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Appendix G condenser water temperature reset in EnergyPlus

This question is following a thread on the EnergyPlus Support group.

Quoting Appendix G:

G3.1.3.11 Heat Rejection (Systems 7 and 8). The heat rejection device shall be an axial fan cooling tower with two-speed fans. Condenser water design supply temperature shall be 85°F or 10°F approaching design wet-bulb temperature, whichever is lower, with a design temperature rise of 10°F.

The tower shall be controlled to maintain a 70°F leaving water temperature where weather permits, floating up to leaving water temperature at design conditions.

For reference CIT rates cooling towers at 95°F DB/78°F WB with 95°/85°F condenser water temperature. That's what's used for the performance of the cooling tower in E+ too.

First question, is my design wetbulb temperature 78°F - meaning the design approach is 7°F - or is it my design day wetbulb temperature (eg: Ann Clg 1% Condns WB=>MDB) for my specific location, in my case 75.8°F meaning an approach of 9.2°F?


Second, as far as the reset strategy, I think SetpointManager:FollowOutdoorAirTemperature placed on the supply outlet of the condenser loop is appropriate.

Knowing that:

  • 85°F = 29.44°C
  • 70°F = 21.11°C

I would do something like this:

SetpointManager:FollowOutdoorAirTemperature,
    MyCondenserControl,      !- Name
    Temperature,             !- Control Variable
    OutdoorAirWetBulb,       !- Reference Temperature Type
    X,                       !- Offset Temperature Difference {deltaC}
    29.44,                      !- Maximum Setpoint Temperature {C}
    21.11,                      !- Minimum Setpoint Temperature {C}
    Condenser Supply Outlet Node;  !- Setpoint Node or NodeList Name

I'm wondering if this is the right approach and what value of Offset Temperature Difference I should use.

Should I use the calculated approach (7°F or 9.2°F) to keep a constant approach? Should I keep it at 0 and leave E+ to try and do as best as possible?

Appendix G condenser water temperature reset in EnergyPlus

This question is following a thread on the EnergyPlus Support group.

Quoting Appendix G:

G3.1.3.11 Heat Rejection (Systems 7 and 8). The heat rejection device shall be an axial fan cooling tower with two-speed fans. Condenser water design supply temperature shall be 85°F or 10°F approaching design wet-bulb temperature, whichever is lower, with a design temperature rise of 10°F.

The tower shall be controlled to maintain a 70°F leaving water temperature where weather permits, floating up to leaving water temperature at design conditions.

For reference CIT rates cooling towers at 95°F DB/78°F WB with 95°/85°F condenser water temperature. That's what's used for the performance of the cooling tower in E+ too.

First question, is my design wetbulb temperature 78°F - meaning the design approach is 7°F - or is it my design day wetbulb temperature (eg: Ann Clg 1% Condns WB=>MDB) for my specific location, in my case 75.8°F meaning an approach of 9.2°F?


Second, as far as the reset strategy, I think SetpointManager:FollowOutdoorAirTemperature placed on the supply outlet of the condenser loop is appropriate.

Knowing that:

  • 85°F = 29.44°C
  • 70°F = 21.11°C

I would do something like this:

SetpointManager:FollowOutdoorAirTemperature,
    MyCondenserControl,      !- Name
    Temperature,             !- Control Variable
    OutdoorAirWetBulb,       !- Reference Temperature Type
    X,                       !- Offset Temperature Difference {deltaC}
    29.44,                      !- Maximum Setpoint Temperature {C}
    21.11,                      !- Minimum Setpoint Temperature {C}
    Condenser Supply Outlet Node;  !- Setpoint Node or NodeList Name

I'm wondering if this is the right approach and what value of Offset Temperature Difference I should use.

Should I use the calculated approach (7°F or 9.2°F) to keep a constant approach? Should I keep it at 0 and leave E+ to try and do as best as possible?

Appendix G condenser water temperature reset in EnergyPlus

This question is following a thread on the EnergyPlus Support group.

Quoting Appendix G:

G3.1.3.11 Heat Rejection (Systems 7 and 8). The heat rejection device shall be an axial fan cooling tower with two-speed fans. Condenser water design supply temperature shall be 85°F or 10°F approaching design wet-bulb temperature, whichever is lower, with a design temperature rise of 10°F.

The tower shall be controlled to maintain a 70°F leaving water temperature where weather permits, floating up to leaving water temperature at design conditions.

For reference CIT CTI rates cooling towers at 95°F DB/78°F WB with 95°/85°F condenser water temperature. That's what's used for the performance of the cooling tower in E+ too.

First question, is my design wetbulb temperature 78°F - meaning the design approach is 7°F - or is it my design day wetbulb temperature (eg: Ann Clg 1% Condns WB=>MDB) for my specific location, in my case 75.8°F meaning an approach of 9.2°F?


Second, as far as the reset strategy, I think SetpointManager:FollowOutdoorAirTemperature placed on the supply outlet of the condenser loop is appropriate.

Knowing that:

  • 85°F = 29.44°C
  • 70°F = 21.11°C

I would do something like this:

SetpointManager:FollowOutdoorAirTemperature,
    MyCondenserControl,      !- Name
    Temperature,             !- Control Variable
    OutdoorAirWetBulb,       !- Reference Temperature Type
    X,                       !- Offset Temperature Difference {deltaC}
    29.44,                      !- Maximum Setpoint Temperature {C}
    21.11,                      !- Minimum Setpoint Temperature {C}
    Condenser Supply Outlet Node;  !- Setpoint Node or NodeList Name

I'm wondering if this is the right approach and what value of Offset Temperature Difference I should use.

Should I use the calculated approach (7°F or 9.2°F) to keep a constant approach? Should I keep it at 0 and leave E+ to try and do as best as possible?