Designing evaporators without fans

Hi there,
Can anybody help me with this question?

If a system is designed for a certain capacity and the evaporator (with fan) is also selected and say it has 100m² of heat transfer area then, if the same application is to be used with an evaporator without fans then what should be the heat transfer area compared to the one with fans? Is there any relations?
What I heard is that the area with fans should be multiplied by 2.5 or 3 which gives the surface of the evaporator without fans. Is this correct?:eek: :confused:
Cheers:)

What I can tell you is that we changed not so laopng ago a static evaporator in a counter and added small fans at a speed of +/- 1 m/sec, so a very slow flow through the fins.
We needed to double the compressor capacity to stay at the same evaporating temperature.

I'm not too surprised that the surface requirements increase drastically. The static evaporators using natural convection have very low heat transfer coefficients. Using forced convection allows the same heat capacity in a much smaller package.

Another important aspect of this is the difference in humidity and product drying. The forced convection units can rapidly dehydrate food product that is not wrapped well.

Air has a very low heat transfer capability, so the only way to improve it is to increase the air velocity. The air coefficient is the controlling factor, so any increase makes a big difference.

Quote:

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What I heard is that the area with fans should be multiplied by 2.5 or 3 which gives the surface of the evaporator without fans. Is this correct?

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2.5 or 3 , this is the first time i hear that , well it's sure that surface will increase dramatically but calculations should be done , and what's right for a single one can't be a general rule with a lot of parameters involved

Regards

You always can look in the catalogs of evaporators manufacturers for a static ceiling evaporator - like they hang sometimes in flower coolers or meat cutting rooms - and compare the given capacity to the fin surface (don't forget to add the tube surface as well)

A static evaporator must have an unrestricted air flow from the upper down to the lower part of the coil.

So if a drip pan has to be mounted underneath the coil, it has to be a special one so that the natural flow isn't disturbed.

The old warehouses used "gravity coils" as the cooling and had no forced convection or flow. I have some old York data on this subject but it packed as we are moving in a week or so. You might try a Gogle search for gravity coils and see what you get.

The coils will be much larger for a given capacity as you only have natural convection to drive the coils.

Ken

Ken