US20190049163A1

US20190049163A1 - Evaporative refrigerant condenser heat exchanger

Info

Publication number: US20190049163A1
Application number: US15/893,026
Authority: US
Inventors: Nicholas Hesser
Original assignee: Evapco Inc
Current assignee: Evapco Inc
Priority date: 2017-02-09
Filing date: 2018-02-09
Publication date: 2019-02-14
Also published as: WO2018148534A1

Abstract

A coil assembly for an evaporative refrigerant condenser having a plurality of nested pairs of serpentine heat exchange tubes tightly packed adjacent to one-another; each nested pair of serpentine heat exchange tubes having an outer serpentine tube and an inner serpentine tube having an identical number of straight lengths and having parallel vertical and horizontal axes.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to evaporative refrigerant condenser coils.

Description of the Background

A conventional evaporative refrigerant condenser is comprised of a fan system, water distribution system and heat exchanger. The heat exchanger is generally comprised of multiple circuits of tube. Individual circuits are formed into serpentines with a specified number of passes, typically by bending a continuous length of tube. Individual circuits are joined in parallel by common inlet and outlet manifolds.
The predominant methods of increasing heat rejection capacity for a given plan area are to increase the airflow rate over the heat exchanger by increasing the fan speed and power, and to increase the heat exchanger surface area by creating serpentines with more passes from longer lengths of tube.

SUMMARY OF THE INVENTION

Manufacturers of conventional evaporative refrigerant condensers have largely applied the aforementioned methods up to the practical physical limitations concerning power consumption, fan blade speed, and material cost and weight. In addition to the physical limitations mentioned above, there exist significantly diminishing returns on thermal performance and efficiency for conventional serpentine coil evaporative condensers. For example in a given plan area, a 350% increase in surface coupled with a 400% increase in fan power results in only 300% increase in heat rejection capability. Table 1 further illustrates the limitations and diminishing returns of increasing circuit length as a means of increasing heat transfer surface area and thermal performance. The capacities provided are normalized for plan area and fan power.

TABLE 1

Thermal Capacity vs. Surface Area Increase for Conventional
Serpentine Coils

Serpentine Coil Row	% Heat Transfer	% Normalized Thermal
(Pass) Increase	Area Increase	Performance Increase

4 to 6	+50%	+30%
6 to 8	+33%	+10%
8 to 10	+25%	+7%
10 to 12	+20%	+6%
12 to 14	+17%	+2%

An analysis of the data used to create Table 1 indicates thermal capacity gains can be realized for a given heat exchanger when the circuiting is simply modified to create more parallel circuits of shorter length compared to a conventional serpentine coil. For all intents and purposes, the factors of tube geometry, tube spacing, external coil surface area, air flow rate, and spray flow rate would be unchanged between the conventional coil and the improvement described above.
A simplified version of this concept is shown on the right in FIG. 4. The improved coil design includes a 100% increase in circuit quantity and roughly 50% decrease in circuit length while maintaining a constant surface area.
The improved circuiting concept, when applied to the fullest extent, can result in a heat exchanger consisting of entirely one-pass circuits as shown in FIG. 5. This heat exchanger can be assembled from modular sections stacked to achieve a range of different heat transfer surface areas and internal volumes.
Increasing the number of circuits and reducing the circuit length has several benefits. First, it reduces the potential travel distance required for condensate to exit the coil. For example, refrigerant that condensed at the mid-point of the first pass must travel through the remaining passes in order to exit the coil, potentially occupying internal tube wall area that could otherwise be used for further condensing.
Second, the presence of additional circuits at the inlet of the heat exchanger will improve the distribution of vapor throughout the coil. This is clearly evident when comparing a conventional four-pass serpentine coil circuit to the equivalent coil with the proposed improvements in FIG. 5. For example, condensate that forms at the mid-point of the bottom pass of a convention serpentine coil will be replaced with vapor that must travel through 3.5 circuit passes. By contrast, vapor must travel through 0.5 circuit passes to reach the equivalent location in the improved coil design.
The improved coil circuiting design will increase the available internal tube wall area available for condensation and decrease the pressure drop through the coil. The result is an increase in condensing capacity, a reduction in compressor power and an overall increase in system efficiency. These benefits may be most notable for high mass flow, low enthalpy refrigerants, which typically exhibit very high pressure drops in longer, narrower serpentine coils comprised of a small number of long, multi-pass circuits. Unit configurations and coil sizes applicable exclusively to low pressure drop refrigerants may now be expanded into new applications when the improved circuit design is implemented.
The circuiting improvements described above can be applied to all currently available evaporative refrigerant condensing equipment designs with minimal modifications to the existing structure, fan system or spray water distribution system, regardless of the tube geometry or the orientation of the heat exchanger with respect to the airflow and spray water flow direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cutaway perspective view of a evaporative refrigerant condenser.

FIG. 2 shows the principal of operation of an evaporative refrigerant condenser.

FIG. 3 shows a prior art evaporative refrigerant condenser coil.

FIG. 4 is a representation of a comparison between a prior art evaporative refrigerant condenser coil (left) and an evaporative refrigerant condenser coil according to an embodiment of the invention (right).

FIG. 5 is a representation of an evaporative refrigerant condenser coil according to another embodiment of the invention.

DETAILED DESCRIPTION

This inventions relates particularly to condenser coil bundles used in evaporative refrigerant condensers 10 of the type shown in FIGS. 1 and 2 configured to indirectly transfer heat between a superheated refrigerant and ambient air, operative in a wet mode or a dry mode as described below depending on ambient atmospheric conditions, such as temperature, humidity and pressure.
The apparatus 10 includes a fan 100 for causing air to flow through the apparatus, and as shown schematically in FIG. 1, sitting atop housing 15. At normal ambient atmospheric conditions where freezing of the cooling liquid, typically water, is not of concern, air is drawn into the plenum 18 of the apparatus via air passages at the bottom of the unit through the open air intake dampers, and enters the evaporative heat transfer section 12 where heat transfer takes place involving the distribution of water from a water distribution assembly 90 driven by a pump 96. When the ambient temperature and the temperature of the cooling liquid fall to indicate a concern of freezing the cooling liquid, the distributor assembly of cooling liquid is turned off.
Prior art refrigerant coil assemblies 20 have a generally parallelepiped overall shape of six sides retained in a frame 21 and has a major/longitudinal axis 23, where each side is in the form of a rectangle. The coil assembly 20 may be made of multiple horizontal closely spaced parallel, serpentine tubes connected at their ends to form a number of circuits through which the refrigerant flows. Each individual circuit within the coil assembly may be a single, continuous length of coil tubing that is subjected to a bending operation which forms the tubing into several U-shaped rows that are in a generally vertical and equally-spaced relationship from each other, such that each circuit has a resultant serpentine shape. Finned tube coil assemblies are preferred.
The coil assembly 20 has an inlet 22 connected to an inlet manifold or header 24, which fluidly connects to inlet ends of the serpentine tubes of the coil assembly, and an outlet 26 connected to an outlet manifold or header 28, which fluidly connects to the outlet ends of the serpentine tubes of the coil assembly. The assembled coil assembly 20 may be moved and transported as a unitary structure such that it may be dipped, if desired, if its components are made of steel, in a zinc bath to galvanize the entire coil assembly.
The refrigerant gas discharges from the compressor into the inlet connection of the apparatus. Heat from the refrigerant dissipates through the coil tubes to the water cascading downward over the tubes. Simultaneously, air is drawn in through the air inlet louvers at the base of the condenser and travels upward over the coil opposite the water flow. A small portion of the water evaporates, removing heat from the system. The warm moist air is drawn to the top of the evaporative condenser by the fan and discharged to the atmosphere. The remaining water falls to the sump at the bottom of the condenser where it recirculates through the water distribution system and back down over the coils.
The invention constitutes a change and improvement over the prior art as illustrated in FIG. 4. Where the left side of FIG. 4 represents the coil assembly 20 used in evaporative refrigerant assemblies of the prior art, the right side of FIG. 4 shows a coil assembly design in which each tube/circuit of coil assembly 20 is replaced with a nested tube/circuit pair 102, each tube in the circuit pair having approximately one-half the length of the prior art circuit (six straight lengths connected by five return bends compared to twelve straight lengths connected by 11 return bends), the nested circuit pairs having effectively the same surface area as the prior art single tube circuit.
Accordingly, where the tube bundle/coil assembly 20 of FIG. 3 comprises approximately 40 tightly packed serpentine tubes, each having 12 straight segments, each tube in the coil assembly of FIG. 3 represented by the left side of FIG. 4, the improved coil assembly of the invention would have 80 tubes arranged as 40 tightly packed sets of nested pairs of tubes, as represented by the right side of FIG. 4, each nested pair of tubes of the invention having the same surface area of the prior art single serpentine tube, and each tube of each nested pair of tubes having one-half the circuit length of the prior art single serpentine tube.
The inlet for both tubes in each pair of nested tubes may be attached to the same inlet header. Alternatively, the outer tubes 104 of each nested pair of tubes may be connected to a first inlet header 105, and the inner tubes 106 of each nested pair of tubes may be connected to a second inlet header 106. Similarly, the outlet for both tubes in each pair of nested tubes may be attached to the same outlet header, or the outer tubes 104 of each nested pair of tubes may be connected to a first outlet header 108, and the inner tubes of each nested pair of tubes may be connected to a second outlet header 109.
Manufacturing of coil assemblies according to this embodiment of the invention has attendant material and labor cost increases, but the efficiencies achieved by such configuration over the life of the device are expected to far exceed the increased manufacturing cost.
According to an alternative embodiment, the coil assembly may be constructed entirely of one pass circuits as shown in FIG. 5. According to a preferred embodiment, the coil assembly may be constructed of modular tube units 200 of the type shown in FIG. 5, each modular unit consisting of one or more straight tubes 110 attached at one end to an inlet header module 112 and at a second end to an outlet header module 116. Each header module may be connected to an adjacent module by a header connector 114. Each modular tube unit may be stacked on top of another modular unit to create coil assemblies of varying heights. Similarly, stacks of modular tube units may be packed adjacent to one-another to create coil assemblies of varying widths. FIG. 5 shows a single stack of 4 modular tube units, each modular tube unit having two straight tubes, one arranged above the other. The inlet header modules of adjacent stacks of modular tube units may be connected to one-another or they may be independent. As with the embodiment shown on the right side of FIG. 4, manufacturing of coil assemblies according to this embodiment of the invention has attendant material and labor cost increases, but the efficiencies achieved by such configuration over the life of the device are expected to far exceed the increased manufacturing cost.

Claims

1. An evaporative refrigerant condenser comprising:

a housing defining a coil section situated above a plenum section;

a fan situated on top of said housing and configured to draw ambient air said plenum section through openings at a bottom of said housing, through said coil section and out through a top of said housing through said fan;

a water distribution assembly located in said housing and above said coil section for selectively distributing water over said coil section;

a water collection section located at a bottom of said housing for collecting water distributed by said water distribution assembly;

a water pump for pumping water from said water collection section to said water distribution assembly;

a coil assembly located in said coil section, said coil assembly comprising a plurality of nested pairs of serpentine heat exchange tubes tightly packed adjacent to one-another; each nested pair of serpentine heat exchange tubes comprising an outer serpentine tube and an inner serpentine tube having an identical number of straight lengths and having parallel vertical and horizontal axes;

each of said nested pairs of serpentine heat exchange tubes connected at a first end to at least one inlet header and connected at a second end to at least one outlet header.

2. An evaporative refrigerant condenser according to claim 1, wherein said serpentine heat exchange tubes are finned.

3. An evaporative refrigerant condenser according to claim 1, wherein a first end of said outer serpentine tubes are connected to a first inlet header and a first end of said inner serpentine tubes are connected to a second inlet header.

4. An evaporative refrigerant condenser according to claim 1, wherein a second end of said outer serpentine tubes are connected to a first outlet header and a second end of said inner serpentine tubes are connected to a second outlet header.

5. An evaporative refrigerant condenser comprising:

a housing defining a coil section situated above a plenum section;

a coil assembly located in said coil section, said coil assembly comprising a plurality of modular tube units, each modular tube unit comprising a plurality of straight tube lengths connected at a first end to an inlet header connector and at a second end to an outlet header connector, wherein said modular units are stacked on top of one-another to create vertical stacks, and a plurality of said vertical stacks are packed laterally adjacent to one-another to result in said coil assembly.

6. An evaporative refrigerant condenser according to claim 5, wherein inlet header connectors of modular tube units in a vertical stack are fluidly connected to one-another.

7. An evaporative refrigerant condenser according to claim 5, wherein inlet header connectors of modular tube units in adjacent vertical stacks are fluidly connected to one-another.

8. An evaporative refrigerant condenser according to claim 5 wherein outlet header connectors of modular tube units in a vertical stack are fluidly connected to one-another.

9. An evaporative refrigerant condenser according to claim 5, wherein outlet header connectors of modular tube units in adjacent vertical stacks are fluidly connected to one-another.

10. A coil assembly for an evaporative refrigerant condenser comprising: a plurality of nested pairs of serpentine heat exchange tubes tightly packed adjacent to one-another; each nested pair of serpentine heat exchange tubes comprising an outer serpentine tube and an inner serpentine tube having an identical number of straight lengths and having parallel vertical and horizontal axes.

11. A method of improving the heat exchange efficiency for an evaporative refrigerant condenser having a housing defining a coil section situated above a plenum section;

a first coil assembly located in said coil section, said coil assembly comprising a plurality single serpentine heat exchange tubes tightly packed adjacent to one-another;

each serpentine heat exchange tubes connected at a first end to an one inlet header and connected at a second end to an outlet header,

said method comprising replacing said first coil assembly with a replacement coil assembly comprising a plurality of nested pairs of serpentine heat exchange tubes tightly packed adjacent to one-another; each nested pair of serpentine heat exchange tubes comprising an outer serpentine tube and an inner serpentine tube having an identical number of straight lengths and having parallel vertical and horizontal axes;

each of said nested pairs of serpentine heat exchange tubes connected at a first end to at least one inlet header and connected at a second end to at least one outlet header, said outer serpentine tube and said inner serpentine tube each having a length that is substantially one-half of a length of said serpentine tubes in said first coil assembly.