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
Hi Kevin,
I see this kind of warning a lot. The warning usually comes from the
design load calculations, not from the simulation of actual hourly
requirements. As long as the thermal zone in question does not have any
(or minimal) unmet load hours (check your SS-R report), there is no
worry. For code compliance, I would use the SS-R report showing the
lack of unmet load hours as a justification that the unit is in fact not
undersized.
I hope this helps. Have a nice day.
Sheila Sagerer
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.
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
With your air flow and heat load you need a differential
temperature of 23.7F to remove the 300kW of heat from the room. With a
set point in the pump room of 90F your economizer will be at 100% OA at
about 66F OA temp, this approximately the temperature you want for the
cooling coil to take over and Equest will switch it over, you don't need
to schedule that. With your economizer low end temp set to 55 you have
nothing happening below 55F, equest has the economizer shut off. You
say you also have the cooling coil off. This is where you are probably
getting the insufficient cooling warning. You don't need to schedule
the economizer proportions, equest will do that on its own in the
temperature ranges you allow it to operate. Equest will want to supply
around 66F air to the pump room. Your minimum supply temperature won't
do anything in this situation. Take a copy and set the economizer low
end cut-off temperature to 0F. That should get rid of your warning at
least down to 0F anyway. Equest will probably allow you to freeze the
coil, it would assume you have a glycol mix to prevent this. In reality
you don't but the solution may be to drain the coil in the winter and
run the economizer at all temperatures below 66F. Water loops can be
tricky as well judging from some of the difficulties others have talked
about in posts, make sure you have pump flow in the water loop. You
also have to watch your water supply, small ponds will heat as the
summer progresses raising your incoming water temperature. Now if you
are drawing water from Lake Ontario below the thermocline you should
have a consistent EWT to your coil. I have not done a loop like this in
Equest but it seems that it is working in your model.
Bruce Easterbrook P.Eng.
Hi Kevin,
I see this kind of warning a lot. The warning usually comes from the
design load calculations, not from the simulation of actual hourly
requirements. As long as the thermal zone in question does not have any
(or minimal) unmet load hours (check your SS-R report), there is no
worry. For code compliance, I would use the SS-R report showing the
lack of unmet load hours as a justification that the unit is in fact not
undersized.
I hope this helps. Have a nice day.
Sheila Sagerer