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Economizer High Limit Controls and Why Enthalpy

November 2010 ASHRAE Journal 1 O utdoor air economizers use controllable dampers to increase the amount of outside air drawn into the building when the outside air is cool or cold and

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By Steven T. Taylor, P.E., 
Fellow ASHRAE; and 
C. Hwakong Cheng, 
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Figure 1:
 Outdoor air economizer controls.
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Figure 3:
 Optimum high limit logic: dry coil.
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High Limit Control Strategies
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Figure 2:
  Optimum high limit logic: wet coil.
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Fixed Dry-Bulb Temperature 
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Figure 4:
 Optimum high limit logic:
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Figure 5:
 Fixed dry-bulb high limit error: setpoint 72°F.
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Figure 6:
 Fixed dry-bulb high limit error: setpoint 65°F.
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Figure 7:
 Differential dry-bulb high limit error.
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Differential Dry-Bulb Temperature 
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Figure 10:
 Fixed enthalpy high limit error.
Figure 8:
 Differential dry-bulb high limit error: San Francisco.
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Figure 9:
 Differential dry-bulb high limit error: Atlanta.
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Figure 11:
 Fixed enthalpy high limit error: Albuquerque.
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Combination High Limits 
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Figure 13:
 Differential enthalpy high limit error.
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Figure 14:
 Error for a combination high limit of differential 
dry bulb and differential enthalpy.
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Figure 15:
 Error for a combination high limit of �xed dry bulb 
and �xed enthalpy.
Figure 12:
 Fixed enthalpy high limit error: Chicago.
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Figure 17:
 Electronic enthalpy controller error: “A” setting.
Figure 16:
 Electronic enthalpy controller.
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Figure 18:
 Iowa Energy Center NBCIP study: best humidity sensor.
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Figure 19:
 Iowa Energy Center NBCIP study: one of the worst 
humidity sensors.
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Table 1:
 Seven high limit controls and combinations modeled and summarized.
Figure 20:
 High limit control performance: Climate Zones 1 and 2.
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Figure 21:
 High limit control performance: Climate Zone 3.
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Figure 22:
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Figure 23:
 High limit control performance: Climate Zone 5.


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Figure 24:
 High limit control performance: Climate Zones 6 to 8.
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Table 2:
 High limit control recommendations for integrated economizers.
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Figure 25:
 Required maximum differential enthalpy error to match �xed dry bulb 
with ±2°F (±1°C) error.
Enthalpy Sensor Error (Btu/lb)
Approximate Humidity Sensor Error (% RH)
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