Heat bank plate exchanger question

1 Aug 2019
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United Kingdom
I am trying to determine how much 45C water can be generated with a water cylinder and a plate exchanger under the following assumptions
- 300 liter cylinder at 70C
- 210Kw plate exchanger (rated @40C delta?)
- plate exchanger primary flow of 30l/minute
- fresh water infeed temperature of 10C
- simplify by assuming the cylinder is evenly mixed
- is no additional heat added to the cylinder

Anyone know of on online app for that? or how to do the calculation directly?
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Some of the heat exchanger sales sites do have calculators, maybe the more classy makes. Take a look at the sites that sell them on the web, I did see one, but can't remember which model it was.
On some of the specialist hydronic sites they give quite a few equations, too.
Well the cylinder water temp is going to drop from 70 to 45 = 25 C, and the heated water is going to rise from 10 to 45 = 35 C, so if everything were 100% efficient you could make 300 x 25 / 35 = about 215 l.

Are you sure about the 210 kW claim for the heat exchanger? That is a lot.
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I think the “210 KW” in that Amazon listing is nonsense.
The table in the last product image has some more believable numbers, though it’s not clear which of the four rows is this product.
I”m thinking that 210 kW is too high because these are broadly similar sizes to the heat exchangers in combi boilers and they are of the order of 20-30 kW.

But maybe flow rate is the reason for the difference; that one quotes 4 cubic m per hour, which is about 1 l per second, which is much higher than a combi can deliver at a useful temperature.

(I know nothing about this. I just saw your “how much hot water can you get” question and gave a back-of-the-envelope estimate.)
I think they are quite a bit bigger than those that are in a typical combi. The combi can supply a steady stream of 80C water to the primary side of the exchanger, but here the tank will start at 70C and quit at maybe 50C?, so I suppose the exchanger will need to be much larger to account for that.

The 300 liter version of the tank below has a coil with a surface area of 4 square meters:

In the specs it says it can make 175 liters of 4C hot water starting from a 65C tank temperature. I estimate that exchanger is around 200kW, so that is where I started, but it would be nice if one could calculate it to know for sure.


  • OEG 300l Specs.pdf
    36.2 KB · Views: 267
If you just want to know how much hot water you can make, the rating of the heat exchanger isn’t important. That’s only useful to work out how quickly you can do it.
True. But I estimate I need to make it at 10 litres/minute to be useful. The bigger the exchanger, the lower the tank temperature can be and still keep up. As you pointed out with an infinite exchanger I could get 210 liters. So I want to know how much I can get with 210kW at 10 liters/minute.

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