MCC Room Ventilation / cooling

[m2e]

My experience is mostly in building HVAC, but recently found myself working on a ventilation design of a MCC room.

The room is about 3 meters side by 10 meters long, and one wall of the room is attached to the main building (ie, 3 walls are exposed). The roof is also exposed, and about 3.5 meters high.

I calculated that the VFD's and transformers are giving out about 2 tons of heat. The outside design temperature (2.5%) is 84.2. I'm not too sure what the inside temperature should be, maybe 86~90?

The owner is not putting in cooling units, and wanted to cool the room by ventilation only (because it's what they have been doing and proved adequate).

How would you approach this design, and what kind of fan would you use?

My initial concept is to put a mushroom fan on the roof, and then simply put a louver on one wall (with bug screen) for the makeup air. Would that be good enough? or should I duct the makeup air, and put motorized damper, etc?

In the owner's other MCC rooms, they have been using inline fans and putting the fan inside the room. Then they have the outlet of the fan ducted to an exhaust louver. On the other side of the room they would put a louver and a motorized damper.

What do you think is a good approach?

Thanks.

 

[ChrisConley]

Either approach sounds fine to me in general. Fan in the room will last longer and is easier to maintain, fan on roof is out of the way.

I'd get the manufacturer's to confirm the exact maximum allowable operating temperature of the equipment. If your outdoor design is 84.2 I'm sure your peak design is higher. I'd use the 0.4% design number and then size the 'free cooling' between the max operating number and peak cooling.

Cold winters where you are? A big fan (depending on your calculations above) will bring in a ton lot of very cold air very quickly overcooling the space. A VFD might not be a bad idea on the exhaust fan.

 

[m2e]

Thanks Chris for your reply.

My concern is if the outside temp is 84.2 or even higher, how much air do I have to bring in to cool the room? Assuming the max temp is 88 and the outdoor air is 86, with two degree difference, the air flow would be huge? And, I haven't taking account of the solar loads yet. What about latent load?

You also mentioned:
"size the 'free cooling' between the max operating number and peak cooling"
What do you mean by that? And how do you calculate that?

Thanks.

 

[ChrisConley]

2 tons of heat (24,000 Btu/hr)

CFM = Q/1.08*deltaT

CFM = 24,000/2.16

CFM = 11,111

Pretty large. The only option is to increase the allowable deltaT (allow the room to get hotter than 88) or add mechanical cooling (AC).

The 'free cooling' is the cooling that you are doing with air. Max operating temp is the room temp, peak cooling is the maximum heat rejection required.

Solar and external loads could be significant and you should do a heat gain calculation. Practically speaking, a 322ft2 space without windows shouldn't have more than ~0.75ton of heat gain from the outdoors.

Latent loads should be insignificant in an MCC space (unless you are concerned about the latent load from all the fresh air you are using). Operating conditions of the electrical equipment should be verified that they don't have an upper RH that is below your outdoor design WB.

 

[KiwiMace]

In industrial projects I have usually air-conditioned MCCs that have VFD's.

Still if your client doesn't want it that's fine, but standard design is 10degF above outdoor temp, so 94degF on your design day.

Put a thermostat on the fan as minimum, or 2sp fan might be a cheaper (than vfd) option given your fan will only be a couple of hp at most.

 

[mechanicaldude]

Just as a added note, if you do use the fan approach where you are using 100% OA for cooling you should put a filter on the intake louver. I have used both methods for MCC rooms. In many cases if the air flow rates get too big and hence the louver sizes get too big because of the low delta T, I went with a ductless split system. The problem the you might have with mechanical cooing is where to run the condensate.

 

[marcoh]

Noticed that Danfoss have VSD's with ducted outlets that let the heat from the VSD's be ducted directly to outside which would significantly reduce the heat load in the MCC. Might be worthwhile to look at his as an option?

 

[EEJaime]

From the Electrical side, it is not uncommon that MCC spaces are not cooled but only ventilated. With the advent of electronics in the VFD's however, other factors come into play. It is not unusual to allow these spaces to run at 95-98degF, they can take it. However if the VFD's serve critical loads, added mechanical cooling might be justified. At the very least insulating the roof, providing intake air filters, and thermostatically controlled ventilation should be adequate. You might also include in your specification for the VFD's that a cooling fan be included in the enclosure to get the heat out into the space.
Just a view from the EE side.
Regards

 

[m2e]

Thanks. EEJamie