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72 TRG-TRC014-EN
notes
period five
Application Considerations
Coils can be protected from freezing in many ways, such as:
n Draining the coils during cold weather
n Keeping the water in motion
n Adding glycol to the water
n Using face-and-bypass dampers to limit the amount of cold airflow through
the coil
n Preheating the outdoor air with either a conventional heating coil or heat
recovered from the exhaust air stream
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.
dry
dry
-
-
bulb temperature
bulb temperature
humidity ratio
humidity ratio
Part-Load Space Humidity Control
part
part
-
-
load space conditions
load space conditions
CV system =
CV system =
VAV system =
VAV system =
mixed air
mixed air
75°F
75°F
[23.9°C]
[23.9°C]
60°F
60°F
[15.6°C]
[15.6°C]
55°F
55°F
[12.8°C]
[12.8°C]
75°F, 66% RH
75°F, 66% RH
[23.9°C]
[23.9°C]
75°F, 58% RH
75°F, 58% RH
[23.9°C]
[23.9°C]
A
AA
A
A
AA
A
B
BB
B
B
BB
B
C
CC
C
C
CC
C
D
DD
D
D
DD
D
Figure 100