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Thread: Make up air unit
07-26-2004, 10:59 PM #1Professional Member
- Join Date
- Feb 2004
- Madison, WI
I recently got a call on a small kitchen make up air blower that had a slab DX coil "patched" into it. The compressor had shorted windings and the owner wanted me to replace it. I told him I didn't think it would last due to the high suction pressures you would likely see on a very hot day. He said the unit worked fine for years.
I would guess this thing to be about a 4 ton A/C (model # unreadable), which would run when the make up air blower was on. The only other control was a low ambient t'stat that would cut it out below 65 degrees F. How could this thing have possibly survived for "years" with 100% outside air and temperatures that can come close to the century mark and high humidity? I don't want to do the job because even if the thing did last for years I don't know if he would be so lucky again.
Would it be possible to add a CPR valve and set it around 70# to protect the compressor?
07-27-2004, 12:15 AM #2Professional Member
- Join Date
- Feb 2003
You have to hate those kind of calls, but give the customer the option, discuss your concerns and get it in writing that you will not warranty the compressor parts or labor as it is likely operating outside of its tolerances.
What is the txv size on the system? Be sure in the compressor size needed through measurements, tx size, etc. Add on the CPR and set to FLA max.instead of pressure.
Chances are you could still get a warranty replacement in 1 year but dont count on it.
Watts New, Ohm My, I been Electrically Commutated. Are U2.
07-27-2004, 01:15 AM #3
do your home work
Explain to the customer that proper design will save him money. You need to figure out how much cfm of outside air. entering dry and wet bulb and what does the customer want to achieve by cooling the make up air? Just condition it slightly or do they want comfort cooling as well as make up air. If you must install another 4 ton you might also want to slow down the blower speed to bring down your superheat or you will fry another compressor. Just had a unitary rooftop with same problems 100% outside air with full blower capacity, hot days would kill the unit. Manufacturer wanted us to slow down the air...works so far... GOOD LUCK
07-27-2004, 01:55 AM #4Professional Member
- Join Date
- Aug 2002
Find out if there are numbers of any kind on the evaporator plate itself.
If there isnt, do a search of mfr's to locate one or more who mfr such items. Then phone the mfr and speak with an application engineer. Tell him your dimensions and he will tell you which BTU category that plate probably falls into.
I mean, this cannot be THAT complicated!
Then once you have your BTU's figured out based soley upon the evaporator capacity, then you can choose your TXV. And this wil be based upon temperature range along with BTU's.
In this particular scenario, hi temp A/C range.
Now that you know your evaporator load, in BTU's and you have chosen your TXV ... you can safely choose your condensing unit.
if that old system happened to work, perhaps it was sized correctly.
But never assume it was.
Once your sure of your condensing unit size, you can price out the repair to the end user.
And he might want to have you purchase the extended four year for his warranty. Giving him five years on his unit. He will love that.
If you have all this ready, then all you have to do is put it all together. And this time, do so with some added features lie suction accumulator and pressure safeties. An ambient control. Something to shut the unit down when the incoming air is chilled enough.
Anti short cycling timer.
Then when you bring it online the first time, take some pix. This should be interesting.
But in reality, it is not all that different than a refr truck which uses chill plates to freeze down overnight.
I really dont know why they dont just use a more standard coil type and just add extra filtration to maintain the coil's cleanliness. (as well as the cleanliness of the kitchen, duh).
In fact, that may just be a selling point for you to suggest he go with a different type of coil versus the kind he already has.
That's up to you.
As far as this thing not being safe or reliable, not so.
Just make sure that while you are starting it up, you maintain the temp and pressure parameters youwould for any other system.
Keep the subcooling within certain parameters.
Keep the discharge temp within certain parameters.
Keep the suction temp, at the unit, within certain parameters.
I mean, this is NOT rocket science and it is certainly NOT Supermarket Refrigeration!!!
So we out to be able to do it! Right?