FLAT PLATE EXCHANGER

Hi all, I need to purchase a flat plate heat exchanger to heat my shop. The shop is 150' from the house and the plate will be in the house by the boiler. The shop has a heat load of less than 25,00 btu, and the primary load will be the infloor tubing. I also installed a 40,000 btu high temp unit heater. I understand it will be easy to supply the infloor , but how big of a plate do I need to transfer enough heat to supply the high temp unit. The shop side of the loop will be glycol filled and all underground piping and interior piping in 1-1/4"? So all of that said how big of a plate do I need and where do I buy it from? Preferably 1-1/4 inch ports?

Just a thought but what about having a small buffer tank in the shop?

You could draw off the middle for the radiant and have the hottest water at the top for the unit heater.

Anyone?

gg

jimdec, you wouldn't need a very large HX. Look up on supplied tables for sizing these exchangers. I found most flat plate HX manufacturers have a very high price on their exchangers because they tend to be way overbuilt for the purposes we use them for. I bought a 50 plate 5" x 12" unit with 1/1/4" MPT connectors for my system and the "book" says that it can exchange about 400,000 btu/h if you put the right amount and temp of fluid through one side and pump enough fluid through the other, so it really becomes a little more complex calculation than just "what size HX?". I'm pumping up to 25 gpm through the source side and 25 gpm through the supply side on this 50 plate and getting a good exchange. Bought it "cheap" through OFS, outdoor furnace supply, for about $280. Sounds like you wouldn't need anything this big (small). Problem I had was that the unit is so small that 1/1/4" black iron created some very small clearances in the plumbing around the HX.

The unit htr rated at 40,000 btuh is based on supply temp of ***F and flow rate of ***gpm. If that is the output you want, then you need that info. Next you need to look at the selection tables to find a plate hx which will provide 40,000 btuh output at side B output temp = unit htr supply temp and flow rate of ***gpm on unit htr side of hx. The selection table also then will give you side A supply temp and required flow rate (boiler side of hx). The selection table also will give you pressure drop (psi) on side A and B at the specified flow rate. Pressure drop x 2.34 = feet of head. Next calculate feet of head at the specified flow rates (sides A and B) in the hx and the piping, and then spec the circulator you need to provide that gpm at the calculated head.

Keep in mind that btuh = deltaT x gpm x 500. So if deltaT = 20, you only need a flow of 4 gpm to provide 40,000 btuh (deltaT = 10, flow needs to be 8 gpm).

Note that typically boiler supply temp (side A) needs to be about 10F higher than unit htr output (side B) from the hx, although you can size the hx to provide a closer approach temp.

For selection software, see
http://www.gea-phe.com

Thanks for the info , I dont want to try and do this if it is not practical seeing that I do have the infloor. Jebatty do you know who would be a person to call to find a exchanger similiar to what you are talkin about.

Oops I just saw your last post.

The main issue you have in your situation is that it appears you need two temperatures out in your shop. Low temp water for the floor and high temp for the "unit heater".
This affects HX sizing much more than the given btu load.
The unit heater will work best at 170-180*, in fact if you check the ratings, I would just about bet that its rated output is delivered at 180 or even 200* water temp. That being the case, you will have to size your heat exchanger so that when your boiler (side A) is at 180 for example, you will be able to get close to that on side B. This simple fact of physics means that you will have to bump the HX size above what is normally recommended as most are quoted with a 20* split temp from side A to B.

That radiant floor in the shop will be your best friend.

Jim,

How warm do you keep the shop? The infloor, if installed correctly and once it's caught up, should easily maintain it at 60-70 degrees, without the overhead heater. Any warmer, and you probably won't want to work in there. At least thats the consensus of the few people I've talked to with infloor heated shops. Also if you used a buffer tank in the shop, you should be able to wring all the useable heat from your water before it flows back to the house, as GG sugested. Just a thought.

I will never be over 60 and as heaterman says I just would like advice on what size plate to accomplish what I want without another buffer tank. 20 ,30 , 40, plate with 1-1/4" ports?

You are in NE WI, I'm in north central MN -- probably similar climate unless you have lake effect impact. Using winter of 2010-11 as an example, since it was a cold winter, this is what I experienced. I have a sensor imbedded in the concrete about 4' in from the exterior perimeter; 2" of foam under the entire floor; 2" of foam around the perimeter and extending vertically down about 3'. Shop has R-19 in the walls, R-40 in the ceiling. Boiler and 1000 gal storage in the shop. Storage is insulated R-30 to 50.

I have the floor set at 61F, 1F differential. At outside temps above about 10F, interior air temp will be about 57-63; at outside temps down to -30F, interior air temp will be about 51-63F. The high 63F will result during a boiler wood burn, the extra heat coming from the boiler and flue in the shop. The infloor radiant makes the shop "feel" warm almost regardless of the air temp. I rough plumbed for adding a unit heater, thinking the same as you, but after two full winters (cold in 10-11 and very mild in 11-12) I never have wished I would have installed it. If I want the shop warmer than indicated, I can set the floor sensor to a higher temp. The shop always has been comfortable to work in, and actually if it was much warmer, it would be too warm for working.

The infloor tubing is glycol, the boiler and storage is plain water. I have a 5000 watt, 240v heater that I use if I am gone for an extended period, but I have found that I can be gone for about 12 days, even with below 0F weather, and the shop stays well above freezing, although in these situations I heat storage up to about 192F before taking off. During extended absences I set the electric heater at 45F. I also have a control on storage that shuts off infloor draw on storage when tank temp falls to 80F, just to guard against any possibility of the tank being subject to freezing.

I totally agree,but if I am going to be working out of town for numerous weeks on end wouldnt it make sense to shut down the shop until I get back. My wife and son would have less draw on the system and get by with one fire per day. When I return home I would then fire up the unit heater to get by for a couple days until the infloor heats up. Like you I would never need it warmer than 61. How big is your PLATE?

Forgot about your long absences - I agree with you. My hx is 5 x 12 x 30 plate, 1" ports. The mixing valve for the infloor is set at 100F. Typical draw for infloor is: supply 100F, return 70F, deltaT = 30, flow is about 2.75 gpm, which equates to 41000 btuh. I have six runs of about 275' x1/2" pex. Winter load at -10F to 25F is in the range of 12,000 to 15,000 btuh. The highest load I've measured was about 17,000 btuh. Building is 32 x 48 x 14, 1500 sq ft. I've never had to burn more than once per day, about 6 hour burn of pine/aspen. Boiler/storage only heats the shop. This last mild winter had many time of one burn every 2nd or 3rd day rather than every day. I usually do a burn when storage is about in the 110-130F range, then bring it up to 185-190F, top to bottom.

I have infloor radiant and love it. I plan to build a unit heater this summer. I will then keep the shop at 50 degrees F
and bump it up to 60 with the unit heater when I'm working out there. Sometimes I don't work out there for a week so I figure no sense in keeping it 60 all the time.