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?
