ABSORPTION CHILLERS Determining the Chilled Water Supply Temperature

ABSORPTION CHILLERS

Determining the Chilled Water Supply Temperature

Determining the Chilled Water Supply Temperature

Determining the Chilled Water Supply Temperature:

The first step in designing or evaluating a chilled water system is to determine the required chilled water supply temperature.

For any HVAC system to provide satisfactory control of both space temperature and space humidity, the supply air temperature must be low enough to simultaneously satisfy both the sensible and latent cooling loads imposed in that  space. Sensible cooling is the term used to describe the process of decreasing the

temperature of air without changing the moisture content of the air. However, if moisture is added to the room by the occupants, infiltrated outdoor air, internal 

processes, and so on, the supply air must be cooled below its dew point to remove this excess moisture by condensation. The amount of heat removed with the  

change in moisture content is called latent cooling. The sum of the two represents the total cooling load imposed by a building space on the chilled water cooling coil.

The required temperature of the supply air is dictated by two factors:

1. The desired space temperature and humidity setpoint

2. The sensible heat ratio (SHR) defined by dividing the sensible cooling

load by the total cooling load  On a psychrometric chart, the desired space conditions represent one endpoint of a line connecting the cooling coil supply air conditions and the space conditions.

The slope of this line is defined by the SHR. An SHR of 1.0 indicates that the line has no slope since there is no latent cooling. The typical SHR in comfort HVAC applications will range from about 0.85 in spaces with a large number of people to ~0.95 for the typical office.

The intersection between this “room” line and the saturation line on the psychrometricchart represents the required apparatus dew point (ADP) temperature for the cooling coil. However, since no cooling coil is 100% efficient, the air leaving the coil will not be at a saturated condition, but will have a discharge dry 

bulb temperature of about 1–2°F above the ADP temperature.

While coil efficiencies as high as 98% can be obtained, the economical approach is to select a coil for about 95% efficiency, which typically results in the supply air wet bulb temperature being about 1°F lower than the supply air dry bulb temperature.

Based on these typical coil conditions, the required supply air temperature can be determined by plotting the room conditions point and a line having a slope equal to the SHR passing through the room point, determining the ADP temperatureintersection point, and then selecting a supply air condition on this line based  on a 95% coil efficiency. Table 1.6 summarizes the results of this analysis for

several different typical HVAC room design conditions and SHRs.  For a chilled water cooling coil, approach is defined as the temperature difference between the entering chilled water and the leaving (supply) air. While this approach can range as low as 3°F to as high as 10°F, a cost-effective value for HVAC applications is ~7°F. Therefore, to define the required chilled water supply temperature, it is only necessary to subtract 7°F from the supply air dry bulb temperature determined from Table 1.6.

Refrigeration Machines:

Typical Supply Air Temperature

Establishing the Temperature Range:

Once the required chilled water supply temperature is determined, the desired temperature range must be established. The required chilled water flow rate is dictated by the imposed cooling load and the selected temperature range. The larger the range, the lower the flow rate and, thus, the less energy consumed for transport of chilled water through the system. However, if the range is too large, chilled water coils and other heat exchangers in the system require increased heat transfer surface and, in some cases, the ability to satisfy latent cooling loads is reduced.

Historically, a 10°F range has been used for chilled water systems, resulting in a required flow rate of 2.4 gpm/ton of imposed cooling load. For smaller systems  with relatively short piping runs, this range and flow rate is acceptable. However,

as systems get larger and piping runs get longer, the use of higher ranges will reduce pumping energy requirements. Also, lower flow rates can also result in economies in piping installation costs since smaller-sized piping may be used. At a 12°F range, the flow rate is reduced to 2.0 gpm/ton and, at a 14°F range, 

to 1.7 gpm/ton. For very large campus systems, a range as large as 16°F (1.5 gpm/ ton) may be used (though low flow rates may introduce problems in selecting cooling coils).

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