Part-Load Space Humidity Control

period five

Application Considerations

notes

Coils can be protected from freezing in many ways, such as:

nDraining the coils during cold weather

nKeeping the water in motion

nAdding glycol to the water

nUsing face-and-bypass dampers to limit the amount of cold airflow through the coil

nPreheating the outdoor air with either a conventional heating coil or heat recovered from the exhaust air stream

part-load space conditions CV system = 75°F, 66% RH

[23.9°C]

VAV system = 75°F, 58% RH

[23.9°C]

	D	mixed air
A	C	B
A		B
55°F	60°F	75°F
[12.8°C]	[15.6°C]	[23.9°C]

dry-bulb temperature

humidity ratio

Figure 100

Part-Load Space Humidity Control

VAV systems do a good job of “indirectly” controlling space relative humidity (RH) at full-and part-load conditions. A typical VAV system supplies air at a constant, low dew-point temperature at all load conditions.

In this example, both the constant-volume and variable-volume systems deliver the same quantity and temperature (A = 55°F [12.8°C]) of air to the space at design load conditions. However, at part load the VAV system still supplies 55°F [12.8°C] air to the space. This results in a lower space relative humidity

(B verses D) than the constant-volume system that modulates the coil capacity and supplies warmer (C = 60°F [15.6°C]) air at the same part-load condition. Realize that adding reheat to this constant-volume system will improve its ability to control space relative humidity, but will also cause it to use more energy.

This demonstrates the danger of using primary air temperature reset on VAV systems! Increasing the primary air temperature on a VAV system will save some cooling energy, but will require more fan energy and will result in higher space relative humidity levels. ASHRAE Standard 62–1999 recommends that the space relative humidity not exceed 60%. If temperature reset is used, it should be allowed to be overridden by a space relative-humidity sensor.

72	TRG-TRC014-EN

TRG-TRC014-EN specifications

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