RETURN LOCATION

[antony09]

WE HAVE ARE COOLING A CORRIDOR WITH PARTITION IN THREE SIDE, GLASS WITH 10 MTS OVERHANG AND STEEL ROOF WITHOUT INSULATION. THE AREA IS 200M2 WE INSTALLED 25 TONS. THE CORRIDOR HAVE 13 MTS OF HEIGHT AND WE ARE USING FLEXIBLE DUCT 3MTS BELOW ROOF, THE RETURN GRILLE IS IN 1MTS BELOW ROOF. WE HAVE 1600BTU/MT2 OF COOLING CAPACITY BUT THE RETURN TEMPERATURE IS 80°F, EXTERIOR TEMPERATURE IS 91°F. WE THINK THAT THE PROBLEM IS THE RETURN GRILLE POSITION, WE NEED TO RELOCATE THE GRILLE MINIMUM 3MTS BELOW ROOF. ALSO WE HAVE THE FOLLOWING QUESTIONS. IF WE MADE THE LOAD ESTIMATING INCLUDING ALL HEIGHT OF THE SPACE, WHY NEED TO RELOCATE THE GRILLE, IF WE HAVE THE COOLING CAPACITY THE MACHINE CAN HANDLE THIS TEMPERATURE, BUT THE MACHINE COULD NOT HANDLE THE ROOM TEMERATURE, WE RECOMENDED INSTALL INSULATION TO THE ROOF. ANY PEOPLE CAN EXPLAIN IF THE RETURN GRILLE NEED TO RELOCATE, I DONT UNDERSTOOD, BECAUSE THE LOAD ESTIMATING GIVE ME 25 TONS, WE THINK THAT THE GRILLE CAN INSTALL IN ANYSITE. BUT THE RETURN TEMPERATURE IS HIGH. THE PROBLEM WILL BE THE POSITION TIGHT WITH THE ROOF, I DONT UNDERSTOOD.

 

[MintJulep]

HOT AIR RISES.

 

[antony09]

ok, we understood that the hot air rises, but the machine have the capacity to cooling, why the air is hot.

 

[lilliput1]

What is the thermostat set for & is the thermostat located in the occupied space? Is the ouccupied space comfortable? If so who cares if the return is at 80°F? With the occupied space low & the return high, there will be stratification. Is the AC unit drawing full power (all compressor stages on) at peak load? Some designers actually use stratification (locate supply & return low) to reduce equipment size. The hot stratified air if above the occupied zone is allowed to store heat or reject it to ambient.

 

[antony09]

THE THERMOSTATA ARE LOCATED IN THE RETURN DUCT, AND THE THERMOSTAT SET IS 60°F, WE DONT USE STRATIFICATION, BECAUSE THE SUPPLY AND RETURN DUCT ARE HIGH. IN PEAK LOAD ALL COMPRESSOR ARE RUNNING, SO THE RETURN TEMPERATURE IN 80°F

 

[remp]

Sound like your having problems with getting the cold supply air down into the occupied space.
You have a very high ceiling 13metres ..yea?
What kind of ceiling diffusers have you? Do they have the required throw. Low level return air would deffinatly help get the supply air down to the occupied level or supply air diffusers with propper air diffusion for hihg spaces.

However you did say the the return air temp set point is 60 F and the actual temp is 80 F...sound more like your plant is undersized.

 

[MarauderX]

It sounds like billyq is right on with the supply and return both being too high. The only thing they would cool would be the air at the top of the ceiling. You CANNOT rely on warm air rising/cool air falling to properly condition the space. Try to install diffusers with a high throw to get the cool air to the location you need it.

And if all the compressors are running with properly sized equipment you should have no problem cooling the space, or at least the part you are distributing the air to.

Could you post total dimensions of the space, location, & glass facing so we can get a better idea?

 

[IRstuff]

How clean are the ducts and are you sure you don't have any leaks?

How clean is the condenser?

What's changed since installation?

How long since installation?

Are you sure that A/C coolant is still at viable charge?

TTFN

 

[IRstuff]

Actually, I left off the most important question:

Did it ever work right?

Also,

How did you size the pump required to move the air?

TTFN

 

[antony09]

hello IRstuff:

the duct is clean and the leaks are little.

the condenser is clean

the installation have 1 years.

the A/C charge is complete.

the systems never work right

the fan was selected with external estatic pressure to be 0.6 inchs of water.

Any questions more please contact me

 

[Stormxtc]

Don't quote me on this but sometimes a dity or blocked return air filter will reduce capacity.

 

[HVAC68]

You have talked about the capacity. How about the air quantity ? What's the design air flow ? Does the AHU deliver the same ? What's the outside air quanity ? Any leak in the roof leading to infiltration of outside air ?

 

[friartuck]

Seems you have a number of problems here

a. Your roof needs insulating. With bare steel roofs you may as well have a single glazed green house. The gain would be horrendous.(Even in the UK..but you appear to be somewhere very hot)

b. The mode of input could be better. Cold air will dump and so the problem would be to distribute it properly and effectively. We use swirl diffusers in the UK which do not cause drafts but you would have to take advice from a decent grille manufacturer such as TROX or KRANTZ to get a good selection.

c. For high spaces, displacement ventilation methods (which have been developed in Sweden give a very effective solution as you deliberately supply very low velocity air at floor level. The air 'creeps' along the floor and when it hits a warm object ii.e. you or me, the air then rises around the body to keep it cool. These are very effective systems. See Halton, TROX, Flakt-Woods or similar manufacturers.

The cooling load is also greatly reduced. You would need to install what are effectively perforated dustbins (trash cans)which allow the suppy air to percolate.

d. The corridor you describe may not be well sealed. This will also have a dramatic effect.

Hope this was of some use

Drapes

 

[pk786]

i think your plant is under size you have to do insulation or cealing relocat the supply and returen grillwhat typr of insulation on duct for supply and returen check your door and windows for out door air leak.Also check your steel roof for out door air leak.make your room air tight you will get required returen air tempture.