Psych Problem

[purduedon]

I have a factory/plant with no air conditioning. The owner wants to add some a/c to help the employees. However, the add will be well short of what is needed and wants to know how much help will be seen from an RH perspective.

Problem: Plant - 200,000sf with 30ft roof. Present condition approximately 90DB/70% RH - calculated load is +/-800 tons.

If we add two (2) 120 ton Air Rotation Units (modified to full recirc), how much will the RH in the plant drop. It's been awhile since I visited the psych chart and would appreciate any input or suggestion at starting the calculation.

There are 16-20 overhead doors that are presently open for ventilation, but could be closed.

Thanks in advance.

 

[Dan1978]

I think a good place to start would be to look at the manufacturers data on the equipment you are planning to use. Determine the latent cooling capacity of the equipment. Once you have that, look at the total volume of air in the building and determine per cubic foot how much moisture will be removed.

On psychrometric chart, locate current conditions in the plant, 90DB, 70RH. Determine grains of moisture per cubic foot you will be removing, and locate your new point.

There may be a better way to go about this, but I would try something like above.

Good Luck

 

[imok2]

Where do you get 800 tons when the amount of air = 200,000 sq ft x 30 ft = 6,000,000 cu. ft I think that your estimate if 250 sq. ft /ton is way too generious ,try about 160 sq.ft./ton
With that amount of tonnage you prepose and the amount of cu. ft of air your dealing with I gusstamate at best 1% R.H. drop/hr assuming you keep the infiltration to an absolute minimum. Hardly worth the effort.

 

[HVAC68]

If you have already decided what capacity unit to put in (2 x 120 TR - you probably also know how much air quantity, that these 2 units can deliver - at least approximately), then work back on the heat load calculations by changing the inside temperature. Keep doing an iteration by changing the inside design temperature, till the time the total capacity (2 x 120TR) and the air quantity match. The output should give you the RH expected.

HVAC68

 

[ChrisConley]

Have you considered spot cooling? Small ducted rooftop units to the areas where people work. Use a high throw diffuser to dump the cool, dry air right on the workers. I've done a couple of industrial plants in this fashion and had success.

 

[ChasBean1]

One other possible suggestion if it's not too late:

Factories usually have a bunch of task exhaust (snorkels, hoods, spray booth, etc.). The goal would be to try to achieve slightly positive ventilation and dehumidifying the make-up air by matching the make-up quantity with the exhaust quantity.

It would take a little homework to inventory all exhaust, but in the end you might win. For example if you find the factory exhausts a total of 20,000 cfm, put in 2-4 units on the roof that supply 22,000 cfm of outdoor air that can cool 90°F db/76°F wb to 60°F. This would only be <110 total tons under design conditions.

The units might struggle a little during the hottest conditions you mention (90°F/70% RH) but should normally bring the dew point in the building down quite nicely. The dew point drop is what would be noticed and the main key to allow you to do this is to balance the building.

The problem I'd see with just getting two 120 ton units (without matching the minimum make-up air quantity to exhaust quantity) is the place will still be under negative ventilation and you'll still be drawing in a lot of moist air from outside, plus possibly creating mold issues where your new, cool surfaces meet the unconditioned air being drawn in...

Good luck, -CB