RT-SVD03G-EN 105
Economizer Operation Enthalpy Changeover
Reference Dry Bulb Method
OA Temperature is compared with a reference dry bulb point.
• The Economizer is enabled when OA Temp
≤ reference dry bulb point.
• The Economizer is disabled when OA Temp
≥ (reference dry bulb point + 5.0)°F.
• While reference dry bulb point < OA Temp < (reference dry bulb point + 5.0ºF), the Economizer
enable/disable status is not changed.
Note: If on a unit with Comparative Enthalpy control the OA or RA Temp is greater than 120°F or
less than 32°F, the economizer control will revert to Reference Enthalpy Control if the OA
temp is within 32°F and 120°F. If the unit is configured with Comparative or Reference
Enthalpy control, if the OA temp. is not within this range, the economizer control will revert
to Dry Bulb Control. Once this occurs, the "out of range" temperature(s) must fall to 118°F
or rise to 34°F to again enable Comparative or Reference Enthalpy control. If either OA or
RA Humidity is measured to be greater than 90% or less than 10%, the value of Humidity
which is reported by the RTEM and used for enthalpy calculation will be 90% or 10%
respectively.
Internal Calculations
Air enthalpy is a value calculated from the air temperature and relative humidity using the
relationship:
Enthalpy= 0.24 x O.A.Temp(deg. F) + h(R.H.,OAT)
Air enthalpy is calculated (Btu/Lb. dry air) using:
H = 0.24 * T + W * Hv
Where
T= dry bulb temperature, deg. F
W= humidity ration, lb. water/lb. dry air
W = 0.622 * P/ (14.696 - P)
Hv= enthalpy of water at T, Btu/lb.
Hv= 1062.1 + 0.43 * T
Also,
P= partial pressure of water at T, psia
P= (0.421 - 0.01503 * T + 0.000202 * T**2) * RH
Figure 63. Dry Bulb Enthalpy Enable
Dry Bulb Hysteresis
Economizer Enabled
OAT - reference point (°F)
Economizer Disabled
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