Can Your Plant Benefit From Free Cooling?

KENNETH M. ELOVITZ, Energy Economics, Inc., Foxboro, MA

here are several direct and are referred to as “fre cooling’’ tech- indirect ways to use outside Whatever technique is niques, they are not completely free air in place of mechanical applied, freeze because a and pumps cooling. A number of these must be used to cool a water circuit. winter free-cooling systems protection is important However, they do require little or no :fer significant energy savings. if a cooling tower additional cooling tower or pump- This is the final installment in a ing energy beyond what would be :ries of articles on free-cooling tech- is involved required if the mechanical cooling iques. Airside and waterside (hy- system were operated. .onic) , beat exchangers, I rect interconnections (strainer cyc- Migration ), and glycol runaround loops were tower considerations important to Refrigerant migration is sometimes scussed previously. Here, refrigerant every installation are reviewed. applied to a centrifugal . In this igration is explained, and cooling Although the methods described arrangement, refrigerant from the is transferred to the con- denser without using the comvressor Refrigerant Migration - (Fig.1). i&*a% ”-3 ,y When the weather is cold enough, a cooling tower produces condenser water several degrees colder than the required for the plant. When this cold water runs through the condenser section of the chiller, the saturation temperature end pressure in the condenser is pushed below the sat- uration temperature and pressure in the evaporator section. The pressure difference is a driving

Fig. 1. In a refrigerant migration arrangement [applied to a cen- trifugal chiller], refrigerant from the evaporator is transferred to the condenser without using the . Refrigerant vapor is then forced from the evaporator through the idle compressor or a special valved refrigerant gas line to the condenser. Air Handling migration can be used only on cen- Unit Coil tral chilled water systems and cen- trifugal that can be retro- Other Loads fit with the required valves and accessories and when the winter chilled water load is fairly small.

76 April 1994 Plant Engineering FILE 2550 force that conveys refrigerant vapor from the evaporator through the idle compressor or through a special, valved refrigerant gas line to the con- denser. The chiller may need a valved refrigerant liquid line to allow liquid to drain from the condenser to the evaporator during the free-cooling cycle. The chiller also needs additional refrigerant charge and a receiver to store the extra refrigerant needed for free cooling during lhe mechanical cooling cycle. It may need a refriger- ant pump to promote circulation. Some installations work on natural 1. Water freezes a1 32 F. The water Fig. 2. A11 free-cooling systems thermal circulation. Others require a in cooling towers that reject heat from except the airside small refrigerant pump. mechanical cooling in winler is in the involve operating a cooling tower Refrieerant migration amlies onlv 70 to 80 F ranee and in little danger of in winter. Because of the cold tem- I I. L to buildings with central chilled water freezing. The danger increases at the peratures that occur, the tower systems and centrifugal chillers that lower circulating water temperatures should always be protected can be retrofit with the required valves necessary for free cooling. Some against freezing. [Photo courtesy and accessories. Buildings must also water in parts of the tower, depending The Marley Co.1 have a fairly small winter chilled water on the type and the load, can approach load. or fall below freezing even if the aver- evaporates. All city water piping can Although some manufacturers age water temperature leaving the be indoors if the system has an indoor &!$...*f.., 631 report higher figures, free cooling by tower is in the. safe 40-F range. Good sump and the condenser water piping refrigerant migration can realistically water distribution and tower airside is arranged so that the sump is always be expected to provide approximately control practices minimize the varia- in the pumping circuit. 25% of mechanical cooling capacity. tion in water temperature within the 4. The possibility of freezing is The more free-cooling capacity tower. reduced if tower capacity is controlled required of the chiller, the lower the 2. Cooling lower outdoor sumps on the air side instead of the water condenser water temperature must be. should be insulated and supplied with side. The tower should always have On the other hand, the higher the electric or steam-fired basin heaters to design water flow over it. Towercapac- condenser water temperature at which keep idle water in the sump from ity is controlled by reducing fan speed the system works, the more hours each freezing. Because circulating water (two-speed motor or variable speed year free cooling can displace com- should always be above freezing, drive) or installing a fan discharge pressor operation. The smaller per- heaters only need to operate during off control (on centrifugal fans). centage of chiller capacity needed cycles. An indoor sump eliminates the Fans on induced draft, crossflow tow- when the system is in the free-cooling need for basin heaters. ers used for winter operation should be mode, the higher themaximum allow- During off cycles, water from the reversible. Reversing airflow is one able condenser water temperature and tower drains into the indoor sump and way to deice a cooling tower. the more hours the system can be used. is safe from freezing. Water in the 5. Large axial fans on induced-draft indoor sump can act as thermal stor- cooling towers should have vibration Cooling Tower Considerations age or a buffer against water tempera- sensors to shut off the fan and sound All free-cooling systems except an ture swings. Indoor sumps typically an alarm if unacccptable vibration airside economizer rely on operating a hold 3 - 5-min circulation through the occurs. Ice can accumulate on fan cooling tower during winter (Fig. Z), system. blades unevenly, producing enough often at temperatures below freezing. 3. Outdoor city makeup water pip- vibration to damage the fan, cooling Protecting a cooling tower from freez- ing should be insulated and heat tower, or its surroundings. If an imbal- ing is important and not difficult. Sev- traced. Cooling towers work by evap- ance occurs, the fan must be deiced. eral principles are involved. (Airside orative cooling whether they run in economizers were discussed in the winter or summer. Makeup water is Cooling Tower Capacity first article in this series, PE, 10/21/93, needed to replace the approximately The tower is the source of cooling in p 61, File 2550.) 1% of the circulating flow rate that all water-based free-cooling systems.

FILE 2550 April 1994 Plant Engineering 77 ------. .. .- Free cooling potentially reduces operating costs, but a proper economic analysis is needed to consider parasitic losses

Therefore, any analysis of free-cool- The required chilled water supply 15,000 Btu/hr/ton. ing potential should begin with an temperature together with the In many cases (other than process analysis of cooling tower capacity. approach in any heat exchangers in the applications), the winter cooling load The design rating of the cooling tower system determine the highest outdoor is much less than the summer cooling operating at summer conditions does temperature at which free cooling can load. At constant water flow, the not apply to winter cooling applica- be used. smaller the load, the smaller the range. tions. The approach of the cooling tower Therefore, the winter range may be '/I As a rule of thumb, the winter free- leaving water temperature to outdoor to [/z the summer range. The smaller cooling capacity in nominal tons air wet bulb temperature increases as range allows the tower to make a clos- (12,000 Btu/hr/ton) is about half the outdoor wet bulb temperature decreas- er approach. nominal cooling tower summer heat es. This occurs because the cooling rejection tons (15,000 Btulhrlton). tower approach is a constant Cost Savings of Free Cooling However, actual data from the cooling difference, not a constant wet bulb The purpose of free cooling is to tower manufacturer should be used for temperature difference. reduce operating costs. The potential this important part of the analysis, not On the psychrometric chart, Ihe wet for doing so is great. However, free a rule of thumb. bulb lines are much closer together at cooling is not truly free. A proper eco- Savings available from a free-cool- winter temperatures than at summer nomic analysis must consider parasitic ing system depend on the mechanical ones. At 78-F ambient wet bulb tem- losses. Cooling tower fan energy cooling load being dis- increases considerably over placed and the number of I the amount required to cool hours each year the system - condenser water enough to The design rating of the cooling operates in the free-cooling 1 I run a chiller. mode. The number of houri I tower operating- at summer I All water-based systems of possible operation conditions does not apply to winter require pumps. The airside depends on the required economizer does not. When chilled water supply tem- cooling applications systems are compared, &hi perature, leaving tower I 1 pumping energy must be water approach to the win- recognized. Increases from ter outdoor wet bulb temperature, and perature to 85-F leaving tower water heat exchangers or coils installed in approach of the (on a temperature, a 7 deg F wet bulb series with other equipment should be chilled water system) or cooling coil approach in the summer is the same considered. Also important are (on hydronic economizer or strainer enthalpy difference (7.85 Btullb) as an increases in supply fan energy to over- cycle systems). 18 deg F wet bulb approach (26-F come the pressure drop through addi- The chilled water supply tempera- ambient wet bulb to44-F leaving tower tional or deeper cooling coils that the ture required for winter cooling may water) in the winter. free-cooling system might require. CI be slightly higher than the design sum- If cooling tower water flow holds mer chilled water temperature. This constant, the cooling tower range is increased chilled water temperature is normally quite a bit less in winter free- For more information ... possible because a given cooling coil cooling applications than in summer. I provides the same sensible cooling Maintaining constant tower water Row Two previous articles discussed free capacity at higher water temperatures is strongly recommended to reduce cooling concepts. The first part of when there is no dehumidification the likelihood of freezing and main- "Can Your Plant Benefit From Free load (100% ratio), tain consistent tower performance by Cooling" (PE, 10/21/93, p 61, File However, if the air handling units keeping uniform water distribution 2550) covered air and water-side serve large interior zones, they need to over the fill. economizers. The second part (PE, produce the same supply air tempera- If the winter cooling load were March 1994, p 106, File 2550) con- ture summer and winter. The only part equal to the summer cooling load, the sidered heat exchangers, direct of the load that falls off enough in win- tower range in winter would be interconnection, and glycol run- ter to permit increasing the chilled approximately 80% of the summer around loops. water supply temperature is the dehu- range. The winter range is less than the Questions about this article may midification load. summer range because there is no be directed to the author at 508- Therefore, the free-cooling system compressor heat to reject in winter. 543-2447. design should not anticipate satisfying Therefore, cooling towers used for ...Jeanine Katzel, HVAC Editor, 708- the load with higher chilled water teni- free cooling reject only 12,000 390-2701 peratures in winter than in summer. Btulhrlton instead of the nominal

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