DETERMINING CHILLED WATER SUPPLY

Temperature The first step in evaluating a chilled water system is to determine the required chilled water supply temperature. For any HVAC system to provide simultaneous control of space temperature and humidity, the supply air temperature must be low enough to simultaneously satisfy both the sensible and latent cooling loads imposed.

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, etc., 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 and

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 represents one end point 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 approximately 0.95 for the typical office. The intersection between this “room” line and the saturation line on the psychrometric chart represents the required apparatus dewpoint (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 temperature about 1–28F 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 18F lower than the supply air dry bulb temperature.

Based on these typical coil conditions, the required supply air temperature can 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 temperature intersection point, and then selecting a supply air condition on this line based on a 95% coil efficiency.

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 38F to as high as 108F, the typical value for HVAC applications is approximately 78F. Therefore, to define the required chilled water supply temperature, it is only necessary to subtract 78F from the supply air dry bulb temperature.

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