RU2300056C2 - Method for utilizing heat of gas (air) flow - Google Patents

Abstract

FIELD: technology for utilization of heat of gas (air) flows, possible use in ventilation and air conditioning systems, and also other industrial branches.

SUBSTANCE: method for utilizing heat of gas (air) flow includes letting the flow through heat-exchanger - recuperator with intermediate heat carrier, circulating by means of ump in closed liquid line successively through heat exchanger-recuperator (air cooler) and into heat exchanger - air heater. In advance in heat exchanger-recuperator gas (air) flow) is let through a layer of granular absorbent for drying and for emission of absorption heat.

EFFECT: increased amount of heat utilized from gas (air) flow removed from premises.

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Description

The invention relates to a method for the utilization of heat of gas, including air flow and can be used in ventilation and air conditioning systems, as well as in other industries.

Currently, in ventilation and air conditioning systems, heat exchangers, heat exchangers, which are installed in air ducts, are used to utilize low-potential heat of removed air from rooms. The most universal devices are recuperative heat exchangers with an intermediate heat carrier, which are water or antifreeze (aqueous solutions of calcium chloride, propylene glycol, etc.). Usually two heat exchangers are installed: one in the exhaust air duct — an air cooler, and the other — in the supply air to the room (in the supply chamber or central air conditioner) —the air heater. Pipe (or annular) spaces of both heat exchangers are interconnected by pipelines, forming a closed loop for the intermediate heat transfer medium. In the same circuit is a pump for the intermediate coolant.

The heat recovery process is carried out in the following way. The removed air from the room containing harmful substances (heat, water vapor, etc.) passing through the heat exchanger-air cooler gives up part of the heat to the intermediate heat carrier. In this case, the temperature of the intermediate coolant rises, and the temperature and, accordingly, the enthalpy of the removed air decreases. Cooled air after the utilizer is usually released into the atmosphere.

At the same time, the intermediate heat carrier, due to the pressure created by the pump, enters the pipe space of another heat exchanger-air heater, where it transfers heat to the outside air, the temperature of which in the winter season is usually below 0 ° С. Then, the cooled intermediate heat carrier is fed back to the heat exchanger-air cooler of the exhaust air, etc.

The heat exchanger-air cooler can operate in the mode with condensation on all or part of the surface, as well as in the “dry” mode [1, p. 303].

Of greatest interest is the condensation mode or the “wet” mode, because in this case, both the apparent and latent heat of the gas (air) stream is utilized, i.e. heat of condensation of water vapor.

Changing the parameters of the air in the heat recovery system with an intermediate coolant in the mode with condensation is shown in figure 1.

Here, the point H ₁ corresponds to the initial parameters of the outdoor air, the point H ₂ corresponds to the state of the outside air at the outlet of the heater, the point U ₁ characterizes the initial state of the exhaust air, the point U ₂ represents the state of the removed air after the air cooler.

Thus, the beam H ₁ H ₂ corresponds to the heating of the external air in the air heater, U ₁ U ₂ - cooling and drying of the removed air in the air cooler.

Cooling with drying of the exhaust air is possible if the surface temperature of the air cooler t _f is below the dew point of air t _p , i.e. provided t _f <t _p [1].

The presence of condensate on the outer surface of the air cooler causes the risk of ice formation at t _f <0 ° C, which leads to the need to either terminate this process or periodically thaw the ice [1, p. 300], i.e. periodic shutdown of the heat recovery unit.

An extremely low average surface temperature of the heat exchanger-air cooler is recommended to be taken t _f = 2 ° C, which is ensured, for example, at an intermediate coolant temperature (antifreeze) t _af > -5 ° C at the inlet to the air cooler [2, p. 65, 69].

Thus, in heat recovery systems with an intermediate heat carrier, the temperature of the exhaust air after the heat exchanger-air cooler is taken to t _y2 > t _f = 2 ° C, i.e. practically its lower numerical value is limited by the technical capabilities of this method.

. Ориентировочно это количество можно определить следующим образом. Согласно [1]:For this reason, the exhaust air at the outlet of the air cooler contains a lot of water vapor - D, kg / s, with which a large amount of latent heat is carried away (not used)

. Approximately this amount can be determined as follows. According to [1]:

Here r = 2500 kJ / kg is the heat of condensation of water vapor;

G is the amount of air removed, kg / s;

d _y2 - moisture content of the removed air after the air cooler, g / kg.

Taking d _y2 = 5 g / kg as the minimum possible moisture content at t _y1 = t _f + 2 = 2 + 2 = 4 ° C, according to the Id diagram of moist air,

In central air conditioning systems, the flow rate of exhaust air can be L = 100,000 m ³ / h or more [1], i.e.

G _y1 = 100000 · 1.2 = 120,000 kg / h (33.3 kg / h), here 1.2 kg / m ³ is the air density.

, что соответствует большому расходу тепла.

, which corresponds to a large heat consumption.

Thus, a significant drawback of the method of utilizing the heat of the removed air (gas) stream with an intermediate heat carrier and recuperative heat exchangers is the incomplete drainage of the removed air, therefore, the inability to utilize all the latent heat (condensation heat) of the water vapor contained in the removed air (gas) stream.

Also known is a method of drying air using solid sorbents — adsorbents, for example, silica gel [1, p. 65-68, 157-161].

The use of adsorbents (silica gel, aluminum gel, etc.) allows you to get almost absolutely dry air. The use of such substances for dehumidification of air is recommended in cases where the purpose of air treatment is to drain it and heat it.

The adsorption capacity of silica gel with increasing temperature decreases and increases with decreasing gas temperature.

During adsorption, moisture condensation in capillaries is accompanied by the release of specific heat of evaporation and specific heat of wetting. The total specific heat of adsorption is 2930 kJ / kg, of which about 420 kJ / kg is the specific heat of wetting [1, p. 67].

Silica gel with a grain size of 1 to 3 mm is usually used to dry the air. Due to the large specific surface of the capillaries (in 1 kg of silica gel - 400,000 m ² ), silica gel has a high hydrophilicity.

An adsorber with a fixed adsorbent layer [1, p. 159; 3, p. 154].

The adsorbent is placed in several layers on shelves (gratings) inside the apparatus. Drained air enters the apparatus through the end hole and passes through the adsorbent layer due to the operation of the fan.

To ensure the continuity of the process of drying the air flow, two adsorbers are used in operation: one device operates in the stage of air drying (this time can be up to 8 hours [1, p. 158]), the other device operates in the mode of activation of the adsorbent.

The activation of the adsorbent is carried out in order to restore its moisture absorption capacity and is carried out by blowing air through it, heated to t = 180 ... 240 ° C [1, p. 68]. Upon activation, the adsorbent is heated so that the vapor pressure of the adsorbed moisture becomes higher than the partial pressure of water vapor in the air passed through it. In this case, the moisture contained in the capillaries evaporates and is removed together with the air. Upon activation, the adsorbent layer is heated to t = 100 ... 110 ° C, as a result of which it is cooled before reuse by blowing cold air.

The disadvantage of the process of drying air with a solid adsorbent is the need for heat to heat the air used at the stage of activation (desorption of water vapor) of the adsorbent.

To eliminate these disadvantages allows the claimed invention.

The technical result of the invention is to increase the amount of heat utilized from the removed air (gas) stream from the room by taking almost all of the latent heat of vaporization of the water vapor contained therein.

The technical result is achieved in that the air cooler is endowed with the functions of an adsorber and a heat exchanger. In this case, the gas (air) stream is passed through a heat exchanger-recuperator (air cooler) with an intermediate heat carrier circulating with a pump in a closed liquid line in series through a heat exchanger-recuperator (air cooler) and a heat exchanger-air heater, previously in the heat exchanger-recuperator gas (air) stream pass through a layer of granular adsorbent to drain and release heat of adsorption.

To remove the total heat of adsorption of water vapor, as well as to take off the apparent heat of the dried off air, coils are placed in the adsorbent layer for circulation of the intermediate heat carrier or the adsorbent layer (granular silica gel) is placed in the tube space of the shell-and-tube heat exchanger, and the intermediate heat carrier circulates in the annular space. The intermediate heat carrier removes heat from the adsorbent layer, heats up and transfers it to the outside air in the air heater.

Typical designs - pipe and shelf contact devices, the principle of which is based on filtering gas through a layer of stationary granular material (catalyst), are used in the catalytic purification of gases [3, p. 217, Fig. 153].

The activation of the adsorbent is proposed to be carried out by the method of displacement desorption [3, p.153].

To activate silica gel, the absolutely dried air obtained in the neighboring adsorber is passed through an adsorber before it is released into the atmosphere.

Due to the large difference in the partial pressures of water vapor in the pore space of the adsorbent and the air stream blown through it, the water molecules will intensively diffuse into the air stream. In this case, the temperature of the adsorbent layer will be reduced and additional cooling of silica gel (adsorbent) will not be required before the adsorber is included in the stage of draining the removed air.

As you can see, the activation of the adsorbent is carried out without external heat.

Figure 2 shows a diagram of the adsorber-air cooler, figure 3 presents a diagram of the proposed method of utilizing the heat of the removed air with an intermediate coolant, figure 4 shows a diagram of the processing of the removed air in the (I-d) diagram.

Position in figure 2 denote:

1 - housing; 2 - pipes; 3 - tube sheets; 4 - adsorbent layer; 5, 6 - nozzles for air inlet and outlet; 7, 8 - pipes for the inlet and outlet of the intermediate coolant.

Position in figure 3 denote:

1, 2 - adsorbers-air coolers; 3 - air heater; 4 - pump for the intermediate heat carrier; 5 - an expansion tank; 6, 7 - fans; 8, 9, 10, 11, 12, 13, 14, 15 - shutoff valves; 16, 17 - valves; 18 - slide gate valve.

The essence of the proposed method of utilizing the heat of the removed air with an intermediate coolant consists of the following stages (modes).

1. The stage of adsorption of water vapor and the selection of latent and apparent heat of the removed air

The exhaust air from the room using the fans 6, 7 is sent to the adsorber 1, while the valves 8, 9, 14, 15 are open, the valves 10, 11, 12, 13 are closed.

Valve operation is carried out using an automation system.

In the adsorber 1, the air flow passing through the tube space filled with a layer of adsorbent - silica gel is dried by sorption of water vapor to an absolutely dry state. In this case, the heat of adsorption is released.

Then the air is cooled with the selection of adsorption heat and part of the apparent heat with the help of an intermediate heat carrier entering the annular space of the apparatus using pump 4. In this case, valve 16 is open, valve 17 is closed.

The heated intermediate heat carrier enters the surface heat exchanger-air heater 3, where it transfers heat to the cold outside air due to heat transfer and, having cooled, again enters the annular space of the adsorber 1 to cool the air flow, etc.

2. The stage of activation of the adsorbent

Drained to a completely dry state and cooled air from the adsorber 1 with the help of a fan 7 is sent to the adsorber 2, which is at the stage of activation of the adsorbent.

In this case, air passes from bottom to top through an adsorbent layer located in the tube space and causes desorption of water molecules into the air stream due to the difference in the partial pressures of water vapor at the surface of the silica gel capillary and in air. The humidified air stream from the adsorber 2 through the valve 15 is discharged into the atmosphere.

Air purging (activation of the adsorbent) through the adsorbent layer is stopped when the minimum residual moisture content of silica gel is reached, analyzing, for example, the moisture content of the discharged air. After that, the valves 14, 15 are closed, and the gate valve 18 is opened and the drained air from the adsorber 1 is discharged into the atmosphere, bypassing the adsorber 2.

After the equilibrium state of silica gel in adsorber 1 is reached (the indicator may be: indirectly, the duration of adsorption or the moisture content of the exhaust air from the adsorber), the apparatus is placed at the stage of activation of the adsorbent, and the removed air from the room is sent to adsorber 2 for drying and cooling.

For this, valves 8, 9, 14, 15 must be closed, valves 12, 13, 10, 11 open, also open valve 17 and close valve 16 on the intermediate coolant line.

In this cycle, the air removed from the room passes from the bottom up the adsorbent layer in the adsorber 2 due to the operation of the fans 6, 7. At the same time, the air is dried, generating heat of adsorption, and due to the supply of an intermediate heat carrier to the annular space of this apparatus, the air is also cooled.

The intermediate coolant is circulated through the apparatus 2 by means of a pump 4 through an open valve 17. The intermediate heat carrier transfers the selected heat from the dried air to the air heater 3, where it transfers it to the outside air and again enters the annulus 2 of the adsorber for heating.

Absolutely dried and cooled air from the apparatus 2 is sent to the adsorber-air cooler 1 to activate the adsorbent. Using a fan 7, this air through a valve 10 enters the tube space of the adsorber 1, where it promotes the desorption of water molecules from the adsorbent layer into the air. Humidified air from the top of the adsorber is removed through the open valve 11 by the fan 7 into the atmosphere.

To intensify the desorption process, a vacuum pump can be additionally installed to lower the pressure in the adsorber, which is at the activation stage. The vacuum pump can be installed on a common overhead line after valve 11 and 15 (not shown in FIG. 3).

Thus, alternately one or the other adsorber is at the stage of adsorption or activation of the adsorbent, providing continuous selection and utilization of heat from the removed air from the room.

Changing the air parameters in the proposed method of utilizing the heat of the removed air is graphically presented in figure 4.

Here, the rays correspond to the following processes:

Y ₁ A - drainage of the removed air from the initial moisture content d _y1 to 0 (process during adsorption of water vapor by silica gel).

The process occurs when I _у1 - const with increasing temperature due to the release of adsorption heat [1, p. 158, fig. IV. 29].

AD - dry air cooling, carried out using an intermediate coolant during the selection of adsorption heat and apparent heat of air.

DU ₂ - isoenthalpic humidification. The process is carried out at the stage of activation of the adsorbent during desorption of water vapor from the pore space of silica gel into the air.

With parameters t. At _2, air is released into the atmosphere.

Here Y ₁ U ₂ - cooling and drying of air in the traditional method of heat recovery (see figure 1)

From figure 4 it is seen that the amount of utilized heat (heat transferred to the intermediate coolant) is:

a) in the proposed method - Q ₁

where Q _ads is the amount of heat released during the adsorption of water vapor, i.e. during dehumidification of air, W (beam U ₁ A);

Q _cool - the amount of heat taken during dry air cooling, W (beam HELL);

b) in the traditional way

We prove that Q ₂ = Q _cool .

.According to [3, pp. 63-65], the real process of cooling and drying air (beam U ₁ U ₂ ) can be replaced by the conditionally “dry” air cooling mode, i.e. per beam

.

, следовательно,Figure 4 shows:

, hence,

From a comparison of (1) and (3) also shows:

Q ₁ > Q ₂ ; Q ₁ -Q ₂ = Q _ads .

Thus, in the proposed method, the amount of heat utilized is greater than in the traditional method with recuperative heat exchangers by the amount of adsorption heat released during air drying.

According to figure 4, this number is equal to:

Q _ads = D · q _ads = G · (d _y1 -0) · 10 ^-3 · q _ads = G · d _y1 · 10 ^-3 · 2930 = 2.93 · G · d _y1 , kW

Here G is the discharge air rate, kg / s;

d _y1 - moisture content of the removed air, g / kg;

q _ads - specific heat of adsorption, kJ / kg (s.4).

D is the amount of water vapor adsorbed by the adsorbent, kg / s.

The advantage of the claimed invention should include an increase in the amount of heat utilized from the removed air from the room due to the complete selection of the latent heat of water vapor contained in it at the initial stage.

Information sources

1. B.N. Bogoslovsky, O.Ya. Kokorin, L.V. Petrov. Air conditioning and refrigeration. - M.: Stroyizdat, 1985, 367 p.

2. O.Ya. Kokorin. Modern air conditioning systems. M .: Fizmatlit, 2003, 272 p.

3. I.E. Kuznetsov, K.I.Shmat, S.I. Kuznetsov. Equipment for sanitary cleaning of gases. Directory. Kiev: Technique, 1989, 304 p.

Claims (1)

A method of utilizing the heat of a gas (air) stream, in which a stream is passed through a heat exchanger-recuperator with an intermediate heat carrier circulating with a pump in a closed liquid line in series through a heat exchanger-recuperator (air cooler) and a heat exchanger-air heater, characterized in that the pre-heat exchanger is a gas (air) flow is passed through a layer of granular adsorbent to the recuperator to drain and release heat of adsorption.

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