Heat exchanger help needed.

[dtwo]

I have a plate type heat exchanger with the following:

Hot in: 88.5 degF
Hot out: 84

Cold in: 46
Cold out: 62

I believe the flows are close to the original spec of the hex. However the piping on the hot side is backwards from the hex's documentation so the hot and cold side flow in the same direction. The hex is a parallel flow type. My question is all things remaining the same but changing the hot side flow direction can I expect the hot out to decrease? And by how much?

 

[speco]

dtwo,

The answer to your problem relates to the effective LMTD (log-mean temperature difference) between the two streams.

In counterflow (the way it's supposed to be piped) the terminal temperatures look like this:

hot side 88.5 --> 84
cold side 62.0 <-- 46
==== ===
diff 26.5 38 LMTD = 31.9

In co-current flow, as it is piped, the terminal temperatures look like this:

hot side 88.5 --> 84
cold side 46 --> 62
==== ====
diff 42.5 22 LMTD = 31.1

That is, with no change in temperatures, the LMTD drops by about 2.5% in this case. The difference would be MUCH more pronounced if the temperatures were closer, and impossible if they were supposed to cross.

In your case, this 2.5% about how much less heat will be transferred, since it's such a small amount. As the LMTD goes down, the leat load is reduced and the outlet temperatures also change, resulting in a new higher LMTD. So, it's a reiterative problem to find a combination of heat load and LMTD that match.

Hope that helps.

Regards,

Speco (

http://www.stoneprocess.com)

 

[dtwo]

To further the discussion on heat transfer, if the target was 75 degF on the hot outgoing side what could I do if anything to achieve this? Replacing the heat exchanger is not an option due to costs.

 

[vpl]

Is it possible to reroute the piping so you get counterflow operation (as per original design) or are you also stuck with leaving it at parallel flow?

Is it possible to slightly expand the heat exchanger by adding extra plates?

You could try decreasing your flow rates so that your cold and hot fluids stay in contact longer, but, without running any calculations to prove it, I doubt that it would allow you to lower your outgoing hot fluid temperature by that much.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[dtwo]

Patricia,

Repiping is an option for counter flow but at a price of $3K. Expanding with additional plates is certainly an option but I would like to know I can get the 75 degree target before we spend the money.

 

[speco]

dtwo,

So, are you hoping to get three times the heat load out of the system, or will the equilibrium in the system "float" such the the hot side temperatures will be 79.5 to 75?

If you are trying to cool from 88.5 to 75, then you will need a much larger exchanger. You may find it less expensive to buy a complete new pack for your existing echanger. Plates come in different "hardness" configurations, meaning that some have higher pressure drop and heat transfer coefficients in the same size, because the chevron pattern in the plates is more transverse to the water flow.

I suggest that you talk to your exchanger vendor and get quotes for plates, a plate pack, or maybe event a new exchanger of the same size, but with more and "harder" plates.

Some manufacturers charge so much for replacement plates that it's less expensive to buy a complete new exchanger.

Regards,

speco (

http://www.stoneprocess.com)

 

[eaboujaoudeh]

if the calculations allow u to speed up the flow in the hot tubes, and slowing down the cool liquid flow maintaining turbulent flow.
but i don't think its efficient to bring the two temperatures so close to each other.

Elie Abou Jaoudeh

 

[davidgregorybaker]

Please be advised that if this cooler is a sea water application, with titanium plates, the delivery time on new plates can be as long as 100 weeks.