Chilled water coil approach temperature

Technical information Chillers Determining Chilled Water Supply Temperature

Definition Of Chilled Water Inlet

The first step in the assessment of the chilled water system to determine the required supply temperature chilled water.

For any system of ventilation and conditioning for simultaneous control of space temperature and humidity, air temperature must be low enough to simultaneously satisfy both sensible and latent cooling load. Sensible cooling is a term used to describe the process of reducing air temperature without changing the content of moisture in the air. However, if moisture is added to the number of the invaders penetrated the outer air, internal processes, and the other, the supply air must be cooled below the dew point, to remove excess moisture from condensation. The amount of heat removed from the change of humidity is called the latent cooling. The sum of the two represents the total cooling load on a building space of the chilled water cooling coil.

The necessary supply air temperature is determined by two factors:

  1. The right place setpoint temperature and humidity and
  2. Sensible heat ratio (SHR) is determined by dividing the sensible cooling load, with a total cooling load.

On the psychrometric diagram desired space conditions is one endpoint of the line connecting the cooling coil supply air conditions and space conditions.

The slope of this line is determined by the state customs Committee. SHR 1.0 means that the line has slope, because there are no hidden cooling. Typical SHR in comfort HVAC applications will be in the range of about 0.85 in areas with a large number of people around 0.95 for a typical office.

The intersection of this room line and a line of saturation on the psychrometric diagram represents the office of the dew point (ADP), the temperature of the cooling coil. However, since no cooling coil is 100% effective, the air leaving the coil will not be in the saturated state, but will have a temperature of about 1 -2F above the ADP temperature.

While coil efficiency that up to 98% can be obtained, the economic approach is to select the coil about 95% efficiency, which, as a rule, results in the supply air wet bulb temperature of about 1F lower than supply air dry bulb temperature.

On the basis of these typical coil conditions necessary supply air temperature can be determined by the compilation in a room, point and line having the slope SHR passing through the room, the points defining the ADP temperature intersection point, and then selecting the fresh air blower on the status of this line based on 95% of the coil efficiency. Table 1.3 presents the results of this analysis for a few typical HVAC room design conditions and SHRs.

For chilled water cooling coil approach is defined as the difference between the temperature of the chilled water inlet and outlet (delivery) of air. While this approach may fluctuate as low as 3F to level 10F, a typical value for HVAC applications is approximately 7F. thus, to determine the necessary chilled water inlet, you only need to subtract 7F from the supply air dry bulb temperature is determined according to Table 1.3.


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