ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 14-2012 CONDENSER WATER SYSTEMS.pdf

1、14.1CHAPTER 14CONDENSER WATER SYSTEMSOnce-Through City Water Systems . 14.1Open Cooling Tower Systems . 14.1Low-Temperature (Water Economizer) Systems. 14.3Closed-Circuit Evaporative Coolers. 14.4Overpressure Caused by Thermal Fluid Expansion . 14.4S part of the vapor-compression cycle for mechanica

2、l refrigera-Ation, the heat of compression produced must be rejected tocomplete the refrigeration cycle. Refrigerant systems may be cooledby air or water. In water-cooled systems, water flows through thecondenser and is called condenser water. Condenser water systemsare classified as (1) once-throug

3、h systems (e.g., city water, well-water, or lake/groundwater systems), or (2) recirculating or coolingtower systems.ONCE-THROUGH CITY WATER SYSTEMSOnce-through city water systems use water from the citys pota-ble water supply and circulate through the refrigeration equipmentcondenser (e.g., walk-in

4、cooler, computer room unit) to reject heatand discharge water directly to the sewer. Many municipalities pro-hibit this type of direct water cooling because of water conservationrestrictions, although some localities allow its use as a standby oremergency condenser water system for critical refriger

5、ation needssuch as for computer rooms, research laboratories, or critical oper-ating room or life support machinery. If city water is allowed forcondenser water purposes, then allowed flow may be limited to lessthan 3.5 mL/s per kilowatt of cooling. The designer should contactthe local water departm

6、ent to determine the viability of this system.Figure 1 shows a water-cooled condenser using city water. Thereturn (leaving water) is run higher than the condenser so that thecondenser is always full of water. Water flow through the condenseris regulated by a control valve in the supply or discharge

7、line, usu-ally actuated from condenser head pressure to (1) maintain a con-stant condensing temperature with load variations and (2) closewhen the refrigeration compressor turns off. City water systemsshould always include approved backflow prevention devices andopen (air gap) drains. When more than

8、 one condenser is used on thesame circuit, individual control valves are used.Piping materials for these systems are generally nonferrous, usu-ally copper but sometimes high-pressure plastic because corrosion-protective chemicals cannot be used. Scaling can be a problem withhigher-temperature conden

9、sing surfaces when the water has a rela-tively high calcium content. In these applications, mechanicallycleanable straight tubes should be used in the condenser.Piping should be sized according to the principles outlined inChapter 22 of the 2009 ASHRAE HandbookFundamentals, withvelocities of 1.5 to

10、3 m/s for design flow rates. A pump is usually notrequired where city water is used. Well water can be used in lieu ofcity water, connected on the service side of the pumping/pressurecontrol system. Because most well water has high calcium content,scaling on the condenser surfaces can be a problem.A

11、nother once-through system uses lake or river water. Althoughthese systems eliminate the need for a cooling tower, they requireparticular attention to filtering out sediment, particulates, and othermaterials that could foul the system. In the United States, specialpermitting is required from the loc

12、al Department of NaturalResources (DNR) and the Environmental Protection Agency (EPA).Particular attention must be paid to the design of intake structuresand keeping the velocity below 0.15 m/s so as not to intake aquaticwildlife.OPEN COOLING TOWER SYSTEMSOpen systems have at least two points of int

13、erface between thesystem water and the atmosphere; they require a different approachto hydraulic design, pump selection, and sizing than do closed hotand chilled-water systems. Some heat conservation systems rely ona split condenser heating system that includes a two-section con-denser. One section

14、of the condenser supplies heat for closed-circuitheating or reheat systems; the other section serves as a heat rejectioncircuit, which is an open system connected to a cooling tower (seeChapter 40).In selecting a pump for a cooling tower/condenser water system,consideration must be given to the stat

15、ic pressure and the systemfriction loss. The pump inlet must have an adequate net positive suc-tion pressure (see Chapter 40). In addition, continuous contact withair introduces oxygen into the water and concentrates minerals thatcan cause scale and corrosion on a continuing basis. Fouling factorsan

16、d an increased pressure drop caused by aging of the piping mustbe taken into account in the condenser piping system design (seeChapter 22 of the 2009 ASHRAE HandbookFundamentals). Cor-rosion and erosion are not just limited to condenser water piping: thechiller condenser water bundle sees the same w

17、ater. It is not uncom-mon to coat the condenser water box with epoxy paint to prolong itslife. Similarly, adding nylon tube inserts extends the life of the tubeends at the tube sheet walls.The preparation of this chapter is assigned to TC 6.1, Hydronic and SteamEquipment and Systems.Fig. 1 Condenser

18、 Connections for Once-Through City Water System14.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)The required water flow rate depends on the refrigeration unitused and on the temperature of the available condenser water. Cool-ing tower water is available for return to the condenser at a temper

19、-ature several degrees above the design wet-bulb temperature,depending on tower performance. An approach of 3.9 K to the designwet-bulb temperature is frequently considered an economicallysound design. In city, lake, river, or well water systems, the maxi-mum water temperature that occurs during the

20、 operating season mustbe used for equipment selection and design flow rates and tempera-ture ranges.The required flow rate through a condenser may be determinedwith manufacturers performance data for various condensing tem-peratures and capacities. With air-conditioning refrigeration appli-cations,

21、a return or leaving condenser water temperature of 35C isconsidered standard practice. If economic feasibility analyses canjustify it, higher leaving water temperatures may be used.Figure 2 shows a typical cooling tower system for a refrigerantcondenser. Water flows to the pump from the tower basin

22、or sumpand is discharged under pressure to the condenser and then back tothe tower. When it is desirable to control condenser water tempera-ture or maintain it above a predetermined minimum, water isdiverted through a control valve directly back to the tower basin.Piping from the tower sump to the p

23、ump requires some precau-tions because, at this point, the water is basically flowing due togravity. The sump level should be above the top of the pump casingfor positive prime, and piping pressure drop should be minimizedsuch that there is always adequate net positive suction on the pump.All piping

24、 must pitch up to the tower basin, if possible, to eliminateair pockets. It is not unusual to have this piping a diameter or twolarger than the pump discharge or pressurized piping.If used, suction strainers should be equipped with inlet and outletgages to indicate through excessive pressure drop wh

25、en cleaning isrequired. In-line pipe strainers are not recommended for coolingtower systems because they tend to become blocked and turn into areliability problem in themselves. Many designers depend on largemesh screens in the tower sump and condenser heads designed withsettling volumes to remove p

26、articulate matter. If a strainer is deemednecessary, two-large capacity basket strainers, installed in parallelsuch that they can be alternately put into service and valved out forcleaning, are recommended.Air and Vapor PrecautionsBoth vapor and air can create serious problems in open coolingtower s

27、ystems. Water vaporizes in the pump impeller if adequate netpositive suction pressure is not available. When this occurs, thepump loses capacity, and serious damage to the impeller can result.Equally damaging vaporization can occur in other portions of thesystem where pressure in the pipe can drop b

28、elow the vapor pressureat the operating water temperature. On shutdown, these very lowpressures can result from a combination of static pressure andmomentum. Vaporization is often followed by an implosion, whichcauses destructive water hammer. To avoid this problem, all sectionsof the piping system

29、except the return line to the upper tower basinshould be kept below the basin level. When this cannot be achieved,a thorough dynamic analysis of the piping system must be per-formed for all operating conditions, a soft start and stop control suchas a variable-frequency drive on the pump motor is rec

30、ommended asan additional precaution, and all actuated control valves should beslow closing.Air release is another characteristic of open condenser watersystems that must be addressed. Because the water/air solution inthe tower basin is saturated at atmospheric pressure and cold-waterbasin temperatur

31、e, the system should be designed to maintain thepressure at all points in the system sufficiently above atmosphericthat no air will be released in the condenser or in the piping system(see Figures 2 and 3 in Chapter 13).Another cause of air in the piping system is vortexing at thetower basin outlet.

32、 This can be avoided by ensuring that the maxi-mum flow does not exceed that recommended by the tower manu-facturer. Release of air in condenser water systems is the majorcause of corrosion, and it causes decreased pump flow (similar tocavitation), water flow restrictions in some piping sections, an

33、d pos-sible water hammer. Antivortexing devices can be added to the pip-ing to mitigate this occurrence.Pump Selection and Pressure CalculationsThe elements of required pump pressure are illustrated in Figure3. Because there is an equal pressure between the level in the towersump or interior reservo

34、ir and the pump on both the suction and dis-charge sides, these static pressures cancel each other and can be dis-regarded.Fig. 2 Cooling Tower Piping SystemFig. 3 Schematic Piping Layout Showing Static and Suction PressureCondenser Water Systems 14.3The elements of pump pressure are (1) static pres

35、sure fromtower sump or interior reservoir level to the tower header, (2) fric-tion loss in suction and discharge piping, (3) pressure loss in thecondenser, (4) pressure loss in the control valves, (5) pressure lossin the strainer, and (6) pressure loss in the tower nozzles, if used.Added together, t

36、hese elements determine the required pump totaldynamic pressure.Normally, piping is sized for water velocities between 1.5 and3.6 m/s. As stated previously, piping on the discharge of the pumpmay see the higher velocities, and piping on the suction sideshould see the lower velocities. Refer to Chapt

37、er 22 of the 2009ASHRAE HandbookFundamentals, for piping system pressurelosses. Friction factors for 15-year-old pipe are commonly used.Manufacturers data contain pressure drops for the condenser,cooling tower, control valves, and strainers.If multiple cooling towers are to be connected, the piping

38、shouldbe designed so that the pressure loss from the tower to the pump suc-tion is exactly equal for each tower. The basin water operating levelshould physically be at the same elevation. Additionally, largeequalizing lines or a common reservoir can be used to ensure this.However, for reliability, r

39、edundancy, and ease of maintenance, mul-tiple basins are often preferred.Evaporation in a cooling tower concentrates the dissolved solidsin the circulating water. This concentration can be limited by dis-charging a portion of the water as overflow or blowdown. This maybe accomplished by discharging

40、or bleeding a continuous stream ofwater, or by using a conductivity sensor to activate blowdown valvesin the system. A rough estimate of the bleed rate should equal theamount of evaporation occurring in the tower.Makeup water is required to replace water lost by evaporation,blowdown, and drift. Auto

41、matic float valves or level controllers areusually installed to maintain a constant water level. If float valvesare used, the control system should be tuned to desensitize the read-ings from turbulent, foaming water in the cooling tower basin. If areservoir is used, the level devices may be installe

42、d in a “stilling”tube to read less-turbulent water levels.Water TreatmentWater treatment is necessary to prevent scaling, corrosion, andbiological fouling of the condenser and circulating system. Theextent and nature of the treatment depends on the chemistry of theavailable water and on the system d

43、esign characteristics. On largesystems, fixed continuous-feeding chemical treatment systems arefrequently installed in which chemicals, including acids for pH con-trol, must be diluted and blended and then pumped into the con-denser water system. Corrosion-resistant materials may be requiredfor surf

44、aces that come in contact with these chemicals. In pipingsystem design, provisions for feeding the chemicals, blowdowns,drains, and testing must be included. Because cooling towers movegreat quantities of air and the water flow acts as an air washer, tow-ers can introduce a great deal of airborne pa

45、rticulate into the basinand condenser water system. Therefore, side-stream filtration withseparate pumping systems is recommended. For further informationon water treatment, refer to Chapter 40 of this volume and Chapter49 of the 2011 ASHRAE HandbookHVAC Applications.Freeze Protection and Winter Ope

46、rationOutdoor piping must be protected (insulated and heat traced) ordrained when a tower operates intermittently during cold weather.The most satisfactory arrangement is to provide an indoor receiv-ing tank into which the cold-water basin drains by gravity, asshown in Figure 4A. The makeup, overflo

47、w, and pump suctionlines are then connected to the indoor reservoir tank rather than tothe tower basin. This arrangement may also use vertical turbinepumps that save floor space compared to a base-mounted centrifu-gal pump. The indoor basin also serves as a settling basin for anywaterborne particula

48、te; thus, the basin should be drained or cleanedannually to avoid biological growth and potential fouling of thechillers condenser surfaces. If the system is small, many coolingtower manufacturers have remote sump tanks as an option.The reservoir or sump should be sized for the entire system watervo

49、lume that drains back into the sump when the pumps deenergize.A common rule of thumb is to add the volume of water required ifthe condenser water pumps were to run for 3 min as a minimumlevel to have enough water to prime the piping system. The reser-voirs should have high- and low-water alarms and makeup waterlevel controls. Consideration should also be given to the pump suc-tion requirements to prevent vortexing to determine the depth.Because the pipe entering the sump also may contain great quanti-ties of air, the sump should have a vent or open grate installed ofadequate size.For