Cooling Coil question

[hvacn00b]

Hello everyone

I'm a newcomer in the industry and I was hoping that I could get some advice regarding the problem that my boss experienced at one of our job sites.

We have an existing chilled water cooling coil (1000 mm wide x 900 mm high x 200 mm deep). Number of rows is unknown but we think it's 6. Carried out a performance test last week and here are the results:

Air Side
Flowrate: 1500 l/s
Entering temperature: 16.3 C
Leaving temperature: 8.8 C

Water Side
Flowrate: 3.3 l/s
Entering temperature: 7.6 C
Leaving temperature: 8.5 C

Pressure drop across the coil: 68 kPa

What he wants to know is why we are getting very low TD of water.

Any advice would be greatly appreciated. Thanks :)

[Lc_shi]

air side energy increase = water side energy decrease
low TD means air side heat load is lower than nominal

you can calculate and balance it .
BTW,you should get the air humidity for exact calculation.

regards
LC

[hvacn00b]

Thanks for replying, Lc_shi.

Anyway, I've done the energy calculation and it worked out to be roughly equal. But I must admit I haven't taken into account the humidity. I'll have somebody check that out.

[frank]

From the figures you give the coil is rated about 13.7kw. With a flow rate of 3.3kg/s the dt = 0.99K so your measurements are correct.

To adjust the dt you need to reduce the flow rate.

Are you getting acceptable cooling to the air side? - if so, leave it alone. :)

[hvacn00b]

From the figures you give the coil is rated about 13.7kw. With a flow rate of 3.3kg/s the dt = 0.99K so your measurements are correct.

To adjust the dt you need to reduce the flow rate.

Are you getting acceptable cooling to the air side? - if so, leave it alone. :)

Yeah I've told my boss about reducing the water flow rate (if that's what you meant) so we'll see what the outcome will be.

According to him we're not getting enough cooling on the air side at the moment :(

[Abby Normal]

Water side and air side numbers in same ballpark, all sensible heat transfer. Most likely measuring airflow a little high or water flow a little low.

That entering water temperature is typical of air conditioning while the entering air dry bulb is not.

The entering air dry bulb temperature is low, the temperature difference between the air and the water is almost half of what one would expect so that would reduce heat transfer. With an entering air condition typical of air conditioning you would expect 3 PLUS times the capacity.

The face velocity of the coil is low as if trying to dehumidify, yet there is no dehumidification indicated in your numbers, the entering air is cool so the entering dewpoint could be low.

Maybe check that the piping is correct you may be approximating 'parallel' flow.

You may also have a blocked circuit in the coil, air locked perhaps. Doubtful that the inside of the tubes are scaled up.

You gave no indication of the fin spacing, but with tubes at say 32 mm on centre, that could very well be a 6 row coil.

From an air conditioning stand point, the water flow is high. Reducing the water flow may actually increase heat transfer contrary to what common sense suggests.

[Renato RR]

What he wants to know is why we are getting very low TD of water. :)

To get 5 deg C delta T you need flow rate of 0,6l/s.
Your pump is 5 time biger then you need.

Renato

[Abby Normal]

The similarties of the leaving air and water temperatures may be a reason to double check that the coil is not piped for parallel flow.

You want the cold supply water connected down stream with respect to air flow, and the warmer return connection upstream with respect to airflow.

I think your boss is giving you an IQ test perhaps.

[frank]

Air Side
Flowrate: 1500 l/s
Entering temperature: 16.3 C
Leaving temperature: 8.8 C

Water Side
Flowrate: 3.3 l/s
Entering temperature: 7.6 C
Leaving temperature: 8.5 C

What he wants to know is why we are getting very low TD of water.


You are getting a low dt on the water side because the water flow rate is too high


According to him we're not getting enough cooling on the air side at the moment :(

From the figures you posted you have a leaving air temp of 8.8C and a leaving water temp of 8.5C. With an entering temp of 7.6C - how much more does he expect out of this system :confused:

[Abby Normal]

Thanks for replying, Lc_shi.

Anyway, I've done the energy calculation and it worked out to be roughly equal. But I must admit I haven't taken into account the humidity. I'll have somebody check that out.

If it is working out roughly equal, meaning sensible and water side about the same, it would suggest no dehumidification.

[NoNickName]

Attached calculation sheet, for all sensible heat.

[Abby Normal]

That entering air could be close to 60% RH and still no dehumidification possible with that entering water temperature.

The coil pressure drop is low for 6 rows, perhaps not just due to the low face velocity but becuase it is dry as well.

[S.K.VARDE]

From the water side data, you are extracting 12.5KW. When we estimate the coil capacity (1000mmx900mmx6R deep) it comes around 55KW. It does't matches.

Secondly, by the the coil size, we estimate the the air flow to be in the range from 2000 to 2400 lps depends upon the application.

Thirdly, your data for air pressure drop accross the coil is also very high.

You also did not provided sufficient data like, application, desirable temperature & RH conditions, WBT of air temperatures, NB of the tubes, fin type and their spacing etc.

My best suggestion is to call call some expert to sattle right your plant.

[NoNickName]

From the water side data, you are extracting 12.5KW. When we estimate the coil capacity (1000mmx900mmx6R deep) it comes around 55KW. It does't matches.

Please let me know how you are able to get 55kW out from that coil, with that water and that air. It would be a revolution in HVAC. And you will be rewarded a banana.



Secondly, by the the coil size, we estimate the the air flow to be in the range from 2000 to 2400 lps depends upon the application.

The air flow was given as an input. Not my decision.



Thirdly, your data for air pressure drop accross the coil is also very high.

It is not. A standard 6-pole 10/8 or even smaller would be ok.

[MadApprentice]

Hm, is the air coil clean? The water temp into the coil seems ok, but there is little heat transfer. (I would expect a 14degree water temp exiting the coil. Is the air flow ok? How are the filters?

Or as mentioned earlier you could have scale in your pipes which also explain poor heat transfer.

Am i way off the mark? (i am still learning)

Mad Apprentice

[NoNickName]

Hm, is the air coil clean? The water temp into the coil seems ok, but there is little heat transfer. (I would expect a 14degree water temp exiting the coil. Is the air flow ok? How are the filters?


I don't see anything wrong. The measured pd is consistent with the calculated pd.

[MadApprentice]

i dunno i would be giving the coil a good dose of coil cleaner and a pressure spray, and i would try and see whats involved in cleaning scale from internal pipe.

Perhaps that is the problem, has your water dosing been kept up to date, perhaps it is scale, i still maintain that you need 7k difference in water temp and to have less would mean poor heat transfer?

The Mad Apprentice

ps i am looking from a simpletons point of view, for all i know the coil is the wrong size or somthing.

[Abby Normal]

From the water side data, you are extracting 12.5KW. When we estimate the coil capacity (1000mmx900mmx6R deep) it comes around 55KW. It does't matches.

Secondly, by the the coil size, we estimate the the air flow to be in the range from 2000 to 2400 lps depends upon the application.

Thirdly, your data for air pressure drop accross the coil is also very high.

You also did not provided sufficient data like, application, desirable temperature & RH conditions, WBT of air temperatures, NB of the tubes, fin type and their spacing etc.

My best suggestion is to call call some expert to sattle right your plant.

Well I think nonickname was trying to run a coil selection that approximated what the original poster had reported.

The face velocity of the coil is low, as if trying to enhance dehumidification, but there is none happening, the entering air is cool to begin with and just based on dewpoints, the entering air would have to be up at 60% just so some dehumidification would be physically possible.

nonick's air side pressure drop as well as the one given by the original poster is pretty low for a 6 row coil, even for a low face velocity.

That coil could typically handle more air as well as warmer more humid air and the capacity could very well be 3.5 times what is being measured.

[frank]

i still maintain that you need 7k difference in water temp and to have less would mean poor heat transfer?

Not all chilled water/air coils are designed for 7K dt. The design is to suit whatever the application requires not just to achieve 7K across the coil

[S.K.VARDE]

Nonicks,

try to design or select a cooling coil for 55KW. you will get the answer. Consider the other aspect of my original post.

pl send me the banana.

[NoNickName]

try to design or select a cooling coil for 55KW. you will get the answer.


Everyday duty.

[Abby Normal]

Nonicks,

try to design or select a cooling coil for 55KW. you will get the answer. Consider the other aspect of my original post.

pl send me the banana.

To extract 55kW from the original poster's airflow and entering air condition would mean double checking the freeze protection of the fluid, lowering the entering water temperature and adding a defrost system :)

Maybe a nice cold banana split with ice cream :)

[Lc_shi]

How about the surface area of the air cooler? Estimate the coil capacity on Q=KF(LMTD); K usually 30~50W/m2.K. 55kw may be within the coil capacity range,surely it depends on the real situation of entering air and water entering status. It seems that water flow is higher based on normal dt is 5K assumption.
it's a meaningful discussion. Water cooling coil looks simple,if you want to go further,knowledge about psychrometric is necessary.

regards
LC

[hvacn00b]

Hi everyone,

Sorry I haven't been on the forums for a while. Yesterday we went to site and check on this coil. And yes it was piped for parallel as expected! So, if we change the pipe connection to counterflow - what can we expect (%-wise) in terms of increase in capacity?

Anyway, I've also found out that this coil isn't used for air conditioning purposes. It takes in unconditioned air from the factory and blows this air into plastic extrusion machine. The requirement for the supply air is 13-15 degC. It is currently OK at the moment, but on hot summer days the temperature inside the factory might go up to 40 degC I don't think this coil would do the job....

This coil is not brand new. It was moved from another factory and it used to run on glycol at -6 degC.

Yesterday we also had a representative from a coil company who recommend to install an additional 6-row coil... have yet to receive his proposal.

[Abby Normal]

Based on the original conditions you posted, I think re-piping as counter flow would improve performance by 16% as a maximum. Realistically maybe 10, not much of a load on that coil, 16C is cool.

That 40C air could also be humid. You may want to increase face area for a velocity down towards 300 FPM (1.5 m/s ?) and go with something 8 rows.

Get the humidity coinciding with the 40C dry bulb. If it turns out to be say 65% sensible load your water flow rate is now in line. You should be able to get away with a 5C water temp. Talk to the coil sales engineer, but he will need to know the entering air condition that the coil is facing, as well as the water temperature available.

You probably already have a modulating type of diverting valve hooked up to a discharge air controller.

Water flow rate through the coil is reduced under lower loads.

[Abby Normal]

Actually I played with a selection program, 6 rows say 8 fins per inch could handle 40C & 30% RH, depends on your circuitting to some extent