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.

 

[AbbyNormal]

Use something with some good filters and pressurize the room with some clean outside air. Outside of any envelope heat gains, make an allowance for heat off of any transformers and equipment, in addition to the outside air load. I doubt the lights would be on much in the room.

 

[AbbyNormal]

Outside of the outside air, should be an all sensible load, so any dehumidification would be due to the forced ventilation, would more than likely overpower any infiltration.

 

[AbbyNormal]

Your air conditioner will draw in a small portion of outside air, most likely less than 10% of the total air flow. It will cool and dehumidifiy this air.

You need to work out the sensible load and any latent load of the MCC room. You can determine the airflow rate, the supply temperature and the supply air dewpoint from that.

Then you look at how much cooling it will take to cool/dehumidify mixed air to the required temperature and dewpoint, using a pyschrometric chart.

May want to consult with a mechanical colleague

 

[relaxjack]

Thanks a lot for your help Abby. What would most likely be the source of latent heat? could it be the 10% of outside air intake by the air-conditioner? The switchgear should be the largest contributor of the sensible load right?

Also, you're saying that i should size the air-cond. unit based on the cooling from the temperature and RH of the mixed air to the desired temperature and RH of the space/room? Do i have to consider the initial condition whereby the room might totally be consist of the outside air?

 

[AbbyNormal]

pretty much be just in the outside air only, I doubt the room is occupied by people much

 

[AbbyNormal]

You supply air cooler than what you want to maintain the room at. The air flow and this temperature differential between the room air and the supply air is what controls the temperature in the MCC room. It cools the internal heat gain plus heat gains through the building envelope.

You size the system to cool mixed air down to the supply temperature. If you have no internal latent load, then making sure the supply air has a dewpoint under 55F would most likely control the humidity.

There are lots of variables especially your local climate. I only mentioned some outside air as you can filter it and then pressurize the space with cooler dry air. This would keep out ambient air from a factory making fertilizer.

May not be the most scientific approach but enough fresh air to notice cool air leaving the space around a cracked open door could be all it takes. Could work out to less than 10% of the total airflow. If you are in a humid environment then go with intermittent fresh air, meaning fresh air only when the compressor is running.

It will pull down the initial temperature and humidity down from being unconditioned to conditioned you do not have to allow for this, if you plan on keeping it cool all the time.

Is being too dry a bad thing, do you have to worry about static electricity in the winter?

 

[relaxjack]

Im from Malaysia and its basically hot and humid here. so if i were to size an air-cond unit, the btu/hr of the unit should be equivalent to the [rate of sensible heat transfer + rate of latent heat transfer] in changing the temp + RH of the mixed air to desired room temp. + RH ?

 

[relaxjack]

almost forgot. howabout the CFM of the A/c unit?

 

[AbbyNormal]

I have a former employee stuck over your way at present, kids is stuck in Thailand but has been travelling through Malaysia, Singapore, Bali.

CFM will depend on the sensible heat gain inside the MCC room as well as the heat gain through the building envelope.

You need to know this to proceed any further

 

[relaxjack]

Your friend must be having a good holiday around those places. LOL.

On the sensible heat load, electrical heat gain from the switchboard would be about 10,243 BTU/Hr. Assume that infiltration and occupant load is negligible, what other load should i consider as sensible load?

Correct me if i'm wrong, Latent heat laod would only be the load involve in decreasing the RH. (In my case, from outside RN of 85% to required room RH of 45%.)

What else would i need to calculate the total amount of cooling load required to chang the temperature and humidity of the MCC Room (from 35 Deg C / 85% RH to 30 Deg C / 45% RH) ?

 

[AbbyNormal]

You would consider heat gains through the wall, roof, windows if any as a sensible load also. The effect of the fan is also a sensible gain, however be negligible on such a small system. Duct work passing through unconditioned space is also a sensible gain.

While it is hot and humid in malaysia, you may want to check that temperature and RH, as it seems overly humid. Relative Humidity is confusing, and many times people note the humidity overnight or early in the morning and associate it with the day time high temperatures. No doubt you can have 85% RH over night, but temperatures at night most likely low to mid 80s.

Perhaps 95F ambient at 70% RH is more than humid enough for a worst case scenario in Malaysia. That is almost an 84F dewpoint which is very high. I am on a tropical island and only once in 8 years have I ever noticed a dewpoint high like you are suggesting. Usually dewpoint here floats from 78 to 81F during the rainy season.

What indoor temperature are you going to maintain. 45% RH at 80F is not unrealistic, but can be a challenge when room temps are kept below 75F. Would almost need reheat if you wanted 45%RH, with a low room temperature.

 

[relaxjack]

The 85% RH ambient plant condition and the required 45% RH for MCC Room is actually given by the fertilizer plant specialist from India who were going to install all the equipments. It was based on their experience according to them. hmm... seems like i'll need to have a discussion with them again. Thanks a lot for your reply. you've been a great help. Will update once i have more info.

 

[AbbyNormal]

You would be pressurizing with outside ambient air to keep the indoor plant humidity out. The data I have for Malaysia shows a worst humidity for a "George Town" at 158 grains, (at 85F dry bulb)meaning a dewpoint of about 80.5.

You would be removing the humidity from the ambient air when you pressurized the MCC room

 

[AbbyNormal]

Assume a total sensible gain in the space from the equipment and through the envelope at 15,000 Btu/hr, minimal latent gain in space.

You will maintain the space at 75 F and 45% RH. Outdoor ambient is 95F with an 80 dewpoint---> 95F @ 62% RH.

Most likely if you pressurize the MCC with 10% outside air, it will add about 50% to the space sensible gain.

A link to an example for 749 CFM of supply with 70 CFM outside air. 679 CFM of room air mixes with 70 CFM of outside air. The system cools the air from the "Mix" to "Supply" condition. Fan heat is ignored in the example.

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

 

[relaxjack]

Thanks again Abby. Your help is very much appreciated. will take a look at the example and talk to my Client again.

 

[relaxjack]

if i were to use pressurize the room with outside air, would i need a filter to stop dust and particles from being sucked in? if so, what kind of filter is normally used? howabout those filter that are normally used in a AHU unit?

 

[AbbyNormal]

I think insects be more of a problem than dust.

Perhaps a small inline fan with a speed control sucks in the fresh air through its own filter and then discharges the small amount of fresh air to the return side of the air handler. A little 'fantech' type of fan.

if the room is kept clean, perhaps a 2" 30% pleated filter on the air handler is sufficient. It is not like they have given you a clean room condition to maintain is it?

I am curious, are the penetrations where the conduits conduits leave the MCC room well sealed, or does the room have the integrity of swiss cheese? Hard to pressurize a room full of holes.

 

[quark]

I would like to rise a point about redundancy here. The ambient design condition 95F (35C) and 85% is a higher value, as suggested by AbbyNormal. Secondly, I find no clue about why the room RH should be maintained at 45% at 86F (or 30C).

My logic goes like this.

Suppose, the inside area is conditioned to 35DB and 45% RH. If external air at 35C and 85% RH enters this area by accident, there souldn't be condensation happening in this area. So, I consider 10% untreated fresh air coming into the area by accindent (say, chilled water control valve malfunctioning after all the material in the room reached the lowest temperature possible).

The return air at 86, 45% and fresh air at 95 and 85% will give us an approximate mixed condition of 65.3 DP (86.9 DB and 71.8 WB). A DP just below this is what I will consider for my design. So, you can go upto 50% RH.

If you have proper control for fresh air damper, to shut it off when the chilled water temperature fails, I don't find any reason to go to even 50% RH, provided you maintain positive pressure in the area.

Don't go with a fresh air rate of 10% but just maintain enough fresh air to provided the necessary positive pressure in the area. Munters Dehumidification Handbook indicates moisture ingress by diffusion from a low pressure area to a high pressure area. To mitigate this, your leakage should be more than 150 fpm. After checking the possible leakage area, consider 150 fpm and back calculate the pressure required in the area.

You can do with a 10 micron filter (in a layman's terms) or EU-5 grade.

 

[AbbyNormal]

I figured the 30C space temp was a typo, my bad. If that factory envirnoment, and not the ambient outdoor air, is as hot and humid as relaxjack says then the MCC room is still below the dewpoint and you do not really want that air infiltrating in uncontrolled. Besides condensation there could be some corrosive elements (electrolytes)in the fertilzer factory air. The humid air will not mix right away and will contact surfaces below the dewpoint.

The crack method is quite tedious, I have gone through the ordeal twice.

2 Pa is a magical number. -2 Pa and you have humid air infiltrating in, touching a surface below the dewpoint and condensating. +2 Pa and you have reversed it. A good reference is ASHRAE's Humidity Control Design Guide written by the same guy who put a lot of work in on Munters manual. '-2 Pa is documented as creating millions of dollars of water damage in hotel/motel walls'

You will note that I mentioned it would be just enough fresh air to notice air leaving when a door was cracked.

Here because of climate,which is similar to Malaysia, it is very difficult to have a sealed vapour barrier on the 'warm side of the insulation' especially in the attic as you would have to be above the bottom of the truss cords, trying to seal around the webs. So with a trussed roof, it is not well sealed at the ceiling plane. 10% is a good starting point to pressurize the home and it will work out to less than that many times.

You start exceeding 10% outside air on standard equipment in a humid climate and the cooling process line rapidly becomes steep, ADP is out of range and the system will be overwhelmed with moisture that it cannot remove. You suddenly require an 8 to 10 row coil and reheat.

When you have the space pressurized, you have the infiltration controlled. There is no leakage by accident, outside of the system failing. If the building envelope is full of unsealed holes then there is no point pressurizing. Might as well put in a dedicated dehumidifier as well as a system to control space temperature.

Here is a photo of the reverse scenario, what a negative pressure ventilation scheme can cause with an ambient dewpoint of 80F. Air was infiltrating in at 300 FPM by the way. I like to refer to the photo as "It worked great in Arizona"

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

If it is truly a 30C temperature requirement then disregard what I have said. I am used to IT guys wanting it dry and below 70F even though the equipment can run at higher temperatures.