Dehumidification for Motor Control Centre 1

[relaxjack]

The Client has proposed to install split air-conditioning unit in the MCC rooms of a fertilizer factory for the purpose of cooling as well as dehumidifying the space to avoid dust and particles + water in the air to attach itself to the switchboards and therefore affect the insulation. My question is, do you guys think it is a good idea to use split type air-conditioning unit ?

If yes, how should i calculate the BTU/Hr of the air-cond unit in order to dehumidify and cool the MCC room effectively?

If its not a good idea, what is the best method to do the job?

Thanks.

 

[quark]

AbbyNormal,

You have constructed a very clear and clever logic all the while based upon limited inputs by OP. I agree with your method and I justed wanted the OP to discuss with his consultant about the design philosophy.

These forums are excellent for learning different experience. I work in pharma and I do base my leakage calculation on crack areas (deliberate openings rather). There will be difficulty during initial phases of commissioning, otherwise, I am happy.

We had two areas, in the past, separated by a solid wall and connected by a pass through hatch for conveyor movement, with a pressure difference of 30 Pa. The critical area was maintained at 22C and 20 RH and other one with 22C and 55RH. When we were facing problems with high RH during a particular time, I found that wet mopping in the high RH area was causing this. I followed some methods recommended by Munter's book and was successful.

I have few questions for you.

By your experience, is +2Pa good even in monsoon condition also?

Is ASHRAE's Humidity Control Guide worth buying if I have Munter's with me (at such an expensive price)

This question is just out of curiosity. Is your handle something to do with the movie Frankenstein(84 or some other year)?

 

[AbbyNormal]

Monsoons no problem, but all bets are off in a hurricane. Been through Cat 5 myself.

You can download a free chapter at

http://www.masongrant.com/html/humidity_design_guide.html

The documented water damage from 2 Pa originated from a hotel/motel association study.

Florida Solar Energy Center pushes the 2 Pascals as well.

The rainy season here , equivalent to 'monsoons' used to be a problematic time, high dewpoints, no solar gain,reduced ambient temperature, minimal run time. My own office ended up with RH in the 70s so I finally practised what I preached.

I added an simple fresh air intake and as Tropical Storm Isabel went by, my RH maxed at 60%, however the positive pressure was enough to close a door. Without even measuring, I concluded that perhaps I had too much fresh air so I dampered it off so you could just feel a slight cool breeze leaving a cracked open door. One week later as Hurricane Lily went by, my RH hit 55% and was never higher (until Ivan destroyed my office)

So I has hooked on the positive pressure concept ever since. Calculating a pressure differential around a deliberate opening is one thing. Going through the length of crack around numerous windows, door frames etc is another.

Investigated other severe condensation problems and negative pressure consistently -2Pa and itermittently worse was found.

Abby was indeed inspired by Marty Feldman answering the question "Whose brain was it then?"

 

[AbbyNormal]

must be a deep space nine influence here as well

 

[abcmex]

RelaxJack,
I have seen many Motor control centers cooled by Split units in Chemical plants. Typically the air ouside is corrosive and therefore they try to minimize the air exchange. If you have additionally a high humidity/temperature outside (like Malaysia) I think a split unit is a good solution. Regarding the load check with the electrical engineer in charge, he can tell you about the heat load and add some human load for the maintenance crew.

RGS

 

[relaxjack]

Many thanks to abby, quark & abcmex for taking time to reply this thread.

I apologize for the lack of info as i wasn't the one who get to meet and discuss with the Client and Plant Specialist. My superior who went there was as clueless as me if not worst. The limited info were conveyed to me and i was expected to come out with a design. LOL

ok,lets say that i have all the total load. How should i calculate the required BTU/Hr of the Air-cond. unit to be used?

 

[quark]

There is an excellent faq403-1255 by Yorkman. Just go through this and it will be a cakewalk.

AbbyNormal,

Thanks for the link.

After 14 years of viewing that movie, I can still recall that indifferent look by the butler transforming into a innocent quizzical one, just after the answer. Nice handle.

 

[relaxjack]

Abby: After studying your link at

http://i32.photobucket.com/albums/d41/a_bee_normal/Malaysia_MCC.jpg

I have a question.

1) The calculated Cooling Load of 22,249 BTU/Hr @ 749 CFM should also be the minimum capacity of air-conditioning unit required?

 

[AbbyNormal]

In my example, 22,249 Btu/hr is the minimum TOTAL cooling requirement needed, to cool mixed air like I ASSUMED down cool and dry enough to compensate for the sensible and latent heat ASSUMED to be generated in the room.

As it turns out, it graphically demonstrates, the calculation procedures by Yorkman, in a FAQ, to which Mr. Quark has given you the link to. You will be using your sensible and latent requirements along with realistic outdoor air properties to perform a similar analysis.

It is more important to look at the supply air conditions required or to look at the Total and Sensible Cooling that a system will provide based on the entering air conditions you will be facing.

Total cooling includes the sesnible and latent portions.

In my example, it does not state the latent cooling involved in the cooling process from the Mix to the Supply statepoint, but it does state total cooling of 22,249 Btu/hr and my program estimated 4.8 pounds of moisture removed.

This moisture would mean perhaps a latent capacity of a little over 5000 Btu/hr and then a sensible capacity around 17,000 Btu/hr.

In my example the sensible to total cooling ratio is typical of residential grade air conditioning for a similar entering condition.

Note that a lot of standard ratings are based on an entering condition of 80F db and 67F wb. In my example the entering wet bulb is less than 64F, and most likely a standard 2 ton rated at 24,000 Btu/hr would be of lower capacity (22,000 or perhaps less) with only 64 wb entering.

You need to either retain a mechanical colleague or carefully follow the procedures in the FAQ. My link was an example assuming more internal load than you were stating and a less extreme ambient condition than you were stating.

Perhaps contact a Trane representative, the sell laminated pyschrometric chart, and on the back of that chart are instructions similar to the FAQ you have been referred to on this forum.

 

[relaxjack]

Thanks for explanation Abby.

 

[AbbyNormal]

you are welcome jack