transformer. The current systems by natural and forced convection are in many cases insufficient, and the air conditioning solution expensive, inefficient and with high maintenance. A cooling method is proposed that uses a tube of the existing ones in the underground electrical conduit through which air driven by a fan is passed. The air exchanges heat with the walls of the tube that are at the temperature of the ground, significantly lower than the environment. It cools and is driven into the ct. Simple method, without maintenance, sustainable and effective, protects the installation and prolongs the useful life of the transformer. (Machine-translation by Google Translate, not legally binding)

Description

METHOD OF REFRIGERATION OF TRANSFORMATION CENTERS (CTs) OF

INTERIOR THROUGH ELECTRICAL CHANNELS

UNDERGROUND "

5

Field of the Invention

The present invention falls within the field of distribution of

electric power, and more specifically, in the cooling of Transformation Centers

Interior type, that is, all those who are housed inside a room

or enclosure (usually urban centers).

1 o

Its characteristics make it valid and suitable for use in the

refrigeration of other types of premises of electrical installations such as

Distribution (CCRR) and Substations (Sub), and even in other technical premises such as

Computer Rooms / Data Processing Centers that host a large number of

electronic devices that require a high energy supply for a

fifteen

Special cooling continues over time.

Background of the invention

The distribution of electrical energy at the Low Voltage level (BT = 400 V

three-phase -230 V single phase) is carried out through transformation facilities

twenty

called Transformation Centers (hereinafter CCTT), whose mission is to

reduce the tension of the networks that feed the urban centers (20kV, 66kV, ..) to the

operating voltage or Low Voltage (BT).

Increases in electricity demand at the BT level, mainly

domestic consumers and small businesses, which have been registering in

25

recent years as a result of increased use of air equipment

conditioning, computers, video monitors, and all kinds of accessories

electronic, produces in a significant number of cases a saturation of the load

of the Electric Power Transformation Centers (CCTT) that it generates in

consequence greater losses of heat inside the premises of the CT that is necessary

30

evacuate with artificial means to maintain adequate conditions

of environment, mainly humidity and temperature, in the premises that house

This type of facilities.

The aggravating circumstance occurs on most occasions, that this

load saturation coincides with the worst external environmental conditions, it is

35

say, in moments of high ambient temperature, which make the

refrigeration of the CT premises and keep the existing equipment inside the worst operating conditions.

The so-called "effect of climate change" also represents another very important aggravating factor in the heat conditions suffered by CCTT premises, since the increase in temperatures, both medium and maximum, day and night, as well as the increase in the number of days with higher temperatures, it is evident as a worsening in the temperature conditions inside the premises that house the CCTT.

These high temperatures that are reached inside the CCTT premises represent a major problem and a serious risk to the function performed by these facilities. The important problem is twofold. On the one hand, the useful life of the transformer, estimated in values between 20-40% on average, is drastically reduced, with the economic effect of having to replace this machine in advance of the estimated theoretical time. On the other hand, the high internal temperatures reached can lead to the disconnection due to temperature of the transformer, with the consequent power cut to the users, which for the distribution company represents a significant damage to the image and economic penalties for breach of continuity of supply. But in addition to the two problems indicated, it also poses a serious risk because when such high temperatures are reached inside the premises under these conditions of high electrical load, all the interior equipment is subjected to a very high thermal "stress" which means an increase very important of the probability of breakdown with possible spread of fire, which would then have very serious consequences not only economic but also of a personal nature for neighboring homes (as in fact it has already happened on more than one occasion).

To date, this problem is being addressed by electric power distribution companies with more or less effective solutions. Thus, at the present time in the vast majority of facilities, the cooling of the transformation centers is carried out well by natural convection by application of the "chimney effect", sometimes by forced convection through the use of generally type fans axial located in the walls or doors of the CT and exceptionally, when the temperature conditions are extreme, through air conditioning systems.

This poor cooling has the following negative effects:

one.

In small and poorly ventilated premises: breakdowns of the electrical elements housed inside

2.

Premature aging of the CC.TI (mainly of the trafo)

3.

Economic losses

Four.

Technical losses, increase in electricity demand.

At present there are in the Spanish territory about 25,000 transformers in CCTT premises that require adequate cooling for proper operation and for their useful life to be as long as possible. Maintenance problems and the negative effects of high temperatures in areas such as Andalusia pose a considerable cost that could be saved with the correct use of passive cooling systems.

As mentioned, in the most extreme cases the solution used generally involves the installation of conventional air conditioning equipment, which by means of a heat pump produces the cooling of the installation air based on a very important consumption of energy and yields less than 50%, penalizing the performance of the electrical installation itself and generating a non-negligible electrical overconsumption that in turn produces more pollution and consequently more global warming. All this without taking into account the maintenance costs required by air conditioning systems and the need for sanitary control to avoid problems such as the possible accumulation of legionella bacteria in this type of equipment.

The present invention has the purpose of defining a simple, cheap and sustainable cooling method to the CCTI premises, so as to allow the evacuation of the heat generated in its interior mainly due to the losses of the transformer and the contributions of energy from the warm outside climate . Its purpose is to maintain the values of the temperatures inside the CCTIs in non-dangerous margins for the operation of the electrical equipment and extending as much as possible the life of the transformer. Its application avoids the assembly of conventional air conditioning installations, which are expensive, inefficient, complex and expensive to maintain, easy to break down, sometimes annoying to neighbors because of the heat they expel and also not respectful of the environment.

Description of the invention

The proposed method is based on the second principle of thermodynamics: "The amount of entropy of any thermodynamically isolated system tends to increase over time, to a maximum value."

More simply, when one part of a closed system interacts with another part, the energy tends to be divided equally, until the system reaches a thermal equilibrium, which is basically that energy in the form of heat is transmitted from a focus Warm to another cold.

This principle applies universally in multiple applications, the vast majority related to cooling systems: heat pumps primarily. Within these multiple applications there is one that can be used specifically for the case of CCTT, which is the use of buried pipes taking advantage of the electrical conduits through which the power cables that interconnect the ccn with the electrical distribution networks run.

Electrical pipes in the vast majority of distribution companies are built using pipes of different diameters that are buried at varying depths (generally between 1 and 2 m) are used as a means of passage of the electric power cables of the distribution networks.

The arrangement of these tubes at the indicated depths gives them an additional property to the one initially contemplated in their design only as an element of passage and accommodation of electrical cables. This additional property is its application as a valid element to perform the process of heat exchange between the air, which can be forced through the use of a fan, with the ground at the depth to which it is located The tube.

This property can be extraordinarily useful in summer periods, or in any case, on those days when the ambient temperature is high, generally above 25 degrees Celsius. This is because precisely the temperature of the terrain from 1.5-2 m deep is usually below 25 degrees Celsius at any time of the year, of course even in the summer period when outside temperatures can be around 35 and even 40 degrees Celsius.

The use of pipes in electrical pipes specifically for this purpose provides a very significant added value to the cooling of the CT premises where the said pipes arrive or leave.

Obviously it is in the design and construction of a pipeline the optimal time to define a tube for this use. It is most logical to arrange this tube at the bottom of the pipeline to achieve a double effect in this way: greater possible cooling and less interaction with the rest of the pipes of the pipeline, both for the purpose of thermally separating one from another and avoiding mutual influences. during its exploitation or operating life, thanks to the vertical physical separation existing between them.

But it is also possible to apply this method in existing pipes in which there are pipes free of occupation, that is, not occupied by power cables. Naturally in this case it is required that in each of the boxes or registers the continuity of the selected tube is continued by installing pieces of tube attached to both sides of the tube cut in the register and conveniently sealed to allow the passage of air along of the entire duct.

However, both in one and another case, it is essential to establish drains at the lower points in each section where there is a change of slope or inclination, to allow the evacuation of the water that can be formed in these sinks, mainly by condensation of the moisture present in the air that circulates through the interior of the tube during the process of cooling by heat exchange with the ground. If this drain is not done, the possible formation of water and its accumulation at these lower points could completely obstruct the tube, preventing the passage of air and thus rendering the proposed method useless. It is also critical that the drainage system to be used must be such that it avoids at all costs that the water coming from the outside (seepage, water table, etc.) can enter into the tube since in addition to assuming the use of the cooling system could in the event that the external water supply is of considerable volumes, reach the interior of the CT object of the refrigeration with the consequences

extremely harmful (flood) that this could produce in the

electrical installation.

There are several variables that affect the performance and operation of the

use of buried pipes to cool forced air through its interior: -Firstly, the temperature of the ground at the location of the tube. Obviously at lower ground temperatures, more cooling of the forced air that passes through the tube. This value depends mainly on the depth of burial, being shown that up to 1 m levels the temperature of the land is very dependent on the outside ambient temperature, while between 1-5 m is quite independent and between 5 and 50 m is almost totally independent, resulting in these levels lower temperature the more depth. For the first two levels, the type and location of the exterior surface of the land are relevant, such as, for example, that a land in wooded and shaded area with grass, with a high humidity tone; It is generally much cooler than a low asphalt ground with a high level of heat stroke.

-

Secondly the heat transmission between the tube and the ground. Clay soils generally have a lower heat transfer coefficient to limestone soils. In the same way the material of the buried tube influences this transmission, for example Al pipes have a heat exchange coefficient much higher than PVC pipes, due to the thermal properties of the material.

-

Third, the length of the tube, the greater the length, the greater the heat exchange capacity. This variable is obvious, but it is very important to determine the maximum advisable length from which the heat exchange is already minimal and therefore unnecessary.

-

The amount of moisture present in the air that is passed through the tube. At a higher percentage of humidity, the dew point temperature increases, and consequently during the process of cooling the air as it travels through the tube, water will be generated. In the process of exchanging heat from the air with the ground part of this transferred energy is consumed in producing water, instead of being transmitted from the air to the ground, so that the

cooling of the air in its circulation through the tube when the

condensation.

-

The speed of the air inside the tube: usually high speeds generate turbulent flows, which lead to greater friction between the air molecules and the surface of the tube and therefore greater heat exchange between air and tube-ground. On the contrary, laminar flows have the opposite effect, that is, less heat exchange. In this sense, the type of route greatly influences, linear sections versus sections with elbows and variations of the route, the latter are much more efficient in the exchange of heat by the formation of turbulent flow. Related to this section, the diameter of the tube is very important, since for the same air flow at a smaller diameter, greater speed and more turbulent flow, at greater diameter, less speed and more laminar flow.

The forced air passage through the buried tube is carried out by a fan, generally of the centrifugal type to overcome the pressure losses existing in the duct at the calculated flow rate to achieve the number of renovations at the precise time for the desired cooling effect. This fan, housed inside the CT that is to be cooled, will aspirate the air from the buried duct, and will drive it, already cooled to the temperature of the ground inside the premises to mainly cool the transformer.

So important to achieve adequate cooling of the CT is the effect of cooling the air conducted by the ventilation tube itself as a correct arrangement inside the fan premises, the flow of discharge and the evacuation of the air that due to overpressure must be extracted from the premises . For that reason, for each particular case, depending on the characteristics of the CT premises (dimensions, geometry, location of the equipment inside and its spatial location), a specific analysis is required to optimize the process of evacuation of the heat produced in its interior.

Detailed description of the invention

The present invention aims to define a method of cooling electrical installations, such as ccn, effective, efficient, sustainable and with virtually no maintenance.

It is also applicable to any other type of electrical installation (Sub and CR) and technical premises that host a large amount of electronic equipment, such as computer rooms or CPDs.

To achieve this task, the very low temperature geothermal energy that is present in the land where the underground electrical pipes run through the provision of a cable-free tube dedicated specifically for this purpose is used.

The solution consists in passing an air flow through this buried pipe in the length and conditions such that the temperature of the same at the outlet allows the effective cooling of the CT.

The data provided and calculations made below are specified in the case of CCTT. For other facilities such as those indicated above, the data of each of them would be used, applying the method and calculations the same as in the case of ccn.

CALCULATION OF AIR FLOW TO INJECT IN THE CT:

We will take a usual case:

•

Starting Variables:

o Trafo submerged in oil bath, 22 / 0.4 kV

o Strafo (S = APPEARANCE) = 630 kV A

o Dimensions of the premises: 5x3 m2

o Traffic load: 70% as a constant average value (although in reality the load will respond to a daily load curve that will be repeated over time).

•

Calculated variables:

o Traffic losses = Losses in vacuum + Losses in load

Vacuum losses = 1,300 W (according to catalog, typical value)

Load losses = 6,500 W (at nominal power, typical value), then at a rate of 70% of the nominal, the load losses (losses in copper) will be 6500 * (0.7) 2 = 3.185 W

Then for this case Traffic losses = 1,300 + 3,185 = 4,485 W

o The flow that we must force so that there is a correct ventilation can be calculated in several ways, we will use 3 according to different existing commercial references:

-According to Schneider manufacturer method, according to catalog (manufacturer of electrical transformers and booths for CCTT housing): Q = 0.081xP (where P = losses in kW and Q the flow rate. resulting in m3 / s) = 0.36 m3 / s

-According to the S&P method, according to catalog (manufacturer of fans): Q (m3 / h) = Losses (W) / 0.34x (later-later) The difference between temperatures is usually taken indoor and outdoor 5 for hot environments and 10 for cooler environments. In this case we will take 10.

Q = 4,485 / 3.4 = 1,319 m3 / h = 0.37 m3 / s

-

According to the Hc Energía method, according to its technical standards (electricity distribution company),

Vol = K ~~ T m 3 / s

Substituting results: Vol = 4,485 / (1,085 * 10) = 0.41 m3 / s

We will take the latter as a valid value (it is the most unfavorable of the three and therefore the most conservative).

BURIAL TUBE CALCULATION

The diameter of the tube is one of the variables that has a relative influence on the

cooling capacity of this method.

If we go to commercial tube diameters, the following options will be assessed:

Inner diameter 110 mm Inner diameter 150 mm Inner diameter 200 mm Inner diameter 300 mm

The 300 mm tube is discarded as "unmanageable" given its excessive size that would greatly hinder its use both in the canalization itself and in the arrival at the destination CT, where it would be very difficult to machine a socket for that diameter.

The tube of 110 is also dismissed for the excessive speed that the air would take for the required flow inside, which would produce phenomena of audible noise, very high losses in the duct and at the same time a very appreciable turbulence phenomenon in the local CT due to the loss of speed that the driven air should suffer in such a small space.

Consequently, the diameter of the tube to be selected must be made between the values of 150 and 200 mm, which also correspond to diameters commonly used for the passage of electrical cables, which on the other hand simplifies and reduces the installation work of the buried pipe. , both in the air intake to be carried out, as in the route of the tube and in the arrival of the tube to the CT premises.

However, in view of the speeds that the theoretical flow would require, it will be necessary to reduce the flow to be boosted inside the CT premises, since from velocities of 15-20 mIs noises can occur in the passage of air through the tube that would advise against their employment, although when they were buried they would not be audible. Likewise, the air inside the installation must not be injected at excessive speed, since if there are no turbulence and interior noises that are convenient to avoid.

For this reason the choice of the driving fan for driving air

buried must be carried out taking into account these two limitations, but trying as much as possible to stay as close as possible to the minimum air renewal flow parameters that would require the installation of the CT premises for the correct evacuation of heat from losses of the traffic.

The following table shows the results for this generic case based on the different diameters of the ventilation tube.

Calculation of losses, ventilation flows and speeds according to the ventilation tube used

Nominal Trap Power (kVA)

Charge Regimen Losses in a vacuum (W Copper losses (at full load) W Total losses (at the indicated loading rate) W Required flow rate (m3 / s) Schneid method er Required flow rate (m3 / s) S&P method Required flow rate (m3 / s) HC method Salt or term mic o Tube diameter (mm) Section (m2) Speed results (mIs)

630

70% 1300 6500 4485 0.36 0.37 0.41 10 110 0.01 O 38,221

630

70% 1300 6500 4485 0.36 0.37 0.41 10 150 0.01 8 20,554

630

70% 1300 6500 4485 0.36 0.37 0.41 10 200 0.03 1 11,562

630

70% 1300 6500 4485 0.36 0.37 0.41 10 300 0.07 1 5,139

<

15 According to these results, the most suitable diameter is 200 mm, as flow rates result below the limit of 20 mIs. In any case, the 150 mm diameter tube could also be used, since, although the speed reaches. limit value indicated, the effect of noise outside would not be noticeable when the tubes are buried.

20 BURED TUBE .: DIAMETER, MATERIAL AND EXECUTION

Buried pipes with a cooling air passage function must have a section that allows commercial application on the one hand, using commercially available materials in the market. For that reason and for reasons of manageability and compatibility with the pipes used for electrical pipes, the diameters of 150 and 200 mm are, "a priori", the most suitable for this type of applications (as we saw in the previous section ).

But a key aspect of the installation of the buried pipe, in addition to its inner section diameter, is the type of material to be used, which in turn will depend fundamentally on the length to be installed as well as the mounting depth.

As always, the contour variables that can be defined by legal or constructive imperatives must be taken into account.

Thus, in the case of the mounting depth of the buried pipe, this value must be in line with the depths used in the electrical pipes. It is clear that the cooling tube has to be located in the lowest point of the trench, since in this way we achieve the maximum depth that as it has been seen is very convenient to obtain the minimum temperature of the ground, as well as becoming independent as much as possible of the external environmental conditions. At the same time, placing the tube in the lowest part of the pipeline, we will get the possible thermal interference due to the effects of heat losses, usually by Joule effect, emitted by the conductors that run through these pipes, affect less possible The maximum depths of the pipes normally used in the conduits of electrical distribution networks range between 1200 and 1600 mm.

Consequently, the depth at which the cooling tube will be installed will oscillate on average between 1450 and 1850 mm, trying to ensure that at no time is it at a lower depth than the first value.

The material of the cooling tube can be of different types, according to the materials used in commercial tubes. Obviously, the better thermal conductivity it has, the better the cooling behavior will be obtained and, consequently, the shorter pipe length will be required to achieve the minimum possible temperature (which will always be that of the ground at the depth level where the cooling pipe runs).

From that point of view the most suitable material is Aluminum, but the cost of the pipes, the installation labor and the complexity of the joints between pipes, which mostly requires welding, can make the use of This type of material for its higher cost.

The most appropriate "a priori" pipes from a constructive point of view are the PVC pipes used for water drain pipes. Although this material has a thermal conductivity much lower than aluminum, they can have a good behavior and be valid for the desired purpose if the existing length in the pipe is sufficient.

The calculations carried out and the pilot experiences executed confirm that, from lengths greater than 75 m, the PVC pipe behaves reasonably well, which together with its low cost, easy installation and absence in maintenance practice, could be the most used solution.

It is very important in the execution of the buried pipe for the passage of air to keep the joints between pipes properly sealed to avoid water or dirt entering from the outside to the inside, which if they occur would cause losses in the operation of the same and with the time arrived the case its disabling. However, despite this requirement, it is not more than making the joints between tubes according to the existing technique, without additional requirements, since if so, the desired watertight effect is achieved.

CONDENSATION WATER DRAINAGE

In summer or hot days the passage of air through the buried tube will produce a cooling thereof by heat exchange with the walls of the tube that will be at the temperature of the ground (usually between 20 and 22 degrees Celsius, depending on the depth and the characteristics and conditions of the land). This cooling can produce condensation of water depending on the degree of relative humidity present in the air. In the event that this condensation occurs, it is necessary to achieve the evacuation of the water formed because if it does not, over time the accumulation of it inside the tube can block the air flow, leaving the same unusable for the function that is chase

For this reason it is very important that the drainage points necessary to keep the tube free of any possible water that from the condensation of the air could accumulate are taken into account when laying the tube.

The execution of the drainage points in a new pipeline is very simple: it is enough only to define those points of the route where changes in slope from negative slope (descending) to positive slope (ascending) occur ), considered in the direction of air passage.

It will only be necessary to execute drainage in the aforementioned points of slope changes or "funnels" in which the depth of the slope is greater than the measurement of the radius of the tube, since if the accumulation of water inside the tube is lower, never it will get to seal more than half of the section of the tube, being considered in this case valid for the function that is pursued. However, the most advisable, whenever possible, is to run a drain at all the relevant "funnel" points.

The execution of the drainage is very simple: First of all it is necessary to verify that the water table is below the point identified for the execution of the drainage. If this situation is not achieved, the drainage cannot be carried out because the condensation water would never go outside, so the tube path should be varied or the slopes modified if possible at this point. Secondly, considering that the accumulation of water by condensation is very slow and in small quantities, it is enough to drill in its lower part, with a diameter of 1 cm and practice in it a small cannula or drain tube that connected to a non-return valve of very low resistance, for example of the ball type, which will expel by gravity the water accumulated outside, thus preventing the water that could exist outside the tube (due to rain or temporary rise in the level water table), can penetrate inside, which would block the tube for the passage of air and leave it unusable.

Depending on the place, it may be convenient that in the place of the drainage point, the expulsion of water through the cannula and the non-return valve, is carried out to a small area (of very small volume) executed of stones or edges rolled diameter between 2 and 3 cm, which protected with a filter mesh (which prevents the soil from mixing with them), favor the filtering of the expelled water and its absorption by the ground.

TUBE LENGTH CALCULATION AND BEHAVIOR SIMULATION

Defined the section and depth to which the buried tube will go, a fundamental section of this innovative solution is the calculation of the length that is necessary to achieve the desired end.

The length is together with the depth and type of material used in the tube, one of the design values of the tube that most influence its behavior as an element of heat exchange between the air that is passed through the interior with the ground.

As you can imagine, there will also be contour variables that will sometimes mark the maximum length to be used. It is from the general data of the installation and the electrical work to be carried out in each case where we will obtain the maximum and minimum length to be executed. Depending on it we can define the type of material we will use (usually PVC and optionally if the length is short Al) and the exact path will be defined.

To validate the necessary length an algorithm has been developed, in an Excel sheet that also incorporates a user interface to facilitate its use and that calculates the output temperature of an air flow when circulating inside a buried tube.

The input data for the calculation are:

•

Relative humidity of the air; The air in real conditions contains a certain amount of water in a vapor state that will influence the outlet temperature and the existence of internal condensations in the tube. .

•

Atmospheric pressure; together with the relative humidity it allows to calculate the amount of water that the air contains, although for normal atmospheric conditions its influence is scarce.

•

Air inlet temperature from outside.

•

Flow rate; which will depend on the power of the fan and the pressure losses of the tube.

•

Tube material; a tube made of a material that is a good conductor of temperature will improve heat transfer, this is especially important with short length tubes where it is necessary to lower the air temperature in a short space.

5 • Inner diameter of the tube and thickness; they have an influence on the resistance offered by the tube to the heat flow and on the turbulence caused in the fluid when circulating inside.

• Tube length; the longer the air temperature is

it will more closely resemble the outside temperature by having more space to exchange heat.

•

Ground temperature; mark the limit to where the air outlet temperature can go down.

•

Fouling resistance; takes into account the existence of dirt

on the inner surface of the tube that causes a small resistance to heat exchange.

The calculation of the heat transmission is performed, in general, by means of

following equation:

The area (A) can be easily calculated knowing the dimensions of the tube.

The resistances that act in this process are due to heat conduction to

through the walls of the tube and the convection of heat between the air and the wall

tube inside. The conduction resistance is calculated:

,

,The n...!...

'¡¡

Conduction = - ~ ktubo

which depends solely on the dimensions of the tube and the material.

30 Conduction resistance is calculated:

where hi is the interior convection coefficient, which is obtained from the dimensionless number 35 of Nusselt:

AT.

h¡ · D¡

JVU = -kaire

This number can be calculated through several empirical correlations that various authors have been obtaining through experimental trials. For this particular case of flow through the interior of turbulent pipes, the following correlations have been used:

0.25

• Mijeev ~ Nu = cL · 0.021 · Reo. 8 · Pro, 43. (: ~)

Valid if 1 OE4 <Re <5x1 OE6; UD> 50

Nu = S + 0.016 · Re a · Prb a = 0.88 _ 0.24

• Notter and Sleicher ~

4 + Pr

b = °, 33 + °, S · e-O, 6 * Pr

Valid if 1 OE4 <Re <1 OE6; LID> 25

NU = (f / 8} (Re-l000} Pr Gnielinski ~ 1 + 12.7 · (¡/ 8) °, 5. (Pr 2/3 -1)

•

{f = [0.79Ln (Re) -1.64r

Valid if 3000 <Re <10E6

With these correlations, a mean hi value is calculated that will be used to perform the following calculations.

It would be missing to know one more data in the equation, either Q or L \ T, but both are in principle unknown since L \ T is the difference between the average temperature of the fluid and the temperature of the ground, will therefore depend on the temperature output that is precisely the purpose of calculating this algorithm.

The properties of the air are obtained from thermodynamic tables or are calculated from the temperature and humidity. In particular, the viscosity, conductivity and saturation humidity are obtained from tables of values with which the temperature is entered and using linear interpolation if necessary. The value of the Prandtl number and density is obtained based on the above and the amount of water

(X) by:

J-l · C

Pr = - P

fluid

density = (1 + X} (p atm)

T287.04 + X 461.5

To obtain the heat Q exchanged in each length it is necessary to know the outlet temperature, for which hypothesis is made and the equation is solved:

it is verified that the result obtained matches that initial hypothesis, it must also be fulfilled:

Q = m · (input -hout)

The approximation is made with the theorem of the average value. When entering a temperature that is too low, the result of the algorithm will be a temperature higher than the assumed one, and when entering a temperature that is too high, a lower temperature will be obtained. The algorithm then tests with an average temperature between the two and depending on the result obtained from this temperature it will take another value above or below it so that it is approaching the correct value. By

By default, the iterations stop when the difference between the assumed output value and the value obtained is less than 1 E-5 or 100 iterations are exceeded, also giving the value of the difference between these two values.

The algorithm divides the total length of the pipe into smaller lengths, by default of one meter, along which an average temperature is taken and the properties of the air are considered constant: density, conductivity, viscosity, etc., in addition of moisture For each length it is necessary to calculate the turbulence using the

10 Reynolds number:

4'm

Re = -

¡R · D · v. Flow 'Pa, m

m = ___- ¡---- = c ::.: ..-____ 461.5'T {O, 622 -X)

. The necessary calculations are applied to this length to obtain the conditions of exit, which will be the conditions of entry of the following length, storing the results obtained.

The output temperature can be calculated by knowing the value of the heat exchanged and obtaining the enthalpy of output by:

20 Q = m · (hen, rada -hout)

With the enthalpy value, the temperature can be cleared in the case of humid air without liquid water:

25 -h = Cpaire-dry + X (, 1, + Cpvapor'T)

or with water condensation in case X> Xs:

30 h = CPaire-dry + Xs {.l, + CPvapor'T) + (X -Xs} CPagua'T

With this value of the outlet temperature, the outlet temperature initially assumed for the calculation of Q is checked and correct if necessary as explained above.

The temperature and humidity values are saved and a graph is generated that shows the variation of the temperature for each longitudinal position of the tube and the point from which there is liquid water.

In the accompanying drawings in the figures section, results of the algorithm and a block diagram of its basic operation are included.

AIR CAPTURE AND EXPULSION TAKES

Of particular importance are the intake and expulsion of air intakes due to their importance in order to optimize the cooling method. It is of little or even nothing to cool the air at the entrance of the CT if the conditions indicated below at the origin and end of the cooling tube are not met.

The collection must meet the following conditions: It must be located in a place that is not accessible by the general public, thus avoiding accidents or unwanted manipulations and even small sabotage that would alter its operation. Avoid the entry of water through rain or runoff that could occur during episodes of heavy rain, since if that happened, all the water would be sent to the cooling object through the tube, which would damage the fan, and also it would be a fatality since it would almost certainly generate a power cut. To achieve this objective it is necessary that the head or inlet of the tube is not accessible by rainwater, it is sufficient for this to be an elbow in said inlet with the orientation of the air intake towards the ground. Additionally, to prevent the entry of water from runoff or partial flooding that could occur in that place, it is necessary that its location be above ground level, always leaving a margin of 50 cm advisable distance from the ground.

Prevent the entry of insects, rodents, animals and any object, since in addition to hindering the passage of air would affect the integrity of the blades of the centrifugal fan that forces the passage of air, and may cause deterioration and consequent destruction. To achieve this objective, a metal or plastic mesh must be incorporated into the head or inlet of the tube, with a grid section small enough to avoid the indicated passage but not sensitively affecting the passage of air, increased by inadequate form of load losses due to resistance.

The outlet or expulsion of air must comply with the following: The same as the intake or collection outlet. For this, the same measures as those indicated will be considered. To be located inside the premises of the CT to be cooled so as to optimize as much as possible the renewal of air inside the room and achieve the greatest cooling effect. In order to achieve this optimization, the following specific measures must be considered:

o Projecting the cold air jet on the hot spot (in this case the transformer, and in particular on the cooling fins in the case of oil immersed traps) produces the greatest possible cooling effect. The way to achieve this projection of air on the hot spot is to "lengthen" the air outlet on arrival at the CT premises to the vicinity of the traffic. In any case, it is not advisable to bring the air projection to the traffic excessively to avoid unwanted "bounces" and "air recirculations", which would generate turbulence and ultimately losses in the operation of the method. In general terms, it is not convenient to project less than 1.5 m away from the traffic.

o Reduce the airflow exit speed by using diffusers or "truncated conical or similar type wideners, just outside the ejection tube, to values that do not produce excessive noise or turbulence, expanding and adjusting it time the projected channel of the outgoing air flow to the volume of the traffic, thus the jet of cold air will adequately cover the volume of the traffic, producing the greatest possible cooling effect.

o The hot air outlet inside the premises must be carried out in such a way that the hot air collection takes place above the hot spot and is transferred to the outside with the lowest possible head losses.

o Avoid what is known as the "air short circuit" phenomenon between the air inlet and outlet inlets at all times, that is to say that all or part of the incoming cold air exits through the air outlet without having gone through the hot spot, because this phenomenon dramatically reduces the cooling effect sought.

o In general, to respect the basic laws of thermodynamics and in particular the phenomenon of natural air convection, the cold air intake will be located less than 1 m high from the floor of the premises and the air outlet hot, it will be located as close as possible to the roof of the premises.

In the accompanying drawings in the figures section, various explanatory schemes of what is indicated here are included.

FAN

The fan that forces the passage of air through the buried pipe is a key part of this proposed cooling method.

The ventilation of a CT can generally be done in two ways: by overpressure, that is, by injecting air at a pressure slightly higher than that existing in the room or by depression, that is, by extracting the air outside.

In this case of the buried tube method, only overpressure / impulse ventilation will be used, which on the other hand is more advisable, since it has in itself an intrinsic advantage over the extraction system.

This recommendation of the overpressure system versus the extraction system is based on studies carried out by simulating the evolution of temperatures inside a room with specific software for the design of ventilation systems.

Two situations have been simulated for a transformation center with identical conditions, in one of them an extractor fan has been placed and in the other a

impeller fan To make this comparison the fans have been simulated with identical characteristics:

•

Q = 0.2 m3 / s

•

w = 2800 rpm

• D = 0.2 m The results obtained in this study using air temperature simulators reflect an improvement in temperature in the center with overpressure (ie injecting air instead of extracting air) of almost 4 ° C, which translates in an increase of the useful life of the machine of the order of 39% (applying the methods defined by the norms of IEEE and lEC referred to the calculation of useful life of a transformer: in particular the IEEE Std C57.91 and StdC57.100 describe thermal and load effects on the life of transformers immersed in oil).

Taking into account the characteristics of buried pipes, length, elbows, diameter, etc., to overcome the pressure losses present at the required flow, the most advisable option is centrifugal fans.

LOAD LOSS CALCULATION

To determine the pressure to be delivered by the fan it will be necessary to calculate the pressure losses that will occur in the circuit.

The flow of a liquid in a pipe is accompanied by a loss of energy, which is usually expressed in terms of energy per unit weight of circulating fluid (length dimensions), usually referred to as head loss.

In the case of horizontal pipes, the loss of load manifests as a decrease in pressure in the direction of flow.

The load loss is related to other dynamic fluid variables depending on the type of flow, laminar or turbulent, in addition to the linear load losses (along ducts), singular losses also occur at specific points such as elbows, branches, etc.

• Linear losses

They are due to the shear stresses of viscous origin that appear between the fluid and the pipe walls.

The characteristics of the shear forces are very different depending on whether the flow is laminar or turbulent. In the event that the flow is to laminate the fluid layers always run in a direction parallel to the axis of the pipe and without mixing. In turbulent flow there is a continuous three-dimensional fluctuation in the velocity of the particles that overlaps the velocity components.

The type of laminar or turbulent flow depends on the value of the relationship between the inertia forces and the viscous forces, that is, the Reynolds Re number, whose expression is shown below in a general and particular way for pipes of circular cross-section:

Re = p v D = v D = (4 Q / (1Z 'D 2 »D = 4 Q

f.-l f.-l / p v I1Dv

where:

p, is the density of the fluid,

v, is the average speed,

D, is the diameter of the pipe,

f.-l, is the dynamic or absolute viscosity of the fluid,

v, is the kinematic viscosity of the fluid and

Q, the flow through the pipe.

When Re <2000 the flow is laminar, if Re> 4000 the flow is turbulent and between 2000 <Re <4000 the flow is transitional.

In laminar regime, the linear load losses can be calculated with the Hagen-Poiseuille equation, where there is a linear dependence between the load loss and the flow:

32 f.-l L v 128 f.-l L hp1 lamianr = p g D2 = P g 1Z 'D4 Q

In turbulent regime, the dependence between shear stresses and velocity is quadratic, which leads to the DarcyWeisbach equation:

f, is a dimensionless parameter, called coefficient of friction or Darcy that in general is a function of the Reynolds number and the relative roughness of the pipe:

¡= ¡(Re, E,) where Er = The D and E, is the roughness of the pipe, which represents the average height of the irregularities of the interior surface of the pipe.

Moody developed a diagram that bears his name, which shows a family of relative iso-roughness curves with which the coefficient is determined from the intersection of the vertical Reynolds number, with the corresponding iso-curve.

Subsequently other authors expressed the coefficient of friction based on the Reynolds number and the relative roughness with an explicit formula:

_1_ = -210 (Er + 5.1286)

., ¡G 3.7 Reo 89

Barr:

• Singular losses

They are those produced by any obstacle placed in the pipe that supposes an obstruction to the flow passage: inlets and outlets of pipes, elbows, section changes, etc. Normally they are small compared to linear losses, the following expression is usually used for its calculation:

hps

where, is the loss of charge in the singularity, which is considered proportional to the average kinetic energy of the flow; the constant of

proportionality, ~, is the so-called singular loss coefficient.

The most common in installations with respect to the singular losses, is the presence in the elbows, which can be calculated with the help of various graphs that relate the value of the loss coefficient according to the elbow geometry (angle and radius of curvature) for each pipe diameter value .:

However, it could be the case of having additional losses due to the presence of valves or other elements.

• Total losses

If we take both loads into account, we will have a circuit resistance curve of the form h = k * Q2, depending on the flow, and whose "k" is calculated with the equations shown above (ktotal = kpl + kps):

FAN SELECTION

Once the resistance curve is calculated based on the flow rate, the optimum operating point (selected flow rate and working pressure) is calculated, and a catalog of manufacturers is used. The cut of the fan curve to be selected with the resistance curve will provide the actual working point:

For each manufacturer, the model whose curve is immediately above the actual working point will be chosen.

In these fans the air inlet is axial to the impeller and the outlet is tangential.

With relatively small powers (less than 500 kW), we are able to overcome the losses in the entire length of the duct at the requested flow rate, so the performance of the cooling method is extremely high.

The location of the fan in the room of the CT to be cooled will be the one that allows

5 easy access, both for installation and subsequent maintenance tasks that need to be performed. The outlet of the expulsion outlet will be extended within the premises to the most suitable point to achieve the desired task, and the section of the expulsion duct will allow reducing the airflow exit speed to admissible values so as not to generate excessive turbulence or noise.

10 Precisely noise is one of the aspects that can be more problematic if it is not taken into account. The noise caused by the turbulence of the air inside the fan and the vibrations must be taken into account to avoid exceeding the permitted limits. The use of silencers is recommended in case of exceeding the limit values.

15 The noise generated by the fan, as well as the attenuation of the silencer, will be obtained from the various catalogs of the manufacturers. It is recommended not to place the fan on adjoining walls with houses, and as a general rule to prevent the transmission of vibration to the wall due to its operation, it is advisable to mount some rubber studs type

20 "silent-block" that minimize it.

APPLICABILITY OF THE METHOD

Two cases must be differentiated, depending on whether it is new construction (a) or existing pipeline (b).

a) In the case of new construction of electrical channeling, the solution consists in designing a specific tube for this purpose, as indicated housed as deeply as possible within the set of tubes that make up the electrical conduits. The type of tube material to be used will depend fundamentally on the length of the tube travel, if sufficient distance is available the PVC option is much more economical, but otherwise, the Al option will be necessary

b) In the case of existing channeling, it is necessary to have some free tube of the assembly that make up the electrical conduit, being the most convenient to use the deepest tube of all. However, you have to be especially careful in this case. In the first place, the diameter of the free tube is advisable to be in the measurements of 150 to 200. Secondly, the slope of the canalization path must be analyzed to establish the required drainage points, to evacuate the condensation water that may occur during operation. If there are caves or records in the canalization route, it is obviously best to try to locate the drainage point there because of the ease of access and absence of civil works required. But if it is not possible it will be necessary to make tastings at those points where it is mandatory to perform them. Additionally, in each of the boxes or registers it is necessary to execute the extension of the tube used for cooling. This connection is made by counter-tumbling with a similar tube or a slightly smaller diameter the path of the casket or register (generally not more than 1-2 m) and sealing the spliced sides with polyurethane foam to avoid leaks out of the conducted air and possible leaks into water that could accumulate inside the casket or log.

In either case, the steps indicated must be strictly followed

in the detailed description of the solution. However as for the buried tube a

employ it is noted that:

a) The difference in results obtained in air cooling is relatively small, whether a 150 mm or 200 mm inner diameter tube is used. Obviously it is slightly better to use a 150 tube because the regime is more turbulent and consequently the heat exchange with the tube walls will be better. Although on the other hand, the air speed is higher, which from speeds between 15 and 20 / ms is not advisable for the generation of noise inside.

b) For lengths equal to greater in the range of 75 to 100 m, the difference between using the Al or PVC pipe is small, since although the air cools more quickly in the Al because of the better thermal conductivity , from that distance the temperature drop is already almost asindotic with the temperature of the land. Taking into account that in most cases (especially in urban environments), the lengths are given by the distances between CCTT, by the considerable price difference that exists between one option and the other, much cheaper the PVC, make this is the type of tube to use more frequently.

c) The dew point or condensation formation has a great influence on the behavior of the tube, which depends mainly on the degree of humidity of the air, since from it, the cooling performance is greatly affected, being diminished, since The heat exchange energy with the soil is applied not only to cooling the air, but also to condensing water from the degree of humidity it contains. In general in climates such as the peninsular, although temperatures are high (40 degrees of ambient temperature), the operation of cooling in the tube is optimal as long as the humidity level does not exceed 30-40%, which is It usually happens. In particular, it is observed that it is the degree of humidity that most affects the operation of the tube, much more even than the air inlet temperature, since no matter how high it is, if the humidity is low, the cooling in the tube It is optimal.

VARIATIONS OF THE PROPOSED REFRIGERATION METHOD

The method proposed in its basic solution allows combinations that make up installation variants that introduce improvements and optimize performance

Of the different possible variants, the two that are considered most interesting are included.

VARIANT 1: SIMPLE APPLICATION FOR 2 ccn

. A particular case of air intake intake location, which can have a lot of use for its additional performance and for the particularity of electric power distribution networks, is to place it inside a CT. We would then have two ccn interconnected by the buried tube: the CT1 where the inlet or air intake is housed and CT2 where the centrifugal fan operating in suction is housed and where the air outlet is located. In that case, looking for an additional profitability to the solution, in the CT1 the air intake can be located precisely in the area where more heat can be accumulated in the same Taste above the area of the distribution channel). In this way the cooling method will fulfill a double cooling function: in CT1 it will evacuate the heat generated by the machine (air that will be renewed from the outside and that will enter through one of the air intake grilles of said CT1), and in the CT2 will provide cooled air from the buried ventilation pipe. This method for 2 ccn can (and should) be used when the length of the buried pipe is sufficient to be able to cool the intake of hot air from the CT1 to the ground temperature, which can be several degrees greater than the ambient temperature outside on very hot days. Despite being "a priori" more demanding from the point of view of heat exchange in the buried tube, it has the advantage, in climates with a high degree of ambient humidity, that the air introduced into the tube is drier, because it comes from the hot zone of CT1, and this has the advantage of producing less condensation water and, consequently, improving performance in the heat exchange process by geothermal use of the buried pipe.

VARIATION 2: DOUBLE APPLICATION FOR 2 CCTT

It is a variant of the previous one.

We have 2 CCTT: CT1 and CT2, between which there are two buried ventilation pipes, independent and conveniently separated so as not to transfer the heat from the ground between one and the other (advisable a distance of 50 cm).

In each of the CCTT two sockets are housed: one of Uusto air inlet in the hottest zone, that is, above the trap) of the tube that supplies air to the other CT, and an air outlet, in the area suitable for refrigerate the trap.

It is therefore the double application of the method operating simultaneously and in combination.

This method is highly advisable in those cases where the entry of outside air into the CCTT is very complicated. Such can be the case of CCTT housed in underground car parks of neighboring communities or in building levels where due to construction defects or because of not initially having a CT, the air intake is very poor.

This combined operation has the advantage of drying the air a lot, due to the heating-cooling process to which it is subjected continuously so that the degree of humidity of the air and therefore the condensation losses in the buried pipes are the least possible.

The drawback is that it requires at all times that the operation of both teams be simultaneous, since it is necessary to achieve the passage of air. To simplify and guarantee this simultaneous operation, the two fans can be housed in the same CT, one operating in suction mode and the other in drive mode, controlled in a unique way.

BRIEF DESCRIPTION OF THE DRAWINGS

A series of drawings that help to better understand the invention and that expressly relate to an embodiment of said invention which is presented as a non-limiting example of

is. Figure 1 shows an urban house-type CT where the present invention could be applied (valid for any type of indoor CT) Figure 2 represents the usual existing solution for cooling.

Figure 3 represents a general descriptive sketch of the proposed invention.

Figure 4 depicts a series of ditch design commonly used in electrical pipes, in which the ideal position of the ventilation tube location has been added.

15 Figure 5 shows the location of the drainage points according to the existing slope in the underground ventilation pipe. Figure 6 represents the operating scheme of variant 1 of the cooling method: application of a cooling tube for 2 ccn. Figure 7 represents the operating scheme of variant 2 of the refrigeration method: combined operation of two refrigeration tubes between 2 CCTT.

Claims (3)

1.-Cooling method of Indoor Transformation Centers (CCn) characterized by the use of geothermal energy of very low temperature from the use of a free tube through the underground electrical pipes that arrive / leave the CT, and through which a stream of air is forced through so that heat is exchanged with the ground and the air thus cooled is driven into the CT and conveniently guided inside so that the heat evacuation of the premises is as optimal possible. 2.-CCTT cooling method according to claim 1 characterized by the arrangement of a buried tube that joins 2 ccn (CT1 and CT2), so that the air intake is made in the hottest part of CT1 and by a driving fan located in the CT2, the air is passed through the tube path, expelling the "cooled" air to the CT2. With this variant of the method it is possible to cool both ccn: the CT1 for the renewal of air with inlet from the outside and the CT2 for the flow of refrigerated air from the renewal carried out in the CT1; obtaining in this way a considerable increase of the efficiency of refrigeration when being able to act on 2 ccn with a unique system of refrigeration and reducing the relative humidity of the circulating air in the entrance of entrance of the tube. 3.-CCTT cooling method according to claim 1 characterized by the arrangement of two buried pipes joining 2 ccn (CT1 and CT2), so that in CT1 air is taken in the hottest part that is passed through one of the two tubes and is expelled by a fan to CT 2. In CT 2 the reverse process is done, that is, air is taken in its hottest part and is passed through the second tube by a fan located in CT1 that It drives this cooled air to the CT 1. It is a double mode of operation of the proposed closed circuit method whose main advantage is to take advantage of the pipes and that reduces the relative humidity of the circulating air in the intake of both pipes.