How Much Cooling is Required

[new2hvac]

I'm looking at replacing some swamp coolers with mechanical cooling and am having trouble calculating the cooling requirement.

The evaporative coolers are only able to cool the space to about 90 deg F on hot days (outside air around 105 deg F), and they would like it to be cooler.

My question is, how much cooling will be required to maintain 75 deg F and 40% RH?

Given:
Area of room - 18,000 SF
Ceiling Height - 25 feet
Current Indoor Conditions - 90 deg F 50% RH
OA Design Conditions - 105 deg F 20% RH
Desired Indoor Conditions - 75 deg F 20% RH
Ventilation Requirement - 6,000 CFM

Ideally, I would have specs on all of the heat sources in the room (people, lights, machines, etc.) to arrive at a heat load, but all the data is not available.

The heat sources in the room should remain constant, all I want to do is change the indoor conditions. Since the evap coolers are not changing the energy in the space (they're only exchanging sensible for latent heat...right?), it seems like I could just look at the change in enthalpy between the current and design conditions. From a psyc chart, I can calculate the energy of the air based on the enthalpy, volume of the room and specific volume of the air in the room. If I take the difference between the current and desired conditions, I have a number in Btu's...not Btu's/hour.

I also need to consider the OA requirement for ventilation, but that's easy enough to calculate in Btu/hr.

I would also need to consider that heat xfer through the walls and ceiling will increase if the temperature in the space decreases.

This is getting a little long so I'd better stop here.

Thanks in advance! Any input is appreciated!

 

[ChasBean1]

Analyze for an air mixture based on OA design condition and steady-state room conditions desired. Typical flow for cooling is 1 cfm/ft2, but has limitations based on use. And for the ceiling height, I might lean toward higher cooling air flow, but since you want 72°F/40% in the room, the discharge temperature shouldn't exceed 46°F - a very cool supply air which would allow lower volume.

What you desire: Mixing 6,000 cfm of design condition OA (105°F/20% - for ventilating) with 12,000 cfm of steady state room air (72°F/40% - recirculated for energy savings) would give you an 18,000 cfm mix that's 82°F/32%. Conditioning that air volume at these conditions to a 46°F saturated discharge condition would be,

Q = 4.5 * cfm * dH
Q = 4.5 * 18,000 * (27.95 - 18.36)
Q = 777,100 BTU/Hr or ~65 tons

I would recommend against this because people who are used to 105°F, 20% RH heat might actually feel their bones will shatter when they hit 72°F/40%. Let's face it, 85° would feel pretty good after coming in from that heat.

If you shoot for more realistic room conditions, say 75°/55% and raise the delivery volume a little (20,000 cfm) the new steady-state condition looks like:

Q = 4.5 * 20,000 * (31.18 - 23.20)
Q = 718,000 BTU/Hr or ~60 tons

The central AHU(s) should preferably have chilled water; if not then multistage DX. Keep in mind these are approximations based on steady-state assumptions and that this is an inexact science.

 

[jburrows]

It's going to be expensive to cool that kind of space mechanically. That's why the swamp coolers were there in the first place. If mechanical cooling is really the best system, you may look at only conditioning the lower 6 ffet or so in the space. You could provide some relief vents controlled by a thermostat to keep the upper portion from getting too hot. Personally, I would look at improving the swamp cooler design. Your air is dry enough that evaporative cooling can be an effective means of conditioning a tall space like you've got.