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Head Load Calculations for Air Conditioning

-C@jun-

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Hello fellow Green Thumbs,
Been a while since I have been here. My old account something happen to and old posts are posted by me as a guest. Not sure made a new account.

Anyway as things are getting legal everywhere more and more industry comes to the area. I do air conditioning and ventilation. Anyway I have a customer who is preparing for something very large scale. As of now the rooms are on original equipment in the building so the units are not cooling enough. As well as 20x20x10'tall 25 light rooms in the shop area need cooling. What is the formula for heat load calculations on the room? General rule of thumb. 3-5x the cubic volume of the room for ventilation. In the past I always exchanged air 3 times the cubic volume of the room. With the air conditioner you will get the extra. Recycled but still moving air. Do you size the rooms as if you were sizing a commercial kitchen? Hoping for some guys with experience in HVAC. Any pictures of ventilation set ups industrial scale, duct work/trunk set ups? Any help is appreciated!
 

Tattered Old Graywolf

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As a retired facilities/process/equip. engineer I would calculate the loads long hand, rather than depend on a rule of thumb algorithm. I've seen rule of thumbs be seriously flawed at times.

In industry, process parameters are typically held to tighter tolerance than human comfort HVAC, but are more trouble free. Human comfort HVAC is subject to who all is present at any given time and their individual metabolisms and comfort levels. There is seldom uniform agreement on room temperature.

For process, I would first determine the R-Value of the enclosure, so as to compute gains and losses. You can get R and K values from ASHRAE.

I would determine gains and losses from outside versus Delta T using 100 year ASHRAE extremes.

I would add all the watts of light, about 20% of the motor loads, as well as any other heat sources I was adding to the room and convert it to btu's.

I would add the btu gains from any human occupancy, based on their level of activity. Here is an Engineering Toolbox chart on Human Heat Gain

I would determine how much air was being exhausted versus recirculated and calculate heating/cooling losses. It takes 0.24 btu/lb per degree F to heat air and (1) one ft3 of air weighs 0.0807 lbs.

There would also be water in the form of humidity in the air, so I would calculate the waters weight and what it took to heat or cool it based on the humidity extremes I expected to see outdoors, as well as my humidity standpoint indoors. Here is an Engineering Toolbox link to that information: Water Vapor in Air

The Specific Heat of water vapor is 1.996 Kj/Kg/K or about 0.47673641 btu's per pound/F

I would add up the total and plan for 110% of that capacity for critical processes.
 
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-C@jun-

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Thanks for the reply yeah i have most of the info was fishing for more experienced input with the specific application. I would have to go over everything but like 4.3 btu per watt, what got me questioning was most use a calculation of 4 btu's even per watt. Im located in a hot humid climate so undersizing will definetly be a plus. Again i would have to go over everything but humidity 1 gallon equals 1 pint of humidity to air, how many plants and how many times watering etc.
 

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Tattered Old Graywolf

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Thanks for the reply yeah i have most of the info was fishing for more experienced input with the specific application. I would have to go over everything but like 4.3 btu per watt, what got me questioning was most use a calculation of 4 btu's even per watt. Im located in a hot humid climate so undersizing will definetly be a plus. Again i would have to go over everything but humidity 1 gallon equals 1 pint of humidity to air, how many plants and how many times watering etc.
I use one Watt = 3.41214 btu
 

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