chill water airhandler

[licensed to chill]

Trane chilled water ahu(cooling only)no pressure or temperature guages installed or even p/t ports.customer calls because lack of cooling. I get the following readings with accurate digital thermometer.

ewt= 47
lwt= 63
delta t = 16 degrees

return air = 76
supply air = 65
delta t = 11 degrees

3 way valve 100% open
all valves 100% open
circuit setter 100% open
strainer removed and inspected it was clean

any ideas on low air delta t?
low chilled water flow maybe?

Thanks for any responses

[techrider]

Realy need to try to find out the DP. Could be scaled internaly or may be all ciruit setters on other units may have been opened by others in the past and there is now no availible pressure differential.

Example would be a recent install our installation crew put in on water cooled package units. New units tripping on high head with around 100 psi. supply to condensors. Problem was return line had same pressure. The other units on the same line were using the differential before the new ones got a chance.

[lwarren]

Waterside Delta looks high to me. Looks like low water flow.

[orland25]

some by-pass in the loop close" "check GPM" try to put that water in 44deg and loock what happend" what your outside air temp?

[ralphtheplumber]

I suspect low water flow too.

Your circuit setter doesn't have pete's plugs?

[heat/cool/dude]

Try slowing your air flow down, also I would run the CWT down to 43-44 degrees. Does this system have glycol in it?



keep it basic!

[just_opinion]

I agree with getting the EWT lower (I prefer 38-40 degrees).
Look like you coil is not fouled up since you have such high temperature rise.

76 degrees return (is it included fresh air at what temp Outdoor).

If 76 is with fresh air - then I say you need to check you air flow that you may have too much and the building has a lot of humidity

[bertoh]

the difference between lwt 63 and supply air 65 is 2 deg. you are not going to get any lower supply than that. question is too much air or toolittle water? you need a flow measurement of one or theother to start doing the math.easiest is get your airflow , buy a pitot tube and manometer that gives you FPM or feet per minute. FPM x square feet of duct equals CFM. than the basic math(1.08x CFMx delta T= btu..
see if that matches up to design criteria of space being cooled. if not more of your math to get a clue if it is close.about 1 cfm per square foot for commercial space, unless something out of ordinary ie. data room, kitchen etc.
bottom line you need more water or colder water to get lower supply air.but to get it to 55 deg you probably need to lower entering water by 10 deg. to 37.
ps what is it leaving chiller 47 i hope or it is insulation on pipe you need to think about.
good luck, let me know if you need the math for the water side.

[enart9591]

Depending upon the age of the AHU, a 10 degree water side temperature rise is a fairly common design, but in recent years I have seen 12, 14, and even 16 degree design rise thru the coil.

If you look at your leaving air temp compared to your entering water temp, you have a 18 degree approach. A fairly common approach is around 10 degrees. (On a chilled water coil, Approach is defined as the difference between entering chilled water temperature and leaving air temperature)

I too suspect low water flow. Have you checked the strainers at the pumps or for bypassing somewhere in the system.

If you can get a model and serial number, your local Trane office may be able to get you design data.

One other thing, is your chilled water coil piped correctly? The coldest water should be going in on the leaving air side of the coil, if it is not the coil will never perform as designed.

Good Luck

[Edited by enart9591 on 09-21-2006 at 04:27 PM]

[bertoh]

i thought approach was a measurement of the two leaving fluids in any heatexchanger. ie the two temps can approach each other but never cross each other. in other words you cant get 50 deg air out of 60 deg water, you may get 61 deg air out of60 deg water but never 60 out of 60

[rommel21]

Hey guys,if the chilled water coil has a MUD LEG on the inlet with a blow down valve, make sure you can flush it out. Sometime the TEE and the MUD LEG fills up with mud and rusty sediment. You can have good temp. 100% on the chilled water valve, and clean strainers, but the TEE will fill up causing a flow restiction . Happend to me at the EXXON Clinton site and the SCHERING Union site in New Jersey. It would be great if there was a ball valve on the blow down.Then if it was blocked you can get a rod up in to it to break the blockage.

[ericnyc]

How about the possibility of air trapped in the coil?

[jeff 9]

Check to make sure the coil is not airbound also if the system has a backup pump make sure the check is not leaking by. This will cause the chilled medium to recirculate thru the idle pump.Also when was the last time the coil was cleaned? One last thing if the cooling medium is sweet water I would not recommend dropping below 44 Which is where you should be.

[Carnak]

Water flow could be low or airflow high.

Check that the coil is not piped backwards. For cooling, you want the leaving water header up stream with respect to air flow.

Too bad you can't measure air flow and GPM.

[enart9591]

The approach temperature is determined by the design (from the engineers) of the machine and is dependent upon the type of machine in question.

The evaporator approach on a centrifugal chiller with a flooded evaporator is defined as the difference between the liquid refrigerant temperature and the leaving chilled water temp. On such a machine, the approach can be anywhere from 0 degrees to something greater than that, but never less than the evaporator refrigerant temp.

The condenser approach on a centrifugal chiller with a flooded condenser is defined as the difference between the liquid refrigerant temperature and the leaving water temperature. On such a machine, the liquid refrigerant temperature can be less than, equal to, or greater than the leaving condenser water temperature due to sub-cooling. Thus, the approach can be a positive, equal, or negative number.

On chilled water AHU’s the approach is defined as the difference between the entering chilled water temperature and the leaving air temperature. It is possible for such a unit to have a design condition of 44 degree entering and 54 degree leaving chilled water temperature with a leaving air temperature that is 49 degrees. The reason being is that you have an average temperature of the water as it flows thru the coil, whereas in the evaporator of a centrifugal the refrigerant temperature is theoretically the same thru-out.

[bertoh]

Originally posted by enart9591
The approach temperature is determined by the design (from the engineers) of the machine and is dependent upon the type of machine in question.

The evaporator approach on a centrifugal chiller with a flooded evaporator is defined as the difference between the liquid refrigerant temperature and the leaving chilled water temp. On such a machine, the approach can be anywhere from 0 degrees to something greater than that, but never less than the evaporator refrigerant temp.

The condenser approach on a centrifugal chiller with a flooded condenser is defined as the difference between the liquid refrigerant temperature and the leaving water temperature. On such a machine, the liquid refrigerant temperature can be less than, equal to, or greater than the leaving condenser water temperature due to sub-cooling. Thus, the approach can be a positive, equal, or negative number.

On chilled water AHU’s the approach is defined as the difference between the entering chilled water temperature and the leaving air temperature. It is possible for such a unit to have a design condition of 44 degree entering and 54 degree leaving chilled water temperature with a leaving air temperature that is 49 degrees. The reason being is that you have an average temperature of the water as it flows thru the coil, whereas in the evaporator of a centrifugal the refrigerant temperature is theoretically the same thru-out.

[bertoh]

i have never heard of negative approach. i thought the term came out of the fact that you cannot make the two lines on a graph cross each other when graphing two leaving temps on a heat exchanger(the temps will "approach" each other but never cross) in other words you cant make 40 deg air out of 50 deg water.
as for the subcooler on flooded condenser i thought that is was subcooled below its saturation temp.

[Carnak]

It is not a negative approach it is 'counterflow'. The leaving water is cooling the air as the air enters the coil.

The colder entering water is what cools the air just before it leaves the coil.

There is always a significant temperature difference between the air and the water.

When it is 'parallel flow', the coldest water cools the air as it enters the coil. Initially there is a big temperature difference but soon the water and air are near the same temperature and heat transfer stops.

Every time some one posts chiiled water performance, where flow rates seem to be normal, but heat transfer is unsatisfactory, when the leaving water and air temps are very close to one another I always have the gut feeling the coil is piped backwards for parallel flow.

[Edited by Carnak on 09-23-2006 at 08:55 AM]

[enart9591]

I will add that on a flooded condenser the cooler entering water is in contact with the condensed refrigerant before it flows to the evaporator. It is because of this that you can have a negative approach.

[bertoh]

carnak , that makes perfect sense. well explained ,