[Equest-users] 4 Stages of Cooling

[Porter, Fred]

Curt,
I’ve just been opening up this feature to evaluate some retrofit RTU controls. First, there is fairly thorough documentation in the help including the “engineering description.” However, the SYSTEM keywords NUM-COOL-STAGES and NUM-HEAT-STAGES seem to have been left out of the interface as far as I can tell. The correct values can be entered in text mode and they are preserved by eQ upon reopening.  You would need to enter NUM-COOL-STAGES = FOUR.

The fan speed at each stage appears to be determined approximately by the ratio of the coil dT at the stage over the design coil dT. Making the following assumptions about your case:
100% speed = both stages of heating, and three highest stages of cooling
50% speed = first stage cooling, 25% capacity
35% speed = minimum/deadband

If the design heating coil dT is 40F, (e.g. 100F discharge at 60F mixed air at 100% flow) and your low stage heat is 50%, to keep the fan operating at 100%, set the Stage 1 “dT” to -20F. This “dT” is the change or offset of the lower stage coil dT to the highest stage. The negative number will not display, but it will be used! So the low stage discharge will be 80F and the fan should be modeled as running close to 100%. A positive number can be input, but will not work correctly. Perhaps a few items that could be fixed easily by the developers. Currently the same in 3.65.

If the cooling design coil dT is 24F, and your “FOUR” stages are 25% each and you are keeping the fan at 100% on the top 3 stages, it seems you would let the high stage default to a “dT” of 0, set the 2nd stage “dT” to 6,  3rd stage “dT” to 12 and 4th stage “dT” to 12 also. However the maximum “dT” allowed by eQ for this input is 10F. Close enough probably.

Set the System/Fans/Flow Parameters/Min Flow Ratio to 35%, I’m not totally clear on what to do with the Heating Min Flow Ratio. I think it could be set to 100% in this case. But it shouldn’t make much difference.

Since the mixed air temps/enthalpies will not be at design values all the time, eQ will actually vary the fan speeds at a given capacity level to achieve a calculated leaving air temp. But probably not by too much. Unfortunately, no specific output variables are provided for the varying modes, just the average hourly airflows and temps. But the program should be modeling the AHU jumping between these modes. SS-R and hourly reports seem to indicate the simulation works the way the documentation indicates.

Fred
Fred Porter, BEMP, LEED© AP
Principal Engineer
Sustainability Services
NORESCO
2540 Frontier Ave, Suite 100, Boulder, CO  80301
fporter at noresco.com<mailto:fporter at noresco.com>
www.noresco.com<http://www.noresco.com>


From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Curtis Robbins
Sent: Tuesday, January 06, 2015 11:00 AM
To: equest-users at lists.onebuilding.org
Subject: [External] [Equest-users] 4 Stages of Cooling

Hi

I am trying to simulate a building (eQuest 3.64) that has rooftop HVAC units with VFD's installed on the blower set for 4 cooling stages and 2 heating stages.  The VFD on the blower has three set points. 1st is for blower only, 2nd is for  Stage 1 cooling, third is for heating and stage 2-4 cooling. Where can I enter the three VFD setpoints for the blower?  Under Air-Side HVAC System Parameters/Fans/Fan Power and Control, there is a drop down for Fan Control that is variable speed, but where do I enter the speeds?

For cooling: When I go to Staged Volume (within the Cooling tab) I can only get the option to enter Capacity fraction and Delta T for 2 stages of cooling and heating. Can anyone please tell me how I can get the option to enter additional stages?

I am new to eQuest, so thanks for your help.

Curt
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