100% Outside Air Gas/Electric Packaged Unit

[steveski]

I'm investigating the operation of an existing 30-ton 100% outside air package unit (nominal 12,000 cfm). Our design conditions are around 105F. I was told that if we try to cool 12,000 cfm to 55F we can ruin our compressors. Our options are therefore to either lower the cfm or increase the supply temperature. Can you please explain how this could create a situation where we would reuin our compressors? Thanks.

 

[Yorkman]

Just running quick numbers you'll never be able to cool the air from 105 to 55 degrees with a 30 ton unit at that CFM.
I think some of the concern is that the temperature of the suction gas may not provide enough cooling for the motor windings(if this is a hermetic compressor) and the oil temperature could get very high. Also consider that you are going to be trying to condense the refrigerant with this same 105 degree air. The head pressures are going to be extreme. Disreguarding any latent heat which is likely the best you could do is about 77 degrees but your suction temperatures are still going to be 85-90 degrees at best this will still put your discharge temperatures way up there, threatening oil break down.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Yorkman]

Again rough numbers only with no latent heat load, so take that into consideration, if you reduce the air to about 6600 CFM you could produce a 55 degree discharge air. The other why to cool things down is to take the exhaust air from the building and pre-cool the supply air before cooling it mechanicaly. This could be done with a heat wheel (air to air heat exchanger) or with some form of run around coil. This way if you could get the air down to 80-85 degrees. You could then use mechanicl cooling to take it to 55. An added bonus is in the winter the same process can be used to pre-heat the air before cranking up the toasters. This way you could still keep the CFM at design.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[imok2]

You might get about 60 degrees at 8000 CFM but then you start to get to dew point depending on OSA relative humidity (about 22%) at most and that would affect the dry bulb I agree with Yorkman that 55 degrees is unachieveable at 12,000 CFM and my heart goes out the any system that would be placed in that delima :>)

 

[steveski]

Thank you for your reply, I appreciate it.

 

[EEJaime]

Our HVAC guys used to specify an evaporative pre-cooler on the outside air, but that was out in the SoCal desert. They've been installed on many schools and office buildings out there. I don't know the technical aspects of the installations, as you can see, I'm an electron head, but they seem quite effective, and that's with summer ambient air temperatures that often top 115-118 degrees F.

Just buttin' into your thread.

Regards

 

[Yorkman]

Buttin' in is what forums are all about. An evaporative cooler could be an option depending on the climate zone, using this approach however will increase the latent load on the coil. Not a real problem if you make sure you take it into account when sizing the capacity of the unit.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[Yorkman]

EEJaime,
Man it's bad when at 2:00am you wake up and realize that you (me) made a large error in posting a response to your (EEJaime's) post. If you use an evap cooler to pre-cool the air in the above scenairio you will still not be able to achieve the desired 55 degrees. You might lower the entering air temperature enough to avoid damage to the compressor due to high discharge temperatures, but 30 tons is 30 tons.
Evaporative cooling is an adiabotic process were your changing temperature of a substance but you have not change in the enthalpy, heat is heat.
Well that being said, now I can get some sleep tonight, G-D I love this sight!!!

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[steveski]

As I've continued to delve into this topic I've come across the indirect evaporative pre-cooler option, which can, for example, bring the dry bulb temp down from 105F to 80F without adding moisture, then the DX coil can take it from 80F to, say 60F. With this set-up a 27.5 ton unit can purportedly supply 12,000 cfm at 60F with outdoor temp at 105F (in Northern California).

Also, if/when conditions allow, the compressors can stay off and the pre-cooler can do all the work.

If anyone can corroborate or dispute this please do so as we are considering this option.

I love this site too, you all are very helpful.

 

[Yorkman]

Now your talking, the indirect method sound like it will work. Could you give a little more description of the process your considering?
I'm thinking that you can cool exhaust air using the evaporative cooler then run that exhaust air through an air to air heat exchanger to pre-cool the make up air. I would look at several different options and plot them out on a psychrometric chart or use a program if you have one. With out being too bold I have a FAQ posted on the HVAC forum on psychrometrics if your interested.
Back to the unit, I might, with out looking at the numbers too close lean toward a run around coil as it can function as a pre-cooler and pre-heater. But too be sure I would do an analysis on all options. The key is keeping the latent load down and the discharge temperature of the refrigeration system low as well.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[lilliput1]

Checkout SEMCO. They make heat wheel heat recovery units that does sensible and latent heat recovery from exhaust air. What is the condition of the exhaust air? Confirm it is not odorous or chemical ladden.

Other tack would be to reduce conditioned air makeup. Can't untreated OA be used for direct exhaust makeup similar to kitchen hoods with integral direct OA makeup up to about 70% and the makeup only requiring heating to 60°F, no cooling?