need help for heat exchanger designe

[junumon]

Dear Friends,
I need to design a heat exchanger to cool hydraulic oil used in a plate bending machine.If the temperature goes above 60 C the machine will stop.Right now (in summer) the temperature goes up to 65 C and the machine stops freequently. I need to cool down the oil to 40-45 C. I have a pump with discharge 185 litre/ minute. The volume of the hydraulic oil tank is 4000 liters.
I have water coming from a cooling tower at around 35 C. Can anybody help me to designe this?
Though I am working in a heat exchanger manufacturinng company, I dont have any experience in design. Can any body send me a sample excel sheet calculation?
The same case explained above, if I am passing this through copper tubes with fins (the type of coils used in air conditioners) and use a fan to cool down, what will be the effect? Which will be more effective?
Please advice.....

 

[TD2K]

If you work in a heat exchanger manufacturing company, there will be someone there who has experience in this, why don't you talk to them? Your technical group has to handle questions like this from customers.

You are going to need to work out/estimate how much heat/energy is going added to the fluid by the machine (it's not 185 L/min being cooled from 60 or 65C down to 40/45C). You can likely estimate it by taking the volume of the hydraulic fluid and how long it takes to increase from 40C (I'm assuming at the start of the shift) up to 60C (when you start having problems). This is a rough estimate so add a design margin to this. This will give you the duty you need. The duty of the fluid plus the flow rate with the heat capacity of the fluid will give you the temperature rise between the hot inlet fluid (leaving the machine) and the cooled fluid leaving your heat exchanger and I bet it's not much.

Your cooling water at 35C is on the high side. I'd look at something like a small spiral wound heat exchanger similar to what is used for some lube oil coolers. If you pick an off the shelf unit and it's too large, you'll just cool the hydraulic fluid down to a lower temperature with your cooling water which likely, since you have 35C cooling water, likely isn't a problem. Is there any time when the cooling water is a lot cooler, say during winter, and would this cooler water give you any problems?

 

[mirchee]

The best answer is to select an exchanger type and look up the proper step-by-step computational method in:

(1) The old classic by Kern, "Process Heat Transfer" (McGraw-Hill, 1950), or
(2) The new classic by Hewitt, Shires and Bott, "Process Heat Transfer" (CRC Press, 1994). This book encapsulates most of the major developments in heat exchanger design and is written by three of the major authorities in the field. It is generally considered by most to be a worthy successor to Kern's book.

If you are dealing with a compact heat exchanger design, see:
(3) Kays and London, "Compact Heat Exchangers", 2nd Edition (McGraw-Hill) for the latest correlations in pressure drop and heat transfer coefficients. I am not sure but there may be a 3rd edition by now. Note their emphasis on the effectiveness-NTU method for heat transfer calculations that eliminates much trial and error. Such charts are included for many complex geometries in their book.

 

[Razzaghi]

Dear friends:
I want to know the effect of fin thickness in compact heat exchanger on heat transfer.
As you know "fin efficiency=tanh(ml)/(ml)" where m=(2h/k*t)^1/2".
So would you please tell me by reducing fin thicknees about 5 microns in compact heat exchanger,how much heat transfer rate is reduced?
best regards