Dont be shy, advice needed.

Big story, hope you will be patient! I am trying to give the whole story all at once.

I have been called in as a consultant to help a friendly competitor with a problem he has been struggling with for over a year. My findings have been knocked by the manufacturer, who has a completely different view. No work has been done yet. The unit was installed a little over a year ago.

The complaints; Condensate is not draining from pan. The fan compartment is filling with water on floor at least 1/2 inch deep. The coil is freezing up. Insufficent air supply to halls. Filters being sucked out of holders.

It is a gas fired makeup air unit, with an induced draft blower fan, with an ac coil placed in the inlet opening, and a bypass damper placed above the ac coil.
The coil is coupled with a 70 ton Carrier condensor, 2 compressor circuits, each compressor feeding 2- 18 ton tx valves operating on R22. Each compressor has an unloader valve on the head, unloading 2 cylinders. The first stage has a HG bypass.

The fan is setup, from factory with pulleys sized to deliver 33,620 cfm of air into the halls of a condominium apartment building.
My first thought when I put my hand in front of the discharge air vent on the top floor is that the filters are plugged. Insufficient air is being delivered. I dont have the design specs on exactly what cfm should be delivered, so didnt measure this, just didnt feel right.
Examining the unit, the inlet opening is 5 ft high, and 10 feet across. The ac coil occupies the bottom 3 ft, with the bypass damper across the top 2 feet. The bypass damper is closed, as this is in summer mode, and the previous tech felt it should only bypass in winter mode.
The fan compartment is full of water, the water is not draining from the pan. A quick inspection of the pan reveals 2 hacksaw cuts at each end, in the side of the pan. I would believe that someone cut them, to try and drain the water out of the fan compartment back into the condensate pan, and down the condensate drain.
A quick calc shows me that the air velocity across the coil...33,620cfm/3x10 sq ft of opening=1000+ ft/min. I am assuming that the condensate is being sucked off the coil. I believe that this happens above 500 ft/min velocity, but cannot verify this, or remember where I got this number from.
The filters (cardboard frame pleated 2in thick) are also bent in the middle and sucked from the frame. Supplier advises me that they are also designed for 500 ft/min max.
I measure air flow at a number of points across the evap coil. A close measurement (2inches away) gives a variety of readings, from 200 fpm, to 2000 fpm.
I deduce that the coil is partly plugged. A measurement taken 18 inches from the coil, gives an inconsistent reading varying from 600-1200 fpm. There is a lot of turbulence in the air.

At this point I am deducing that the air speed is too high! The manufacturer advises me that the design is for the bypass dampers to be permanently open, so that 22,000 cfm are to go through the dampers, and 11,000 cfm are to go through the coil.
I still see that this air speed is too high, and that the air filters will not survive this install...my math is 33,620 cfm/5x10=672.4 fpm. I am thinking that they need a bigger inlet opening to reduce the velocity of the inlet air, reducing the static.
The static in the fan compartment is 1.2 inWC. I think that this is preventing the water from draining out of the fan compartment.
I have not been back to site to test the manufacturer's advice to open the bypass dampers fully.

I am thinking that the air will not get a lot of cooling, when 2/3 of it is bypassed around the coil. The plan according to one engineer is normally to bypass 1/3, but this unit is designed to bypass 2/3.

I am having concerns about the coil. It will only see 11,000cfm. 30 ft sq of surface, means 366 fpm, a bit slow. I saw some freezing on the edges of the coil when all 33, 620 fpm (84F 40RH) were going through it, have big doubts about what will happen when we bypass 2/3 of the air.
(Am questioning all my assumptions at this point...so,,,anyone know what happens to the amount of heat transfer when the air is going to fast? Am I seeing poor heat absorption from this fast air, but perhaps I will see better heat transfer when I alter the air flow as recommended, to the 366fpm).
This coil is hooked to two 35 ton compressors, each one with 2 18 ton tx valves. I am viewing this as way too much mechanical cooling equipment for that much air flow. The rule of thumb that I have grown accustomed to is 400 cfm/ton for AC.

The engineers claim that this unit is designed for 90 deg F ambient air entering, and will deliver a 33 deg F TD for a discharge air temp of 57 deg F.

If I use their number of 57 deg off the coil, and mix that 11,000 cfm with 22,000 cfm of 90 deg F air I get (1x57+ 2x90)/3=79 degF. I dont see that as adequate cooling for supplying common areas in an apartment building.

When I saw the unit it was only operating on one compressor (the second stage has plugged LLFD) with entering air at 84 deg F and 40 %RH. The back pressure was sitting at 65 psi. There was frosting on the ends of the coil (on the fins at each end and return bends, also on distributor tubes) I see this as insufficent load on this unit.
The compressor was unloaded, and the HG bypass would have been bypassing but I shut off the isolation valve.

MY math is 11000 cfm/450 cfm/ton=25tons, or 11000/400=27.5.


My conclusions:

Inlet air opening is too small, needs to be increased from 50 sq ft, to 75 sq ft, to allow 33,620 cfm at 450 fpm air.
My suggestions;
-is to cut a side out of the mixed air chamber, and put filters in it with balancing dampers in it, c/w filter frames.
-remove the 2x10 bypass damper and install a evap coil in this opening above the existing coil, to give a full surface area of 5x10 evap. My math is 450 fpmx 50 ft sq=22,500 cfm and 22,500 cfm/400cfm/ton=56.25 tons, or 22,500/450=50 tons. Use the balancing dampers in the newly installed opening to ensure 2/3 air through coil.

Forgot to post, SH on the operating stage is 15 degF on one tx, and 18F on the other
tx measured at the bulb.

Hi Sledge,

What I feel is that the Coil fins are chocked. The air velocity as measured by you is very high. The coil surface is correct for 500 to 600 fpm velocity. You may also consider draining the fan compartment and the drain pan.

Reducing the velocity will further increase the possibility of ice formations.

The above are just my views which may be wrong.

Hi Sledge,

What you described seems incorrect design from the beginning.

If air velocity is that high, definitely will carry over the condensate. On the other hand coil performance will be effected badly.
With high velocity the air TD will be low and with low velocity air TD will be high.
I always design AHU coils for 500 FPM, except in applications where high air TD is required. In those cases I take low air velocity.

I suggest you talk to the manufacturing company and get the design data as much as possible, of course if they supply these info.

Cheers

I'm struggling to convert the measurements into SI units. Can't seem to find my book with the conversion factors in.

If you could convert and post in SI then I may be able to add my 2 pence worth. :)

Quote:

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Originally Posted by frank

I'm struggling to convert the measurements into SI units. Can't seem to find my book with the conversion factors in.

If you could convert and post in SI then I may be able to add my 2 pence worth. :)

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I will try and find some conversions...unfortunately I am in the same boat as you, I often have to avoid posts in SI because I dont understand those units.

Hi Sledge

Yes the design air velocity over this coil is much overdesigned (the recommended value is 500 fpm (2.5 m/s) smooth plate fins or lower than 600 fpm (2.75 m / s) for corrugated fins). If the design is established over 600 fpm this leads to the carryover of the condensate water droplets and this is obviously occurred in ur case where the fan compartment is filled with the condensate water. The freezing on coil edges did not come from high velocity, it may come from maldistribution from the air over the coil due to some blockage or obstacles in its path or in the air filter cause less amount of air over the coil edge and distributor. In contrary, the higher air velocity the higher coil performance but on the expense of the higher pressure drop, fan power, and carryover factor. In principle, I think ur problem is the CFM is very high and as well air velcoity and pressure so the reduction of the CFM over the coil as u mentioned 400 CFM/ton is suitable for the system is proper solution. Therefore, I suggest u use speed reducer (different pulley diameters) or replaced the pulley transmission by gear box and reduce the fan seed from let us say 3600 RPM (32600 CFM, 1050 fpm over 30 ft2, the bypass gate will be closed in the summer) to almost 1600 RPM (14400 CFM, 480 fpm). This will give adequate frontal air velcoity of 500 fpm with static pressure drop of 0.24 WC and avoid the damage in the air filter. In the winter, to shun the overcooling (as a result of the drop in CFM will be followed by the decrease in off coil air temperature), I suggest u put balanced damper in the air entrance to close partially the air manifold to some extent to bypass large amount of the air over the coil. I wish it could help

Best Regards

Thanks guys.

I was trying to maintain discharge air volume, as I was already viewing the air supply as inadequate. I hadnt considered the idea of slowing the fan, because of this.

The fins on this project are corrugated.

Sledge,

The coil and bypass proportions you calculated seem to be wrong if the initial data is correct. Coil face is 3'x10' and bypass damper face is 2'x10' then the air flow proportion should be 3*33620/5 = 20172cfm. The velocity of 672.4 fpm across the coil (20172/30) is high but not very problematic in terms of moisture carryover. You should check the U loop of drain leg for atleast the cooling coil box static when entire air is flowing through the coil. Nothing would be better if you make the leg difference equal to the fan static.

If you bypass the air then the mixed air temperature should be around (0.4*90+0.6*57) = 70.2F. This may be a bit high. 65F could be ok if the room load is predominantly sensible.

Couple of questions that can throw more light on the issue.
1. What are the design ambient conditions.
2. What is total floor area
3. Any data about the loads?
4. Room conditions required.
5. Is this a once through system?

To SI guys, (I am a mixed one)

33620cfm = 15.87cu.mtr/s
90F = 32.22C
57F = 13.89C
33F (TD) = 18.33C
70Ton(i.e TR) = 246.18kW

Now the most critical unit conversion:p

10ft = 3.048m
3ft = 0.914m
2ft = 0.609m

The best unit conversion software (called Uconeer) can be downloaded from katmarsoftware, free of cost. I already complained about making me more lazier by even providing conversion for temperature difference.

It should be kept in mind that this is a makeup air unit, NOT an A/C unit. It's purpose is to maintain a slightly positive pressure in the building, NOT to heat/cool the building.

If the building is excessively pressurized, the CFM is excessive. If the building is in a vacuum, the CFM is insufficient. The ground floor doors should open easily, not blowing outward nor sucking inward.

Assuming the CFM is correct, the discharge temperature is controlled by the compressor unloaders.

The bypass dampers are for humidity control. What all of the above calculations seem to be missing is humidity estimates.