ASHRAE LV-11-C015-2011 Myths and Realities of Indirect Evaporative Cooling Thermodynamic Performance.pdf

1、 Dr. Des Champs is a consultant to Munters Corporation of Buena Vista, Virginia Myths and Realities of Indirect Evaporative Cooling Thermodynamic Performance Nicholas H. Des Champs, Ph.D., P.E. Fellow ASHRAE ABSTRACT With the aid of a newly developed finite-difference computer program, that predicts

2、 the performance of heat exchangers when used as Indirect Evaporative Coolers (IEC), various flow and heat exchanger arrangements are analyzed for overall system cooling performance. One example investigates the use of cooled, dry air resulting from the first pass of an IEC for use as scavenger air

3、in a follow-on IEC heat exchanger for the purpose of increasing the overall system wet-bulb depression efficiency. A second example investigates using an IEC to generate cooled water, as opposed to cooled air, for supplemental cooling in a chilled water system. A third involves predicting the maximu

4、m performance expected by drawing off cooled, dry air as it progresses through an IEC and using this air to feed into the wetted scavenger air flow so as to eventually consume approximately half of the original inlet, dry air as lowered wet-bulb temperature air to be scavenger air for the evaporativ

5、ely cooled flow channels of the IEC heat exchanger. Other concepts are presented, such as using a direct evaporative cooler to pre-cool air for a condenser coil while simultaneously cooling the sump-water temperature of an IEC to enhance its effectiveness. INTRODUCTION With the tendency toward zero

6、energy design for buildings, there has been a greater interest in the energy-savings aspects of utilizing the cooling effects of evaporating water to reduce energy usage in air-conditioning systems. By far, when using the cooling effects of water, evaporative cooling of outdoor air is produced by ha

7、ving the air come in direct contact with a wetted media where energy from the air is used to evaporate water in an adiabatic process. This process results in a lowered dry bulb temperature. It is a very simple process, but Direct Evaporative Cooling (DEC) may introduce too much moisture to the space

8、 which has led to the use of Indirect Evaporative Cooling (IEC), or a combination of IEC and DEC when space relative humidity is a concern. Implementing the IEC process requires the use of an air-to-air heat exchanger. Any of the commonly used air-to-air heat exchangers, such as plate, heat pipe, or

9、 tube meet the requirements. IEC using air-to-air heat exchangers began to show up in the market place in the mid 70s, initially to utilize the exhaust wet-bulb, as opposed to the exhaust dry bulb temperature, to increase recovery efficiency. During the 80s, the IEC concept moved principally to arid

10、 regions and applied to cooling outdoor air with outdoor air (or possibly returns air) and the resulting cooled, dry air stream delivered to the space. In addition, the use of various combinations of IEC, DEC, and cooling coil to condition the supply air in the most efficient manner for a specific a

11、pplication became the norm. There are two basic methods to cool the supply air within the air-to-air heat exchanger during an IEC process. The first is to use a DEC to lower the “working air stream,“ or scavenger air, temperature to within a few degrees of initial scavenger wet-bulb temperature and

12、then direct this cooled, moist air through one side of the heat exchanger that cools the supply air in adjacent flow channels (this process also allows use of energy recovery wheels). Figure 1 is a schematic of a unit that uses DEC media to cool air prior to it traversing the scavenger side of an IE

13、C air-to-air heat exchanger, showing the temperature entering and leaving the components. The second method is to have the entire thermodynamic process take place within the IEC heat exchanger, thus eliminating the DEC. In this case, water continually sprays on the scavenger-channel surface where ev

14、aporation takes place because of the scavenger air flowing over the wetted surface. The cooled water cascades over the scavenger-side surface and absorbs heat, through the tube wall, from the hot, dry air stream. The water flows by gravity to the sump, under the heat exchanger, from which a pump del

15、ivers it to the spray nozzles above the heat exchanger to complete the water recirculation process. Figure 2 is a schematic of the integral IEC heat exchanger that uses its scavenger-side surface as the evaporative cooler or “cooling tower“ with thermal performance shown for this type of IEC product

16、. LV-11-C015 2011 ASHRAE 1192011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is n

17、ot permitted without ASHRAES prior written permission.An IEC system must withstand the difficult environment of sprayed water, with its scaling and corrosion properties as well as extreme sunlight and temperature. It should also be price competitive and present no greater maintenance cost when compa

18、red to conventional air-conditioning equipment. Figure 1 Drawing of an IEC unit showing typical airflow and temperatures using DEC media to cool air prior to passing through the scavenger side of a plate-type air-to-air heat exchanger. Figure 2 Integral cooling tower and air-to-air heat exchanger. 1

19、20 ASHRAE TransactionsNormally, the initial installed cost of an IEC system is greater than a DX system so the overall energy performance of the IEC must be significant in order for the engineers and owners to become interested in their use. An estimate of the total market for IEC systems using air-

20、to-air heat exchangers, in North America, is less than $50,000,000 per year, a small percentage of its potential sales. There are several reasons, 35 years after its introduction, for lack of consideration of the concept, with the principal two reasons being high initial cost and above average maint

21、enance. Inadequate thermal performance is another important reason. An ongoing problem with IEC heat exchangers is lack of necessary analytical tools to aid in the thermodynamic design process. Predicted IEC performance usually is the result of tests performed on a prototype that is relatively easy

22、to build. The testing procedure should, but seldom does, follow the guidelines set forth in ANSI/ASHRAE 143-2007, Method of Test for Rating Indirect Evaporative Coolers. Extrapolating data from an unreliable testing program leads to unpredictable field results. A principal reason that there is a lac

23、k of analytical tools for use by manufacturers is the relative complexity of the interaction of various physical and thermodynamic effects taking place within an IEC heat exchanger. Using a finite-difference program, developed by the author, that considers essentially all of the twenty-five or more

24、inputs that are necessary to define the IEC system and process, several examples illustrate that the predicted performance is sometimes not what might be anticipated. A LIMITED COMPARISON BETWEEN COOLING TOWERS AND IEC EQUIPMENT Figure 3a is a schematic diagram of an indirect-contact evaporative coo

25、ling tower. Water sprays on the tubes and scavenger-air moving counter to the cooling water flow results in evaporative cooling of the tubes and indirectly cools the process liquid within the tubes. The cooling water drains into the sump and then pumped to the spray headers. The identical process ta

26、kes place in an IEC heat exchanger (fig. 3b), except that the supply air is cooled within the tubes instead of process liquid. In both cases, the temperature of the water in the sump, at equilibrium, is the same as the spray water temperature. Therefore, the temperature of the spray water entering a

27、t the top of the heat exchanger is the same as the cooling water temperature leaving at the bottom of the heat exchanger. Comparing these two forms of indirect cooling highlights the fact that IEC has been around for a long time. Figure 3a Indirect cooling tower for cooling water. 2011 ASHRAE 121Fig

28、ure 3b Indirect cooling tower for cooling air. A principal difference between cooling liquid and cooling air is that the air being cooled, without adding moisture, has its wet-bulb temperature lowered. Therefore, it is possible to utilize a portion of the lower wet-bulb air to produce even cooler ai

29、r than in the first stage of cooling. That fact leads one to envision all sorts of processes where, in the limit, there is the possibility that dew-point temperature air is achievable. REVIEW OF CONCEPTS AND DESIGNS Over the 35-year time span, since the first IEC air-to-air heat exchanger came upon

30、the scene, there have been standard techniques and novel approaches to cool a dry air stream indirectly with evaporative cooling, in addition to the two configurations discussed above. Some that made it to the field or promoted to industry are: a) Cooling-tower water to coil piped systems. b) A pate

31、nted, four-stage system consisting of two cooling coils, two direct evaporative coolers, and an air-to-air heat exchanger. c) IEC heat exchangers that use a portion of the leaving, conditioned dry air as scavenger air in an attempt to increase the Wet-Bulb Depression Efficiency (WBDE), but at the ex

32、pense of increased size and/or power consumption. d) A system that recirculates all of the leaving, conditioned air to use as scavenger air in an attempt to produce cool water in the sump for use as a pre-cooler for a chilled water system or for supplying cool water directly to coils. e) An IEC heat

33、 exchanger that extracts portions of the dry, sensibly cooled air at finite increments as the conditioned air proceeds through the heat exchanger for injecting this extracted air into the scavenger air passageways. This is similar to item c) except done in several finite steps as opposed to pulling

34、the air from the end of the heat exchanger after being fully conditioned. f) Using a DEC to reduce air temperature for cooling a refrigeration condenser coil while simultaneously using the cooled water from the DEC sump to enhance the performance of an IEC heat exchanger. COOLING-TOWER WATER TO COIL

35、 The first system analyzed was introduced to the market in the early 80s and over several years, many of them sold in the southwest U.S. Figure 4 is a patent drawing showing the two cooling-tower water to coils. The first evaporative cooler, 122 ASHRAE Transactionsor cooling tower, delivers its cool

36、ed, moist air over a tubular air-to-air heat exchanger to cool a dry air stream going through the tubes. This process is one of the two basic ways to use an air-to-air heat exchanger to sensibly cool air as an IEC. However, in an effort to make the system more efficient a pump delivers cooled water

37、from the evaporative cooler sump to a water coil (cooling-tower-to-coil). The problem with this “two-stage“ approach is that the warmer water returning to the top of the DEC reduces its performance and consequently reduces the cooling ability of the tubular air-to-air heat exchanger. The correct app

38、roach to achieve greater efficiency would be to use a single, more efficient air-to-air heat exchanger typical of Figure 1. Another option would be removal of the air-to-air heat exchanger all together and design a more efficient tower/coil combination. The next two stages defy the first law of ther

39、modynamics. Figure 5a shows the psychrometric processes for the system. Points A to B are across the air-to-air heat exchanger, B to C through the first coil, C to D through the second coil, and D to E through the second evaporative cooler. During the first two stages, removal of heat occurred becau

40、se of the moist, heated scavenger air leaving the system envelope. However, the third and forth stages remove no heat from the system, but the process on the psychrometric chart shows sensible cooling from C to D. Addition of the second coil serves no purpose other than adding cost and pressure drop

41、 since the process from C to E is adiabatic. With the patented concept in mind, the next step, proposed by the inventor, is to make each of the above stages more efficient with the result that the supply air could approach dew-point temperature. Figure 5b postulates just that a “dew-point“ cooler. F

42、igure 4 Example of Indirect cooling tower using combination of evaporative media, cooling coils, and air-to-air heat exchanger. A B C DE 2011 ASHRAE 123Figure 5 (a) Psychrometrics of Patent 4,380,910, showing the four (4) processes. (b) Psychrometrics if the heat exchangers, media pads, and other co

43、mponents are much more efficient. a) b) 124 ASHRAE TransactionsUSING PORTION OF SENSIBLY COOLED AIR AS SCAVANGER AIR Because the air leaving an IEC has a reduced wet bulb, it becomes very intriguing to use this air, with its lower energy level, in some manner, to enhance the overall system efficienc

44、y. The most direct way to do this is to use a portion of the cooled, dry air to feed back as scavenger air, preferably toward the exit end of the supply air where it would have the greatest impact on the dry-air leaving temperature. A 75% WBDE, basic, tubular IEC air-to-air heat exchanger selection,

45、 with two times more scavenger air than supply air, allows a performance comparison between normal once-through operation and a condition that sends 50% of the cooled, dry supply air back to the scavenger inlet. Figure 6 is a schematic of this arrangement. For this heat exchanger, the normal operati

46、on would be 20,000 cfm (L/s = cfm x 0.472) supply and scavenger flow. The schematic shows 10,000 cfm of the supply diverted back to the scavenger side. In order for this arrangement to produce meaningful results, the design must separate the cooler, sump water resulting from the diverted scavenger a

47、ir from the sump using outdoor (or return) air as scavenger air. A separate recirculation pump, spray manifold, and air baffle is an integral part of the design. Figure 6 IEC with 50% of the initial process airflow diverted back to the scavenger inlet at the leaving end of the heat exchanger. Table

48、1 lists the performance for five separate flow arrangements using the same heat exchanger, with the exceptions regarding separating the two scavenger air streams. The ambient condition for all arrangements is 108/66F (42.2/18.9C) at 2500 feet (762 meters). Columns one through three simply varies the

49、 total supply airflow but holds the scavenger flow constant. Column 1 is for a standard heat exchanger that has an equal flow for supply air and scavenger air and has a base WBDE of 76.6%. Column 2 reduces the supply airflow by 25% resulting in the WBDE increase to 81.7% and when reduced to 50% in column three results in the WBDE increasing to 87.4%. Similarly, diverting 25% of the supply as scavenger air results in a WBDE of the remaining supply air of 84% and if 50% is diverted then the WBDE increases to a very respectable 90.7%. Added costs are associated with divertin