[Equest-users] an inadequate cooling capability warning

Kevin Kyte KKyte at watts-ae.com
Tue Aug 3 07:32:09 PDT 2010


The current controls are when outside air is below 55F the cooling coil shuts down, there is no supplemental cooling only lake water.  Therefore the outside air and return air must modulate to maintain 55F supply air temp.  With 300 kW of internal gain and 40000 CFM supply airflow, SS-F shows up to 200F in the space at minimum outside air below 55F.  We considered letting the lake water coil run at all temperatures though don't want to risk freezing the coil.  I understand that if we run the coil in the winter time and just return air it would probably save on fan energy.  The coil is probably efficient enough and the thought is the main water line will have enough pressure to branch off and circulate through the coil loop without additional pumping at all times.

Considering the current design strategy, I think the only way to accuracy model the OA and RA modulation is to schedule it in the min-OA schedule.  I have set a min-supply-T so the software should use that as a minimum supply air temperature but I notice that when economizing, at least with no low limit, the min-supply-T entry gets thrown into the garbage (yet we still get the warning).  Is there any way to program it in using expressions?  Not sure which entry though, maybe min-air-schedule?  Above 55F, since 100% OA is used, the cooling coil is only used when lake water temp is below outside air temp.  There is no way to model this however, except fiddling with the cool-control range, which is entirely inaccurate.  I think the economizer should be integrated with coil because they are both used simultaneously, at least in the current control scheme.  You might be able to get away with the delta T just using return air, I haven't done the calc though.

The model is pretty much done, I think, and I was hoping to get some feedback from the board on this warning.  Something that I am not seeing?

Thank you.

Kevin

________________________________
From: Bruce Easterbrook [mailto:bruce5 at bellnet.ca]
Sent: Monday, August 02, 2010 4:47 PM
To: Kevin Kyte
Cc: 'eQUEST Users List'
Subject: Re: [Equest-users] an inadequate cooling capability warning

    I think your control strategy is not right or you are misinterpreting how equest works.  You say your room requires cooling at all times of the year and you want to keep your maximum temperature below 104F.  You have selected 90F as your control point which is the setting of your room t-stat.  Your 55F temperature only matters if you have potentially freezing temperatures in the winter because you have a water coil which you don't want to freeze.  Below 55 your economizer is shut off and you should be on minimum fresh air which with no personnel in the pump room should be at .06 cfm/SF, you will be almost at 100% return air.  You should have a differential temperature or enthalpy controller to control the economizer.  Your system will remain like this, considering std equest defaults until the room temperature reaches 92 F.  If the outside air temperature is above 55 the system will use the economizer to control the temperature.  If that fails it will use the cooling coil.  You don't want both the economizer and the cooling cool running at the same time.  Basically you don't want to use water pumping energy if you don't have to as it is additional energy on top of the fan.  Practically you could have 2 temperature controllers controlling part of the same air stream and the system will hunt.  The only other variable here is the water temperature in the summer.  Basically your peak heat generation in the pump room and your peak water temperature in the summer will set your system CFM, coil size, coil flow, pump size.  This is a basic explanation, there are more things to play with to fine tune the system further.
Bruce Easterbrook P.Eng.
Abode Engineering

On 02/08/2010 03:09 PM, Kevin Kyte wrote:
I have been struggling with this scenario.  A pump room does not need to exceed 104F.  Lake water is used for cooling.  The room is expected to contain internal loads requiring cooling at almost all times.  Below 55F the OA and RA modulate to provide 55F supply air.  Above 55F, 100% OA and the cooling coil turns on.  I have the cooling Tstat set to 90F and cooling coil available at 55F.  I am unsure how to even model Below 55F except for economizing which is not really exact but when I set the economizing low limit to 55F I get 200F in the space.  I have set economizing to 90F integrated with the coil and no low limit.  The cooling coil is rarely used and not needed in its full capacity yet I am getting inadequate cooling warnings.

The Min-Supply-T is set to 55F because supply should not be less than this and hourly reports show acceptable supply temperatures into the mid 70s (I don't know if this is acceptable to the program?)  I am not using a lake water chiller because I am not trying to use the same coil temperature.  I am using a simple packaged single zone with the cool-control range set to max and EIR zero'd because there is enough force off the main header to not need any pumping through the loop.

This is only for energy code compliance and I need an explanation on the warnings.  Other than stating several obvious limitations I don't really understand why this warning is occurring.  Does anyone have suggestions on how I may model this system?

I understand that electronic files would help and I will work on that.

Thank you.





Kevin Kyte, LEED (r) AP
Watts Architecture & Engineering, P.C.
95 Perry Street, Suite 300
Buffalo, NY  14203
p:  716.206.5174
f.:  716.206.5199
e:   kkyte at watts-ae.com<mailto:kkyte at watts-ae.com>
http://www.watts-ae.com<http://www.watts-ae.com/>








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