Sizing a Flat Plate Heat Exchanger

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  #1  
Old 03-26-12, 10:09 AM
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Sizing a Flat Plate Heat Exchanger

Hello Everyone!
I need to know if there's anything beyond the obvious when it comes to choosing a flat plate heat exchanger for a hydronic heating installation that includes cast iron rads and infloor piping.

The whole thing is powered by an outdoor wood boiler with a maximum output of 250K btus/hour running a 20% glycol solution. Total maximum calculated heat loss from the building is 80K btus/hr.

Is it simply a matter of choosing a flat plate exchanger to match that 80K btus/hr figure? I want to make the exchanger bigger, but how much bigger is prudent?

Is there any difference in the performance of brazed plated, low pressure,high pressure, or all stainless models that I see out there?

Thanks very much!

Steve
 
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Old 04-04-12, 02:29 PM
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There is lots to work with.
You need to understand the relationship between the water temps on each side of the heat exchanger.
For instance, if you boiler runs at 180 F and your heat emitters are looking for 170 F water temps that will create the need for a much bigger heat exchanger.
You should be looking at flow rate and head loss restrictions, entering and leaving water temps and also remember that gylcol reduces the heat carrying capacity of the fluid.

Lets gets some specs together and we can see what we can size for ya.
 
  #3  
Old 04-05-12, 05:36 AM
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Good Morning TOheating,
Thanks very much for your help. Here are my specifics:

Water coming in to the exchanger comes from an outdoor boiler at about 170F, flowing through 180 feet of 1" ID (actual size) PEX pipe. A Grundfos 2699 is moving this water at about 12 gpm currently.

On the other side of the exchanger, two things are emitting heat:
1. Eleven cast iron rads operating at 165F putting out a total of about 50K btu/hr.
2. Radiant infloor heating network involving 3000 feet of 1/2" Uponor PEX in 11 loops. This installation is under a wooden floor (5/8" ply with 3/4" pine on top), with tubing clipped into Uponor aluminum transfer plates. I expect to have to run this water pretty hot to ram heat through the wood. Let's say 150F as a max.

Since I hadn't heard from anyone on the forum, I went ahead and ordered at Bell & Gossett BP410-20MT. This unit is rated at 350K btu/hr, with a 36 gpm flow rate and 6.3 psi pressure drop on the boiler side, and 35 gpm and 5.6 psi pressure drop on the other side. I figured this exchanger would be plenty big, but how does it look? It hasn't shipped yet, so I could cancel if you don't think it would work.

Thanks so much! I've learned so much from the help I get here.

Bye,

Steve
 
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Old 04-06-12, 07:28 AM
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I'm not an expert by any means, but you bought a heat exchanger 30% larger than the maximum output of your wood boiler. You have wasted your money on the size. I would doubt there is a 50% loss of heat exchange between the two sides, so going with one twice the size of your heat loss at 160k btu would have been way more than enough.
 
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Old 04-06-12, 07:39 AM
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Thanks for your input. Yes, I considered the points you made when I was placing my order, but I suspected that choosing a heat exchanger is about more than just btus. What I don't know is what the temperature loss will be across the exchanger at full flow. The bigger exchanger was a hundred or so dollars than one with a lower btu rating, so it seemed a cheap way to err on the side of caution. Also the exchanger that's coming measures 5x12, with 20 plates. That didn't seem overly large compared with other exchangers I've seen in use in similar situations.
Thanks again.
 
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Old 04-07-12, 08:32 AM
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I expect to have to run this water pretty hot to ram heat through the wood. Let's say 150F as a max.
Ram the heat through? You mean 'cook the flooring' don't you?

I don't think you want to go above 120 or so.
 
  #7  
Old 04-09-12, 11:59 AM
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Trooper is right, you will need to mix that down to 120 or so for the in floor radiant,
As for the heat exchanger, your dealing with a very close approach application. The supply side of circuit a is 170 and your trying to get 165 out of the other side.
This calls for a BIG heat exchanger. a 5x12 is big, but 20 plates may not be enough. Typically plates are sized with 180 incoming on side a and a delta t of 30, and 140 or less with a 20 degree delta t on the side b.
When I fire up my design computer I will run your numbers.
 
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Old 04-09-12, 02:07 PM
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Thanks TOHeating,
I appreciate hearing what your design computer says. I can always up that 170 to 180. 165 is not essential, either. I could drop it to 160.

I only have experience analyzing one other wood floor installation, but there I've needed 150F going into the floor pipes, to create a surface floor temperature of 85F the minimum needed to keep the space warm when it's -20F outside. This floor does not have any heat transfer plates, and the floor I'm currently working on now does. So perhaps I won't need as high an input temperature because of these plates. What do you think?

I'll wait to hear from you on the heat exchanger sizing.

Bye for now, and thank you!

Steve
 
  #9  
Old 04-09-12, 03:06 PM
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Making some assumptions here,
1) 33% propylene glycol in the wood boiler side.
2) Water in the home side
3) 25 BTU/Sqft for the radiant.

Total load is 125,000 btu 165 design water temp.
If you can heat your home with lower water temp, that would be very good.
Primary loop around the "Home" side of the heat exchanger, close spaced tees for CI rads, close spaced tees for In floor (use tekmar 356 variable speed control with ODR).

Using a 5 x 12 20 plate will get.
Primary side flow of 12 GPM will give a delta t of approx. 23 f - 4.2 PSI pressure drop (this will slow your flow down). 175 F entering water temp from boiler
Secondary side, 165 F leaving with a approx. 50 f delta t - .6 PSI pressure drop.

Seems you "guess" was a good one.
Should the radiant need more BTU then you will need to up your incoming water temp. Should you need more that 145 F for the in floor you will need to up your secondary flow rate (currently 5 GPM)

TO
 
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Old 04-09-12, 06:56 PM
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Thanks very much. This is good news.
I really appreciate your help.
Bye,
Steve
 
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