Heat Rejection consumption is so small!!

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Dear all,

What are typical values for heat rejection, if my Space Cooling
constitutes 25% of my school consumption? All I'm getting is 1% or so!

I left eQUEST to autosize the capacity of the tower, and I entered many
default values into the inputs of the Open Tower and the Condenser Loop.
I've heard that Heat Rejection in such climates such as Lebanon should
reach at least 10%.

One more thing, when modeling the baseline case according to ASHRAE
90.1, I read that I should model a condenser water loop for each
chiller. So if I have 2 chillers, will I need 2 Open Towers?

Many thanks,

Omar Katanani

Omar Katanani's picture
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Hi Omar,

here are my thoughts based on your questions and info.

First, when I read the condenser/heat rejection baseline requirements from
90.1-2007 I interpret that it is not indicating two separate condenser
loops, it is stating "individual condenser loop pumps per chiller", there
for you would still have one condenser circulation loop serving both
chillers and one cooling tower.

Next, if you have heard from designers with experience in the Lebanon
climate characteristics that your heat rejection energy should be upwards of
10% then you could/should trust thier given experience. However; you need
to ask them what is this 10% relative too? Are they implying that 10% of
the whole building end use energy should be around 10% for the Lebanon
climate? (my opinion: I doubt it.) Are they implying that 10% of the
Chilled Water System end use energy (i.e. CHW-chiller, CHW-pumps,
CHW-condenser water systems including pumps & tower, etc?) (my opinion: I
would guess that this 10% value is relative to HVAC system(s) end use
energy, not *whole building* end use energy. Or is that 10% relative only
to the chilled water-side of the systems (extracted from the total HVAC
energy which would include heating energy.)

Last, if you would like to attach your .pd2 & .inp file myself, or maybe
others might be available to peruse your project file and give you some
indication if your end use heat rejection energy is falling within a
reasonable whole building energy range.

p.s. if you are using a unique (non-eQuest) Lebanon climate weather file,
please attach that too so that I can run your file on my computer.

pkg

Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 600

Pasha,

Thanks for your support!

Attached is the Baseline scenario. A colleague of mine once modelled a
big Mall and got the values for Heat Rejection even greater than those
of the Space Cooling! Please find the attached small excel file.

As for my school project, the heat rejection is negligible! This is my
concern. Can you please check my tower parameters and the condenser loop
and advise accordingly?

Many thanks to all,

Omar Katanani

Omar Katanani's picture
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Joined: 2011-09-30
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Dear all,

Can anyone please answer my question that I sent last week? Your continuous support is much appreciated!
In addition to that, I have this small question below:

Section G3.1.3.11 of ASHRAE 90.1 - 2007 says:

"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."

I was able to put the design set point temperature and the loop design DT of the Condenser water loop to 85F and 10F respectively.

What about the tower? Where can I input this leaving temperature range from 70F up to the leaving water temperature?

Many thanks!

Omar Katanani

Omar Katanani's picture
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Cooling towers are just fans, so it's no surprise that their percentage relative to the total consumption is small. Make sure your cell KW or total fan EIR is accurate.

And as for your question on reset: I usually just specify the condenser water loop control strategy to be LOAD RESET, with a minimum reset temperature of 70 degrees.

James Hansen, PE, LEED AP

James Hansen's picture
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Also, the pumping energy for the cooling tower ends up in Pumps and
Auxiliary. This number usually constitutes much of the energy used by the
cooling towers.

--
Karen

No Username provide's picture
Joined: 2011-09-30
Reputation: 200

James,

Thank you for your advice.

I did what you told me regarding the LOAD RESET for the condenser water loop. Please find the attached excel file with the results.

I am happy that the heat rejection is now a significant number. However:

1. The space cooling for the proposed case is now higher than the baseline case, which is unacceptable due to the better HVAC system
2. The space cooling for both cases has decreased significantly. Can you please check the numbers and the percentages of the space cooling with respect to the whole building consumption?

Unfortunately, I may keep the condenser water loop controls at "Fixed" and "setpoint = 85F", because of the first reason up.

Best regards,

Omar Katanani

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