Flat plate heat exchanger

[offthegridandy]

I'm wondering if anyone can offer thoughts on the use and sizing of FPHE.

Our heating system consists of an oil boiler (80,000 BTU), indirect (vented) DHW tank (160 Ltr) wet UFH. The UFH is also or can be heated via a 12Kw WBS.  At present the WBS is connected to a direct (vented) 200 litre buffer tank; this is then connected to the UFH circuit via temp operated valves.

In practice when the primary room stat calls for heat, if the WBS is alight and tank temp is over 50' then the temp operated valves open allowing a flow to the UFH manifold and the UFH pump operates.  If the buffer tank temp is low then the oil burner will operate and is run by its own zone controller.

I am about to wire in a a 750W immersion heater to the buffer tank as a dump load from the WT.

The present set up works ok mostly.  There are 3 pumps in the system, boiler main circ pump, WBS pump plus UFH pump.  There are circumstances when all 3 pumps are running when I get water back into the expansion/header tank via the expansion pipe, not ideal and I'd like to eliminate this problem.  

I am planning to take the output from the WBS/buffer tank and feed it thro a heat exchanger to get the heat indirectly into the UFH system.  This will separate the output from the WBS from the rest of the system and should eliminate the above problem.

We have natural untreated water which comes into the house off the hills so I am about to replace our copper indirect DHW cylinder with a stainless steel one and will also be replacing it's immersion heater with an acid resistant jobby. So as I'm going to be draining the system down now would be a good time to add in the heat exchanger. On the assumption that the max output from the WBS is 12 Kw any ideas what size FPHE to look for ( in stainless steel ideally)

The existing pipe work connection from the buffer tank to the UFH  is 22mm speed fit.

Any thoughts anyone?

Andy

[offthegridandy]

Supplementary question.  The immersion heater in the buffer tank is 750W DC.  The cable run is about 14 mtr.  Any advice on cable size? 6mm?

Cheers.

Andy

[Tinbum]

Supplementary question.  The immersion heater in the buffer tank is 750W DC.  The cable run is about 14 mtr.  Any advice on cable size? 6mm?

Cheers.

Andy

Voltage?

[offthegridandy]

24 volt battery. Divert voltage relay is currently set to switch at 29.4VDC.

[martin W]

I take it that you gravity heat the water tank with the wbs? If so running the pipe work though the heat exchanger may effect the free flow due to some flow restrictions in the heat exchanger...

what about converting the DHW tank to a thermal store with the heat exchanger for your hot water. The negatives are that you would have to run a pump when you wanted hot water demand... simply done with pressure switch. Other than that, all heat sources can supply you heating and hot water system..

OR if all your heat sources already heat the hot water tank, put a pumped feed from the hot water tank through the heat exchanger and connect your UFH to it. Effectively you still have a thermal store (as such). you can remove the motorised valve from your oil burner heating circuit and just heat the hot water cylinder on temperature demand.

I have a stainless steel flat plate heat exchanger connected to my thermal store. It runs my central heating radiators quite happily. (around 6kw in total) I think it is a 3/4" Bsp model with 40 plates... I'd have to check exactly which one. My store is around 60-70 deg C (depending on what's heating it), I use a mixing valve to run the radiators at around 50 deg (mixing valve is on the heating side of the heat exchanger.

I have WBS (2kw jacket), gas system boiler, solar hot water and a PV immersion diverter connected to heat the thermal store, so similar to what you want to do.

hope this makes sense and helps..

[offthegridandy]

Hi Martin.  The WBS is connected to the heat store and is pumped, with pumped being controlled by 2 stats. 1 in the flue and 1 on the boiler return. The store is a 200 ltr direct tank with immersion element fitted awaiting wiring in. From the store I have flow and return which link in to the UFH system adjacent to the manifold.  Both flow and return have motorised valves.   The heat store has a tank stat and when temp  exceeds 50'C the valves open allowing stored heat to feed the UFH. I intend to fit the FPHE between the heat store and the motorised valves.

I was thinking that as the WBS output is 12Kw, that would indicate a heat exchanger rated about 12Kw.  Does that make sense.

I don't plan on altering the sytem at present (apart from FPHE) as it works as designed although further immersion heaters  for the WT, to be added.  But thank you for your ideas.

Cheers.

Andy

[martin W]

Hello Andy,

try these guys for a heat exchanger, its where I buy mine from... they are on eBay as well. I have no connection to the company, just used around 10 of these heat exchanges so far over the last 5-8 years..

http://lednet.info.pl/press/plate-heatp-exchangers-products/



on eBay
https://www.ebay.co.uk/usr/orson_123?_trksid=p2047675.l2559



The heat exchangers go up in size from BA12 to BA60 and then all have a different number of plates..

Generally the bigger BA number means a longer heat exchanger which is also good.

You need to look at the heat exchange surface area when comparing the heat exchangers.

a BA16-22 plate heat exchanger has 0.352m squared surface area and the BA12-30 plate has similar at 0.360m squared.

So if there are similar priced and will fit your space, I'd go for the BA16 as it will flow better. Although the BA23 is more comparable unit to the BA12 as they are both 3/4".


I would think  a BA12-12 would be fine, and at around £50 delivered not too expensive, but for £7 more I'd go for the 20 plate version.

http://www.ebay.co.uk/itm/Stainless-steel-PLATE-HEAT-EXCHANGER-NORDIC-TEC-25-65kW-Ba-12-INSULATION-BOX/253218337036?hash=item3af4fd310c:m:m2HP_EZMaDw4nkEJhoSan-Q

But the BA23-20 for £85 would eat it and then some..

http://www.ebay.co.uk/itm/Stainless-steel-plates-PLATE-HEAT-EXCHANGER-brazed-NORDIC-TEC-3-4-90-210kW/262799805204?hash=item3d3016cb14:m:mTdXSNh0aZwtOepSyRSckIQ


They do sell covers for the heat exchangers, but they are not very close fitting. A shoe box, cling film and a can of spray foam would be cheaper and more effective, although taking the fittings off afterwards would be difficult.

I believe there are many same sized products out there by different suppliers, and you might get it cheaper, or even contact LED direct.. When I needed 3 of the big ones, for a project, it was much cheaper direct.

hope this helps.

Martin

[offthegridandy]

Thank you Martin that's most helpful.  I'll look up the Nordic flea bay offering and let you know how the job progresses.

Andy