Make Up Air Systems Using DX

[RodButler]

I feel a little stupid asking this question. I have been designing HVAC systems for over 25 years and I have never really dealt with this situation before.

I have to provide make-up air for a facility in Texas. The ambient air is 102 db/75wb and I need to cool it down to 55 db. (saturated of course)

Can I do this safely with an off the shelf split system using 134a or similar? I know it has to do with the charging of the system but I have always thought that taking a temperature drop that large would result in high head pressure problems and the like.

Can any of you tech types help please?

[dougheret0]

I'm really surprised and disappointed that no one has answered this question. I've been practicing for 30 years, and would never try to do 100% outdoor air with a stock split or packaged system. But to be honest, I'm not sure why.

First, consider that a stock system cannot be run at an air flow lower than 300 cfm/ton. At that air flow, with 102/75 evaporator entering conditions, the SHR of a typical packaged unit is .53, so the best delta T it can do will be 20 degrees F. So it will be delivering 82 degree air to the indoor unit, or space. In fact, if you plot the coil process line on the psychrometric chart, it ends up at 82 db/ 64 wb. This is not a comfort condition, but if you are supplying an indoor unit, it will provide more moderate outdoor conditions.

Basically, you should plot the entire process on a psychrometric chart, and see if it works for your application. Also, discuss the implications with your rep. In my book, which is available for free download, I have a chapter on psychrometrics which would be applicable to this case. You may download my book for free at
www.nettally.com/doughert (http://www.nettally.com/doughert). The title is HVAC Design Manual – A Mechanical Designer’s Guide to Design of Small Commercial and Institutional HVAC Systems

I love the idea of hot gas reheat and hot gas bypass, so I can run way below 300 cfm/ton, wring tons of water out of the air, and deliver the air at a comfort condition, so all the indoor unit has to do is take care of the space load.

[Andy]

I'm really surprised and disappointed that no one has answered this question. I've been practicing for 30 years, and would never try to do 100% outdoor air with a stock split or packaged system. But to be honest, I'm not sure why.


I would have answered but the units used were imperial not SI. If the poster is too lazy to convert the units so am I.

It would be good manners to quote SI units in brackets if you wish someone in europe to post a reply.

Kind Regards Andy:)

[Gary]

Here's the oversimplified answer: In order to get that much temp drop (47F/26K dT) across the coil, the condensing unit would need to be at least double the capacity of the evaporating unit. But then as the outdoor air temp drops the coil would freeze... unless the compressor were unloaded to match current evap load.

IOW, no you cannot use an off the shelf split system. You need a system that is designed for make-up air.

[Brian_UK]

I'm really surprised and disappointed that no one has answered this question.
Why are you disappointed?

Surprised, well not every question gets an answer straight away, sometimes you need to be patient.

We might ask why you did not reply more promptly?

[dougheret0]

Why are you disappointed?

Surprised, well not every question gets an answer straight away, sometimes you need to be patient.

We might ask why you did not reply more promptly?

I was surprised because I know there is a straightforward technical answer, and disappointed because I wanted someone quicker and smarter than me to lay it out. I sort of explained the answer in my post, but could have done better. I replied as soon as I read the thread, but should have sat on it a few hours to do a better job.

[US Iceman]

It is not only the full load cooling capacity and heat rejection requirements, but the issue of doing a high cooling load with DX. As the load begins to decrease off of the design condition the system capacity required will also decrease. If you do not have adequate capacity control on both the compressor and TXV's you will have a mess.

Not quite as bad as using TXV's on a VAV system, but almost...

[Abby Normal]

I feel a little stupid asking this question. I have been designing HVAC systems for over 25 years and I have never really dealt with this situation before.

I have to provide make-up air for a facility in Texas. The ambient air is 102 db/75wb and I need to cool it down to 55 db. (saturated of course)

Can I do this safely with an off the shelf split system using 134a or similar? I know it has to do with the charging of the system but I have always thought that taking a temperature drop that large would result in high head pressure problems and the like.

Can any of you tech types help please?

you will be about 190 to 200 CFM per ton

you will need stages to be able to handle the air when it is not quite so hot out. Not a very humid load, maybe throw hot gas bypass on the first stage.

This will be off the shelf, like off of aaon's shelf or (choke cough) addisson's shelf.

Company like engineered air can custom build it for you.