1 reply to this topic

[drdave]

Hi every one,

In air water cooler, Air outlet temperature is not being controlled in specified range.Though it is known fact that gas side film coefficient is controlling in such a system,It is psychological attempt to increase CW flow to establish normalcy.My query is :
How can one get gas side improvement to perform given cooler normal as specified?
Conditions present for given case :
[a] CW flow is 20% higher than specified.Flow is reduced severaltimes,but air temperature does not get affected anyway.
[b] CW temperature range is 7 deg_C against specified 9 deg_C.
[c] Outlet header butterfly valve is open by 65%.

Thanks in advance

[Art Montemayor]

DRDave:

First, let’s establish some essential basic data:

1) Is this cooler a shell & tube type – similar to an air compressor intercooler? Is the air (presumably compressed) in the tube side and the cooling water supply (CWS) on the shell side?

2) Or is this a unit where the air is on the shell side (as in a duct-type application)?

The reason I ask for these basic data is that, as you correctly cite, the gas film heat transfer coefficient is notoriously very low as compared to the water film coefficient and, as a result, sets (or controls) the amount of heat transfer area required for the design heat load. The conventional manner of combating the bad, low gas film coefficient is to add heat transfer area to the gas side in the form of fins to the tubes in question. This is easily and often done where the gas is on the shell side; when the gas is in the tubeside, it becomes a mechanical problem of putting in the internal fins. It can (and has been) be done – but with special tubes and equipment. The resultant internal fins are not easy to clean out, as can be expected.

Your choice of wording in (a) leaves a lot to be desired. I can’t understand what you mean after you state the CWS is 20% more than what is specified for the service. Can you further explain what follows? I believe you are trying to say that the outlet air temperature does not reduce noticeable even when you reduce the inlet air flow by several times (what factor is this?). Not knowing what the TEMA type of exchanger you are using and its configuration (vertical, horizontal) leaves us not being able to say much here.

You state the CW range (the difference between the outlet and inlet water temperatures) is only 7 oC as compared with the 9 oC specified by the fabricator. This is one strong indication that you either have an undersized (wrongly designed) cooler or that the existing heat transfer surface is fouled. You don’t tell us if the cooler ever did work successfully in the past and if this failure to cool sufficiently is a recent finding. This is important to know. If the unit originally worked according to specifications, then the existing condition is due to fouling – either on the shell or tube side. If the air is relatively clean, I would suspect the water side as fouled.

The fact that you are throttling the CW outlet by 65% open is not relevant to the problem, since you already have stated that you are supplying 20% more CW than is specified as required.

What is one important point to mention, together with the TEMA information mentioned above is the temperature approach of the outlet air to the CWS – assuming the unit is in counter-flow. Additionally, from a critical design point, is a temperature cross involved here? All this information can’t be addressed since you don’t furnish the specific stream temperatures. Recapping, I feel you can’t simply assume that you need additional heat transfer surface area (in the form of fins or otherwise) from the data you’ve supplied. Your problem may be as simple as water-side fouling. But one can’t tell from the sparse basic data furnished.

I hope these comments help to resolve your concerns and problem.