PGC, Inc
500-1000 CFM Air Handler
May 2006
Appendix A
A-6
2.
Water Temperature Set Point Limits
Minimum and maximum Water Set Point limits are factory pre-set in order to prevent
freezing or overheating the pump.
3.
Dew Point
Humidity is a function of air temperature and dew point, and the water temperature is
directly related to dew point The controller limits the Water Set Point to 1ºC < Air
Set Point to prevent a runaway temperature control. Without this limit, a runaway
condition could be experienced when operating the unit with a high humidity set
point. Such a setting could cause the water temperature to keep increasing in order to
achieve the humidity set point, which would have the side effect of increasing the air
temperature; this increase in air temperature would increase the capacity of the air to
hold moisture, such that the system would never reach the humidity set point.
4.
Measured Water Temp
If the Rh Set Point is set above or below the current Rh control band, the Water Set
Point Rate of Change will automatically maximize to change the water temperature
as quickly as possible. Many variables affect how fast the water temperature will
change, such as the efficiency of the pump, how much water is in the sump, how
often the test chamber is opened, etc. These are beyond the capability of the control
system to measure; however, the controller measures how quickly the water
temperature is actually changing, regardless of the uncontrollable variables. When
the measured Rh is outside of the control band, the Water Set Point is changed
automatically:
Water Set Point = Measured Water Temp + Control Bandwidth
For example, assume that the unit is operating with a Water Set Point of 25ºC and the
water control bandwidth is 5ºC, with the humidity controlling at 50% with the control
bandwidth at 10%. If the Humidity Set Point is set above the upper level of the
control band (>60%), then:
Measured Water Temperature
+
the Water Control Band
=
new Water Set Point
In this example, the new water Set Point would be (25ºC + 5ºC=) 30ºC. This will
cause 100% water heater output, and the Water Set Point will continue to increase at
a value equal to the rate that the water temperature is changing. As soon as the
measured humidity value is back within the control band (in this example, when the
measured Rh is 65.01%), the Water Set Point will resume changing at the normal
rate.
Summary of Contents for 9354-4250
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