water cooling coil

[Lc_shi]

Hi sirs
Need to build a water cooling coil for fresh air dehumidification ( from 34C/ 21.4g/kg dryair to 13C/8.38g/kgdryair). I'm going to build a 8 rows deep which seems not enough to deal with the humidity demand. I'm not sure if we can add the rows to more than 8 rows. I've not seen the practical application more than 8 rows water cooling coil. Who knows more about it, pls teach me thanks

regards
LVC

[US Iceman]

LC,

Industrial refrigeration coils using ammonia are normally built with up to 10 rows deep. My feeling is the additional 2 rows (going from 8 to 10 rows) does not add very much capacity as a percentage.

As the coil becomes deeper (more rows) the additional rows do not provide the same proportion of cooling.

At the same time, the additional 2 rows also add static pressure to the fans. The combination of the 2 extra rows and the additional static pressure would make the last 2 rows "less effective".

[Victorman]

Hola Li :
That is a very interesting project you have there
Are you making a coil or specifying one ?
Do you know the quantity of air through the coil?
What about the physical size ? perhaps 2feetx2feet ?
There are several factors that determine the number of rows in a coil , basic capacity , thermal resistances , number of fins per inch,.....etc..etc...
If you gives us more information , we might be able to help you
Regards
Victorman

[Vincent Yu]

LC,

Maybe you can try some other ways to increase the coil capacity. What kind of tube and fin do you use? Recently, I try to use enhanced tube in evaporator. In general, the coil performance increases at least 10%. The down side is enhanced tube generates more restriction in refrigeration side. Raised lance fin is also more efficient than flat fin. But it generates more restriction in air side.

LC, when you design dehumidifiers, how do you control both the temperature and the relative humidity of outlet air? I am interested in it.

Thanks
Vincent

[Lc_shi]

Hi Victorman
I'm glad to know experienced guy like you. The air flow volume is 500 m3/h (i'm not familair with US unit). The fin tube HX is :
# tube outer dia 9.52mm,thinkness 0.35mm
# AL fin thickness 0.15mm
# against the air flow : 12tubes ,tube distance 25.4mm
#along the air flow : 8rows ,row distance 22mm
# face velocity 1.2m/s

water in/out 7C / 11C volume flow is 1.56m3/h
Pls give your comment and corrections.thx!

Hi Vincent
How are u? It's long time not to see u here
The dehumidifier we design is not for precision environment control ,so we only need the temp and humidity is within the required range. If you need to precisely control both the temp and humidity,you should control the cooling coil ( chilled water or DX coil)capacity and the reheating ( electricity or recovery of condensing heat) capacity which means both the capacity are adjustable precisely. Only for your ref.
Have a good time!

best regards
LC

[S.M.Gokhale]

Hir Lc,
I have successfully designed and commissioned a few fresh air dehumidification systems. Firstly you need to carefully estimate the heat load. Plot your incoming and leaving air conditions on a psychrometric chart and find the enthalpy difference. Convert your air flow from Cu. Mtr./ Hr. to Kgs./ Hr. and multiply with the enathalpy difference to obtain the heat load. Now you need to design the cooling coil to meet this load. The heat load is also important as you need to know how much chilled water will be required to cool the air. In one case where I had to cool air from 34 Deg. c to 2 Deg. C (dew point) the load was about 13 T.R. for 1700 Cu. Mtr./hr. air flow. In this case I used a 10 rows deep chilled water coil and an 8 rows deep DX cooling coil. Also keep in mind that as the condition of the incoming air changes, the condition of leaving air also changes. In the above stated case I had used a hot gas bypass to artifically load the compressor to prevent ice build-up on the coil when incoming air condition was very low. (The compressor can't be switched off)

Also your air velocity across the face of coil appears to be very low. This can adversly affect heat transfer. If you can keep this at about 2.3 to 2.5 m/sec the heat transfer will be better.

We have many a times used 12 or 14 rows deep chilled water coil when the dehumidification load was high.

In case you need any further help you can PM me.

S.M.Gokhale

[Vincent Yu]

Hello Gokhale,

When I design dehumidifiers, I always have trouble to determine the relative humidity of the air right after the coil. To make my life easier, usually I just regard it to be saturated. Actually, since the Apparatus Dew Point is lower than leaving air temperature, the air relative humidity should be less than 100%. Do you have any simple ways to tell the relative humidity of the leaving air accurately?

Thanks

[S.M.Gokhale]

Vincent,

Sorry for the delay in answering as I was out of town.

Unfortunately I have not come across any simple method to tell the conditions of the air leaving cooling coil. Generally it is found that 8 to 10 rows deep cooling coils would provide a 95% to 98% saturation. The performance of a cooling coil depends on many factors such as air velocity across the face of coil, water velocity through the tubes, no. of fins per inch and of course the condition of air entering the cooling coil and temperature of the chilled water / refrigerant.

In case you have a good coil design calculation (prefreably manual calculation), you can try and design the cooling coils for different leaving air conditions for any given incoming air condition.

I have an old catalog from 'Marlo' coils of US which is very comprehensive in terms of design data and also has
design examples ( this was printed in the 1960s )

Perhaps a simple and practical way would be to actually measure the conditions (DB and WB) after cooling coils of different depth.

S.M.Gokhale

[autt]

LC,

Note the message I send you.
If dehumidity is so important, 10 rows should be acceptable, Problem is that you have to use a fan with more HP.

regards,
autt

[Abby Normal]

If done a few 100% outside air units in my day with a North American OEM, and have recently had a few built for tropical ambient air with dewpoints 26 to 27C.

What is the condition of the entering air and how much moisture do you want to pull out of it.

Do you need the air reheated to room temperature?

Ediited in: Stupid me just read the fine print, you state entering and leaving conditions, will take a crack at it

[Abby Normal]

A quick look on a chart makes it look like this would be an impossible process, meaning there is no ADP. Maybe if you over cooled the air to pretty much saturated at the leaving dewpoint you need, a draw through fan and motor could heat that air up to 13C.

[Abby Normal]

If there is not enough reheat from the fan and motor you could look at wrap around heat pipes.

I face a similar type of ambient air, usually cool it to a dewpoint of 10 or 11C, then reheat as needed. DX always has the option of free hot gas.