Subject : A load calc problem

Here's one I just got through working on....

This is a WEIRD system. 100 % roof-top air is sucked into a water-based air scrubber ( we haven't got to the fan yet, so the pressure here is negative ). There is a pre-filter ( 90% pleat ) and steam pre-heat coil ( for winter only, not in effect right now ), both of which I'll ignore. :~)

After the water-scrubber, the air is sucked into the fan, and then expelled into ~ 10 running linear feet of solid charcoal, HEPA, and other bio-active mixed-media filtration, in an 18" x 18 " plenum. Gross static pressure reaches 18" water column due to restriction of water-scrubber and filtration, but I'll ignore that, as final WC is ~ 1", therefore I take all heat of compression / expansion to be a net wash for my purposes when I then get to the heat/cool segments at a still-strong but normal static.

After the above, there is a 2 ton R22 AC unit to 'pre-cool' the roof air. There is then a 5 ton Carrier R22 residential scroll AC, and then a 4KW electric element.

Final target of the system is 70 F air at 50 % - 60 % RH, year round, at 500 CFM. Yes, I said 500 CFM.

I just got through tuning and repairing the system such that it holds 70 F +/- 0.5, and 55 % RH +/- 5 %. I hope to get that RH a little tighter next week, tweaking the Faircloth multiplier down just a bit.

Controls are as follows, in equipment sequence : Steam pre-heat is on a Penn OA stat for winter pre-heat only. Scrubber sump temp is whatever evaporation and OA and makeup temp drive it to, no control.

2 ton unit runs continuous, no control.

5 ton unit has a variable hot gas bypass, which is controlled by pneumatics, which sense RH only. RH sensor ( standard 3 - 15 lb) goes to a Barber-Coleman prop-band controller, then to a Faircloth variable multiplier, with final pneumatic output on the HG bypass being 0-80 lbs pneumatic control pressure, and minimum HG bypass pressure set at ~ 60 lbs suction to the 5 ton unit, to prevent ice. TXV is set to provide minimum compressor cooling when in maximum bypass ( Andy, you're just gonna have to let me slide on this one :~) ).

Electric reheat is under DDC PID control, for sensible heat only ( thermistor input ). There is interconection between the controls of any one piece of equipment to any other piece.

I corrected some initial problems with this 'rig', such as : The low-level pneumatics were being run at 35 lbs ( instead of 20 ). The high level pneumatics were being run at 120 lb ( instead of 80 ). The RH sensor was _BETWEEN_ the 5 ton coil and the reheat element, 6 inches from the evap coil, and routinely had liquid water dripping off it ( once the water falls , does it become 'low' humidity ? :~) ). The sensible thermistor was ~6 inches downstream from the reheat element ( I measured >20 F laminar stratification delta in this region ). After I regained consciousness, I moved both sensors 30' downstream. The 5 ton unit was running 300 lb head on a cool day, 400+ on a hot day. Unit would cut out on head and thermal regularly. Unit has had 3 compressors in 2 years ( whole system is two years old ). Sight glass NEVER came even close to clear with these pressures, and unit has NEVER had a clear sight glass in 2 years, from what I've been told. There is a solenoid in the liquid line, between the guage ports and the condensor. After I evacuated the system with this solenoid _OPEN_ ( which I don't think had ever been done ), the system now runs a clear glass with a 260 head on a hot summer day. Ah, yes, the glories of air ( or nitrogen ) in a system ! :~(

So anyway, on to my question...

My intuition told me that 5 ton was hugely oversized for 500 CFM load. Then I ran the numbers, and the numbers tell me that my intuition needs a long vacation. Would some kind ( engineering type ) soul care to comment on the numbers below, please ? This is a bit out of my usual area. Please remember that some of the below are approximations from my psychrometric chart, I just want to know if my understanding is somewhere in the ball park.

Clean air supply system - sample load

If setpoint = 70 F , 50 % RH

If input air = 85 F, 85 % RH

Volume is ~ 500 CFM at all times

Input = 85 F, 85 % RH = 50 btu / lb, @ 14.25 cu.ft. / lb = 105,623 BTU/HR

because :

1 / 14.25 cu. ft. / lb = .0701... lb / cu. ft.

0701 lb / cu. ft. x 500 CFM = 35 lb / minute

35 lb / minute x 50 BTU / lb = 1754 BTU / minute

1754 BTU / minute x 60 = 105,623 BTU / HR enthalpy of input air

MINUS

Ouput = 70 F, 50 % RH = 25 btu / lb, @ 13 .5 cu.ft. / lb = 55,555 BTU/HR

because :

1 / 13.5 cu. ft. / lb = .0740... lb / cu. ft.

0740 lb / cu. ft. x 500 CFM = 37 lb / minute

37 lb / minute x 25 BTU / lb = 925 BTU / minute

925 BTU / minute x 60 = 55,555 BTU / HR enthalpy of output air

PLUS :

Assume : 4 KW re-heat element runs at 50 % duty cycle to maintain sensible at 70 F, under PID control

Thus 4,000 W x 3.41 BTU / W = 13640 BTUH, x 50 % cycle = 6,820 BTU / HR

105,623 - 55,555 + 6,820 = 56,888 BTU / HR total enthalpic load

56,888 / 12,000 = 4.74 TON total combined sensible and latent load.

Therefor, 5 ton ( plus capacity from the prior 2 ton ) is rock and roll city.

I eagerly await the input of those who do this stuff for a living.

You'll have to excuse me now, I think I'm getting a headache. :~)

Paul

Oops...

I just realized a basic error I made by adding the reheat to total load. My actual proposed total load should be 4.17 ton, not 4.74.

Please continue to educate me from there. :~)

Paul