HVAC design

[mike9046]

Hi there,

I have a 20ft X 8ft X 8.5ft container which I is divided into two rooms. One room which is about 12ft in length is use as a control room with heating generating equipment like computers and transformer etc. while the other room is used as a store. I am planning to store some chemical into the storeroom.
I have and 30,000 btu AC with 1000 cfm handler serving both room via a common ducting with air return in each of the room (presently without fresh air supply or exhaust). My problem is that I am fear that the fume from the chemical in the storeroom will get into the control room via the return.
I plan to introduce an exhaust fan into the storeroom side and just have an inlet at the control room side to solve the fume problem. Can any one please help by advicing me if this will work and if the changes will effect the cooling of the rooms? What size or capacity should I use for the exhaust fan?
Thanks.

 

[Hillbottom]

Mike,

I assume that what you have in mind is to remove the return air inlet from the chemical storage area. If this is the case, you will have to make up the difference in return air to your air handler from some other area. The most logical solution seems to be a fresh air intake equal to the amount of exhaust you are planning to install. You will have to be careful not to over do it with the exhaust or the fresh air because your system may not have the capacity to condition the additonal load of the fresh air.

 

[mike9046]

Hi Hillbottom,

Thanks for the tip. I am actually thinking of installing a 450 cmh exhaust fan at the storage side with a volumn control damper so that I can control somewhat the amount of exhaust. I was not planning for a supply fan but just to have a fresh air inlet. The logic was that since the AHU is common for both the rooms, whatever air that is exhaust from the storage room will be compensated with fresh air drawn in from the control room side which will then be returned to the AHU. What's your opinion?

 

[lspears92]

Mike,
I'm somewhat of a rookie, but I may be able to help. Where is the fresh air coming from? If the fresh air is coming directly from outside without being conditioned, then you may have a problem because that air will be an additional load in your space. If your air is conditioned by your 30 mbtu unit then that will be a load on your unit. If so, can your unit handle that additional load from the fresh air? You might want to make sure your fresh air intake is a little less than your exhaust, to create a negative pressure difference in your storage room.

 

[Hillbottom]

If you are exhausting air, you will need to make it up from somwhere. It is OK to bring in the makeup air through your air handler if there is A) Enough capacity to condition the makeup air B) There is enough air available in the space you are pulling your return from. Keep in mind that you have a 1000 cfm air handler. This means that you will be drawing 1000 cfm from the control room into the return of the air handler. Since you will only be supplying a percentage of this air back into the control room, a negative pressure will result. This negative pressure will be made up either through infiltration or a direct fresh air connection. Also keep in mind that exhausting 450 cfm means you are exhaustng approx. 12,000 btu of your 30,000 btu cooling capacity

 

[IRstuff]

One caveat is that the exhaust and inlet need to be separated. I've seen systems where the exhaust was adjacent to the inlet, so you'd wind up breathing your bathwater, so to speak ;-)

TTFN