Interesting Water coil application

[Garz]

Here is my heat transfer challenge

The air side fixed parameters are as follows
Airflow is 520CFM ( 0.245m3/h )
Air temp 150 deg C
Density 1.27 Kg/m3
Specific heat capacity 1012 j/kg/k (25 c - but don’t think it changes very much with temp)
Mass flow rate 0.3115 Kg/s
Face area 0.0100m2
The system is tolerant to air side pressure drop and can accommodate pressures as high as 2ft water / 7kilo Pascal’s

The waterside fixed parameters are as follows
Water flow rate 1,500.0 dm3/h
Water in temperature 40 deg C
Flow rate 0.4167 dm3/s
Pressure 0.2 bar
Density 30% glycol@40 C 1.040 Kg /dm3
Spec heat cap 30% glycol @40C 3,805j/kg/k (25C)
Mass flow rate 0.433 kg/s
The water pump can maintain 1500dm3/h at a pressure of 0.2bar and 1250dm3/h at 0.3bar

Heat exchanger

Face dimensions 87mm by 115mm ( there is room for turns/end tanks on top the 115mm dimension)
Depth (in direction of air flow) 60mm max
Inlet / outlet should be on the 60x87mm face
Pattern/type rows/fins etc within that envelope are entirely free – e.g could also be plate & fin instead of tube & fin
(I know it’s far too small to be ideal - but the mission is just to optimise - and we do have a large delta T to work with)

Goal
Objective is to lower the temperature of the air by at least 50 deg C - more cooling is better.
I calculate heat transferred will be in the region of 15KW
Wish to be able to evaluate tube diameter/ fin pitch/pattern etc to optimise for heat transfer - the coil manuf calculators are no doing it - so just to get the right equations to start with would be great

I know the air on temperature is high – but water flow rate is sufficiently high as to keep the water in the system well clear of boiling
In fact I think it will only have a temperature rise of around 10 deg C

I have tried various sizing programs - but am either outside limits they can deal with, or they need me to specify air in and air out temperatures - which fixes the heat transfered - where-as i want to determine what coil design ( that fits in the space) gives the highest heat transfer.

If any one knows of a sizing program that can deal with this please let me know

any help appreciated

many thanks

[desA]

http://www.technisolve.co.za

Good software, at an excellent price.

Software developer knows his game. Nice guy to boot.

[Garz]

thanks for your suggestion des A - i downloaded the chilled water coil calculator - and tried using it.
unfortunately i think my inputs are outside the limits it can deal with.
i thought the high air on temperature would be the main culprit - so i changed this to 45 and the water in temp to 0 ( similar temp difference to see if that would allow it to solve )
still would not solve - i think the face velocity is also too high for it - will try to post a screen shot
but if i change thsi i will not be able to relate the results to my application much - if at all.

any further suggestions - for instance - does anyone have the method i can use to solve this from first principals ?

here is my coil


here is the coolit CWC program screen


( picture posts dont seem to be working )
will try to attach

[desA]

E-mail Bruce with your questions. He'll get you sorted out in no time flat.

[desA]

Ok, I've taken a whirl through your example.

The kind of program you need would be better found in the chemical-processing industry. I'm not really sure what to suggest here, though, as most of these will have circular fins on tube. You may try Chemstations, but that is pretty pricey.

Your face velocity will probably have really excessive pressure drop requirements. Not quite sure how you intend to drive the air through the coil at that level of dP. Is it really necessary?

You also don't mention the inlet air RH%. This is important. If you're hoping to blow wet air through at such a high face velocity - then watch out behind...

[desA]

An alternative - it's easy:

1. Obtain the equations for the air-side heat-transfer coefficient;
2. Obtain the equations for the tube-side heat-transfer coefficient;
3. Write a simple spreadsheet to solve the problem.

In your case, simulate the unit as a single long tube, with length double your current coil face width. Coil fin height half the present face height.

Makes for simpler calculation - the old cut & open trick. Gives same heat-transfer, may need to allow additional dP on tubeside for the return elbow.

I think that your fins will get into trouble, unless they are steel. Even copper, or brass will take a pounding - will wear a little early - especially at 25 m/s face velocity.

I think that you're designing in an uneconomic area.

[desA]

Your tubeside velocity is too low. Aim for closer to 1.5m/s. Downsize the tube diameter.

[Garz]

thanks for the extra input DesA - some helpful tips there -

to address some of your points -
1, i cant do much about the face velocity - the flow is fixed - as is the space constraing the coil.
yep, the design is one optimised for the space envelope - at the expense of efficiency - tahts agreed - but thats the mission.

2, the value for water/tube velocity in the snapshot are wrong - could'nt get it as i needed it. With the current pump it will be 1.5m/s with a single 5/8" tube

3, moisture off the coil is no problem - may need to coat the coil for corrosion ressistance though

its clear to me that the coil is lacking surface area for the job - so wanted to maximise this - but dont know what the best tube configuration / fin configuration would be to deal with the high air velocities - and know as i increase surface area - i shoudl expect airside pressure drop to go up also - creating teh need to compromise
that was what i was trying to zero in on.
i am aware that once i have more than 1 row of tubes i cannot model it as a single long tube as air off one will hit the next and temps etc will change.

however, i am familiar with running equations in excel
anyone point me in the right direction for heat transfer equations to suit - or even better - and example for a water chiller coil !!

many thansk for the input once again !!

[desA]

however, i am familiar with running equations in excel
anyone point me in the right direction for heat transfer equations to suit - or even better - and example for a water chiller coil !!

A water chiller won't be of use to you, I'd imagine, as the face velocity will probably be constrained as per CWC. He does it for a reason - to prevent a user putting in an input that will push the design out of bounds...

An alternative may be for you to purchase a copy of PLATE from Prof. Ralph Webb. This may be able to manage the face velocities (not sure, but will check).

Warnings:
1. I believe that your face velocity may very well result in excessive pressure drop, leading to further problems in your system.
2. Coating fins won't help in preventing fin erosion at high face velocities & temperature. You need to change the fin material.
3. 25.4fpi fin spacing is unwise. For this application, around 1.5mm fin spacing should be your limit.

This is, in all likelihood, why your suppliers are refusing to co-operate with you on this project. To be candid, I'd say that you may have better success on another forum, more geared towards chemical engineering.

Best of success.

[mad fridgie]

I see a number of issues using a fin coil. One being thermal length.
I would suggest that you look at a shell and tube/ tube in tube (a long one) Air down the tube side (some what easier to calculate presure drop) I would have the water baffled into the shell side (counter flow of course) keeping the intermnal shell veloicty below 2.2M/s and above 2M/s.

[Gary]

I have tried various sizing programs - but am either outside limits they can deal with, or they need me to specify air in and air out temperatures - which fixes the heat transfered - where-as i want to determine what coil design ( that fits in the space) gives the highest heat transfer

If you specify air in and air out temps, isn't the one that gives you the highest water temp rise the one that has the best heat transfer?... or maybe the least GPH?... or the least CFM?... or whichever is the non-fixed variable?

[Garz]

DesA - i understand - but just to clarify - no one is refusing to work with me - its just that the standard off the shelf programs cannot deal with the inputs - for the reasons discussed, and i cant really ask then to write a special just for my one-off feasibility calculations. I appreciate your input on fin material and pitch.

Gary - thanks for that suggestion - makes logical sense - will try it out

[mad fridgie]

What is the air used for?

[desA]

DesA - i understand - but just to clarify - no one is refusing to work with me - its just that the standard off the shelf programs cannot deal with the inputs - for the reasons discussed, and i cant really ask then to write a special just for my one-off feasibility calculations. I appreciate your input on fin material and pitch.

Apologies if I misunderstood your first post...

Why don't you mock up a sample similar to what you have drawn & just run it. You'll soon see what its capability is. At least, then, you will have a clear idea of what the airside is doing to the rest of your system.

I wonder what kind of noise you're going to get off those fins, I must say...

[mad fridgie]

At those temps why do you not directly inject the water into the air stream (you will not need much water)
Soon knock 50C out

[desA]

Excellent idea...