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I'm modeling a low-temperature operating room being served by a constant volume air handler (it's really only constant volume due to hospital ACH requirements). The model is entirely within DesignBuilder v5.0.1

The air handler's SAT setpoint is 43F and in the design there are two chilled water coils in the mixed air stream that are controlled separately and in series to meet this temperature. The first coil is served by a campus chilled water loop and brings the air down to 54F. The second is served by a low-temperature air-cooled chiller which brings the SAT down to the required 43F (after accounting for draw-thru fan heat). The energy purpose for this arrangement is to use the high-efficiency campus plant for as much as possible, and to then only load the relatively poor-performing air-cooled chiller with the remainder but to also avoid excessive unloading on the air-cooled chiller.

I've tried a number of setpoint manager configurations for this arrangement and the closest I've gotten is to place a 54F setpoint manager after the first cooling coil, a 43F setpoint manager after the second cooling coil, and a 43F setpoint manager after the supply fan. This correctly forces the low-temperature chilled water coil to supply 43F air but the campus CHW coil seems to disregard the 54F setpoint manager and just get the air as close to 43F as it possibly can, which results in temperatures substantially lower than 54F when the mixed air temperature is below the coil's design entering air temperature. In turn, this leads to poor part-load performance and excessive unloading at the air-cooled chiller.

I've also considered using a pre-cooling coil but it looks like this only pre-cools the outdoor air and not the mixed air stream. This is an option but it would be difficult to control the amount of cooling of the outdoor air that would be required in order to bring the mixed air temperature to the 54F setpoint due to variations in the return air temperature at different points in the day. One idea for this approach might be to write an EMS script changing the pre-cooling setpoint to some function of the delta T between mixed air temperature and outdoor air temperature.

Does anyone have any experience with a similar issue? Curious to know if this can be solved with a different setpoint manager/coil configuration, or if I'll need to leave the Designbuilder interface and/or utilize EMS code.

I'm modeling a low-temperature operating room being served by a constant volume air handler (it's really only constant volume due to hospital ACH requirements). The model is entirely within DesignBuilder v5.0.1

The air handler's SAT setpoint is 43F and in the design there are two chilled water coils in the mixed air stream that are controlled separately and in series to meet this temperature. The first coil is served by a campus chilled water loop and brings the air down to 54F. The second is served by a low-temperature air-cooled chiller which brings the SAT down to the required 43F (after accounting for draw-thru fan heat). The energy purpose for this arrangement is to use the high-efficiency campus plant for as much as possible, and to then only load the relatively poor-performing air-cooled chiller with the remainder but to also avoid excessive unloading on the air-cooled chiller.

I've tried a number of setpoint manager configurations for this arrangement and the closest I've gotten is to place a 54F setpoint manager after the first cooling coil, a 43F setpoint manager after the second cooling coil, and a 43F setpoint manager after the supply fan. This correctly forces the low-temperature chilled water coil to supply 43F air but the campus CHW coil seems to disregard the 54F setpoint manager and just get the air as close to 43F as it possibly can, which results in temperatures substantially lower than 54F when the mixed air temperature is below the coil's design entering air temperature. In turn, this leads to poor part-load performance and excessive unloading at the air-cooled chiller.

I've also considered using a pre-cooling coil but it looks like this only pre-cools the outdoor air and not the mixed air stream. This is an option but it would be difficult to control the amount of cooling of the outdoor air that would be required in order to bring the mixed air temperature to the 54F setpoint due to variations in the return air temperature at different points in the day. One idea for this approach might be to write an EMS script changing the pre-cooling setpoint to some function of the delta T between mixed air temperature and outdoor air temperature.

Does anyone have any experience with a similar issue? Curious to know if this can be solved with a different setpoint manager/coil configuration, or if I'll need to leave the Designbuilder interface and/or utilize EMS code.