Heat Exchanger sizing through temperature swings in storage temperatures.

[Sawyer]

Recognizing that my usable water storage temperature will vary through a range of 200*-110*, what is the best way to choose a water to water heat exchanger? I have tried the calculator at http://flatplateselect.com/site/pub/login.aspx?ReturnUrl=/site/hx/chooseapp.aspx . It seems I can vary the HX selection by the input temperature, output temperature on both A & B sides, as well as the flow rates. I am assuming a flow from the boiler through 1/1/4” at 10 gpm, also 10 gpm on the B side. I have a water to air HX in the plenum that will only service down to 140* and radiant in the slab.

When I run calculations I end up with smaller heat exchangers with ¾” inlet/outlet, to the 5x2, 50 plate, 1-1/4” inlet/outlet. Is there a general rule that will get my results more specific?

TIA, George

 

[WoodNotOil]

In my, albeit short, experience with my flat plate hx, it seems to handle transferring from the 180* temp I sized it at on down no problem. I think being able to transfer all the btus at your highest temp is the more important sizing factor and it should transfer anything below that without a problem. I am no pro, so hopefully someone with some expertise can chime in as well.

I have 1 1/4" piping in my system, but put in plate with 3/4" inlet/outlet. Mine will transfer 140,000 btus at 180* at 15gpm with a delta T of 20*. I got this plate (broken link removed) and the guy I talked to there helped me size the plate and pumps. I am using the Grundfos 15-58 (spd 3) variable speed pumps set to high to get the 15gpm. I hope that is helpful information. He calculated it for me.

 

[BrownianHeatingTech]

Heat exchangers work best when there is a large difference in temperature between the two fluids.

When the tank is cold and you are charging it with hot boiler, you'll have no trouble. The most difficult condition for the heat exchanger is when the tank is approaching the same temp as the boiler. Those are the conditions that you need to size for...

Joe

 

[Sawyer]

As I understand it my problem is getting the diminishing storage temps from the Garn to transfer heat to the existing boiler loops (radiant in floor and coil/air HX). The coil/air HX requiring higher temps than the radiant (140* min). The Garn storage will supply heat from 200-120* usable for the radiant and 200-140* usable for the coil/air HX.

I ran the calculator. At side A 140* in and 120* out 10gpm, and side B 100* in and 120* out 10gpm a FG5x12 40 plate, 1 ¼” MPT, load 98,632btu/hr was recommended.

At side A 180* in and 160* out 10gpm, and side B 100* in and 120* out 10gpm, load 97,655btu/hr only a FP5X12-10 plate 1inch MPT was recommended.

I think I now understand what Joe is referring to. Looking at the above calculations I am assuming I am best to size for the narrower, lower temperature differences rather than the high temperature differences which means going to the larger 1 ¼” flow capacity and thirty more plates. I could use a smaller HX if I chose to push the gpm to 15gpm but would rather keep the pump and velocity smaller as I am running a 175’ underground supply.

If I am misunderstanding please correct me, thanks, George.

 

[heaterman]

Basically, the closer you want side B (load) to approach side A (heat supply), the larger the HX has to be. Getting that last 10* into the system (going up from 160* to 170* on the load side) takes a substantially larger HX. Using a larger HX will allow more variation in the supply water temp to a point.