Dehumidification for Motor Control Centre
Dehumidification for Motor Control Centre
(OP)
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.
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.





RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre
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
RE: Dehumidification for Motor Control Centre
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?
RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre
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?
RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre
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
RE: Dehumidification for Motor Control Centre
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) ?
RE: Dehumidification for Motor Control Centre
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.
RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre
You would be removing the humidity from the ambient air when you pressurized the MCC room
RE: Dehumidification for Motor Control Centre
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://
RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre
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.
RE: Dehumidification for Motor Control Centre
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.
RE: Dehumidification for Motor Control Centre
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"
ht
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.
RE: Dehumidification for Motor Control Centre
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)?
RE: Dehumidification for Motor Control Centre
You can download a free chapter at http://
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?"
RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre
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
RE: Dehumidification for Motor Control Centre
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?
RE: Dehumidification for Motor Control Centre
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.
RE: Dehumidification for Motor Control Centre
http://
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?
RE: Dehumidification for Motor Control Centre
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.
RE: Dehumidification for Motor Control Centre
RE: Dehumidification for Motor Control Centre