Mounting AC on PLC cabinet

[aryaf]

So this is a bit off topic, but a client has asked me to do a heat calc and spec an AC for a PLC cabinet - which I have done - but now he's asking me where to mount it and I'm not quite sure what to consider for this. I've seen them mounted on the side of the cabinet near the top. Thoughts?

 

[shooter]

Yes please on the top, when the AC flow is only small the cold will stay down, when the ac is in top the heat will flow in the AC.
beware of any condensation inside the cabinet (never go below dewpoint)

 

[The Plc Kid]

I always avoid the top. Any condensation or issues with the unit down the road and you have water in your panel. Sometimes you have to do top mount but do a side mount if you can.

 

[aryaf]

Yeah I should emphasis that it's on the side, but the near the top as opposed to bottom. So is that fairly standard then? on the side? Is there a reason to put one near the bottom on the side?

 

[Andy6]

I would put it near the top. The hot air at the top will get sucked right into the AC unit and your cold air will drop right to the bottom of the panel. Mounting the AC near the bottom would probably only circulate cold and cooler air while leaving hot air at the top.

 

[leitmotif]

I would favor mounting in bottom to avoid condensation problems.
I admit you are going to lose natural circulation but that can be overcome with a fan blowing cooled air upward to cabinet top.

Whatever you do ensure you put in a drain pan for condensate and a BIG drain line. Bacteria love these and eventually plug small drain lines. No smaller than 1" and bigger the better.

Dan Bentler

 

[JeffKiper]

There are a couple of brands that have intakes up high and supply down low.
Look at Thermal Edge. They have non condensing AC. These units have several different methods to keep from condensing. Wet bulb sensors, variable hot gas bypass, fan circulation without running the compressor.
Sorry the HVAC geek inside of me is trying to come out.

Don't think you have to use the 240VAC unit. The 120 VAC units are fine. Moe sure you don't oversize this unit. You WILL HAVE PROBLEMS.

 

[leitmotif]

Bit of add on to what Jeff said.
Non condensing can be nearly achieved with hot gas bypass. Most HVAC guys I saw removed it - especially so it seemed on commercial refer applications - bit different I admit - these were 10 to 30 (??) ton units.

The expansion valve will always give you condensation and in some cases icing. The liquid is turned into a cold gas. Heat of vaporization in refrigerant will be supplied in part by ambient thus condensing water in the air.

You will need unit to remove heat from equipment. When heat production is low then unit is oversized. I would intentionally ovesize by say 10% to ensure adequate capacity. Going too far on ovesizing will result in unit short cycling.

I would guess the unit size we are talking about is 1/4 to 1/2 ton.

What is really needed for this discussion is the estimate of waste heat from the equipment in the enclosure and ambient temperature.

Dan Bentler

 

[JeffKiper]

Dan as you know I have a little bit of an HVAC back ground. I was at a shop working and they where installing a 100hp VFD in a panel that had little to no extra room for the drive. The put a 10K but unit on the side and went through 3 of them. I asked if I could help and the old timer said it was all because of the damn 120VAC units couldn't handle that demand. He said if he needed any help he would get me. They installed a 230 VAC transformer and still lost 2 more units. The old timer finally let me look at what it was doing. It was short cycling. image that but 10K in a shoe box and wonder why you have temereature swings of 30°F. I added a 4" discharge to another panel close by and programmed the stat to lockout for 15 min. haven't lost a AC on over 4 years.
SO I agree with a small amount of up sizing. I tend to play it pretty close to the size needed only after a full heat gain calculation.

The first system I ever calculated was about 50 small 2HP starters 30 3HP VFD 3 10 slot SLC chassis and 3 200HP VFD. I did all the calculations and sized it on the money. I didn't take into consideration that this panel controlled 3 mixing towers and they could only run 1 at a time. Talk about overkill. You live and learn and hopefully pass some along to others on the road.

 

[leitmotif]

Jeff

I think we are in agreement - you said
"SO I agree with a small amount of up sizing. I tend to play it pretty close to the size needed only after a full heat gain calculation."

The most important part -- full heat gain calc ie KNOW THY LOAD.

Given a good set of calcs I would add 10% just to make sure I had enough but not too much to avoid short cycling.

Dan Bentler

 

[gbradley]

Can someone post a picture so I can see what the heck you guys are talking about?

 

[JeffKiper]

Dan you and I are on the same page.

"KNOW THY LOAD" should this be on the PLC Law over on MrPLC? I think so if is isn't already.


Take a look at http://www.thermal-edge.com/aircon/

 

[leitmotif]

The starting motivation for me to develop KNOW THY LOAD was when a guy asked
"can I drive a 3/4 ton 4WD pickup using a 1/3 HP washing machine motor?
I told him
yes you can - with proper gear reduction.
Dan

 

[GlennA]

Near the top, but on the side. Are you sure that you need to add an A/C unit ? Are you having failures that you think are heat related ? Have you put a thermometer in the panel to see what the temperature is ? A simple way to do a cooling test is to temporarily add a vortex tube = compressed air cooling. A vortex tube will give you more and colder air that just squirting compressed air into the panel.

 

[JeffKiper]

The cost of running a vortex tube is usually higher than the cost of an AC unit. I did the calculations several years ago. The initial cost the Vortex was less expensive. After a couple of years the AC becomes cheaper to operate.