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TRG-TRC014-EN 71
period five
Application Considerations
notes
Freeze Protection for Coils
As mentioned, proper VAV system ventilation often leads to a high percentage
of outdoor air at part-load conditions. Streams of air at vastly different
temperatures are difficult to mix and may cause the mixed air stream to stratify,
resulting in a risk of freezing the water inside the cooling coil.
Air blending devices, mixing baffles, and high-velocity mixing dampers can
provide effective mixing, but usually require more space in the air handler and
more fan power. As an alternative, preheating the outdoor air before it enters
the mixing box decreases this temperature difference, improves mixing and
offers freeze protection for cooling coils.
Even with effective mixing, the temperature of the mixed air entering the coil
may approach that of the outdoor air during cold weather. This may cause
water inside the cooling coil to freeze.
Freeze Protection for Coils
OA
RA
mixed air
mixed air
warm air
warm air
cold air
cold air
cooling
cooling
coil
coil
80°F
80°F
[26.7°C]
[26.7°C]
10°F
10°F
[
[
-
-
12.2°C]
12.2°C]
Figure 98
Measures to Prevent Coil Freeze-Up
▲ Drain coils during cold weather
▲ Keep water in motion
▲ Add glycol to the water
▲ Use face-and-bypass dampers to limit amount
of cold airflow through coil
▲ Preheat the outdoor air (heating coil, heat
recovery)
Figure 99