Plate heat exchanger flow vs. system flow

Mosherd1

I’ve got the dreaded “last man who touched it, now gets to care for it job.” HVAC work is not my specialty, but I do have a degree in it, and do dabble with it. I’m an electrician by trade, and started taking care of this system a few years ago because the local Plumber/hvac co didn’t understand how to trouble shoot powerpile millivolt systems.  The system worked well when it was installed, but I also know the original contractor is a hack. It’s a mechanics shop, 100 x 60 with 6 large overhead doors as zone 1 and zone 2 is a 24 x 24 office. The original “boiler” is/was a millivolt controlled pool heater with 200,000 Btu output. The shop has 12 loops of 1/2 AQUAPEX and the office has 5 loops of 1/2 AQUAPEX. It’s got cast iron circulators. The manifolds are just 1”  copper with 1/2” stubs that the pipes hook on to, there is no flow control, no gauges, very bare bones. I have no idea what the water temp leaving the boiler was as the knob on the heater is just a warmer/cooler dial. The pool heater is now shot, it leaks water like a sieve. So I emailed Badger Pipe out of Wisconsin to help size a flat plate heat exchanger. They suggested using an 80 plate, using 120*-140* input temp to get 110* output temperature 
at 200,000 btus. Its pressure drop is .36 and flow rate is 28 gpm. Does that mean I need to move 28 gpm on the output side? That would mean each of my 1/2” loops would have to move 1.6 gpm and assuming the loop lengths are 250 feet that puts the head pressure way up to 14 if I remember correctly. Typically I would figure about .5 gpm per loop.  I’ve never had yo use a brazed plate heat exchanger before. 

Mosherd1

The current circulators are both Grundfos up15-42‘a and there is no heat exchanger. The loops are just fed off the pool heater  input/output pipes. 

hot_rod

What boiler will feed the plate HX? Another new pool heater. If so why a plate HX?

A pool boiler is not a bad way to go with non barrier tube. And they have return protection built into them. The issue is they are not an ASME “H” stamped vessel. So not officially legal.

Ideally you would want to know the building heatload to size the HX. YES .50- .75 gpm max on those 1/2” loops

17 loops? X .65= 11.5 gpm. So the HX would be sized by boiler input and required gpm & temperature delta. On the b side, system requirement in temperature and 11,5 gpm.

80 plates in what dimension? 5x12- 80?

Mosherd1

@hot_rod the new boiler will either be a triangle tube or Lochenvar mod/con.  Below is the specs on the heat exchanger. They have 28 GPM listed. How does that pertain to my design? The boiler output will be around 200,000.

Mosherd1

@hot_rod also I’d like the “B” side output temp to be 110* while being able to keep the boiler in condensing mode all season if possible 120* (ideal) but would go up to 140* supply temp if required.  Is it possible to oversize a flat plate heat exchanger or is the only down side economic?

hot_rod

I suppose you could back into an answer by determining a realistic amount of heat those loops could transfer. 17 loops. At .75 gpm would be 12 gpm. So the b side is 12 gpm, 20 delta on a shop is fine.

The load might be 150- 170k?

Most all the HX manufacturers have online sizers if you wanted an exact answer. Although that seems like a bargain price for that size HX. No harm in over sizing to keep the approach temperature low and a mild con condensing.