Maths Calc teaser

[darkart]

Can anyone tell me how to work out the Kw usage of a plate heat exchanger using Primary and return flow rates when no demand for hot water or heating is needed. (Basically a valve is passing and want to work out the heat loss from the plate, due to the valve passing).

So lets assume the following:

• Primary flow rate to the plate is 80c and the return temperature is 75c.
• the flow meter fitted to the Primary return is registering 700 litres per hour.

What else would I need to calculate temperature loss from the plate heat exchanger In Kw per hour?

Thanks

 

[D_Hailsham]

You need the Specific Heat Capacity of Water, which is measured in Joules/kg/K. This is approximately 4.18 J/kg/K.

700 litres/hr is 700/3600 = 0.194 litres per sec. The rate of heat loss = 0.194 x 4.18 x (80-75) = 4.05 kW.

So, in one hour, the heat loss will be 4.05 kWh.

I have assumed that the density of water is 1 kg/litre.

 

[darkart]

You need the Specific Heat Capacity of Water, which is measured in Joules/kg/K. This is approximately 4.18 J/kg/K.

700 litres/hr is 700/3600 = 0.194 litres per sec. The rate of heat loss = 0.194 x 4.18 x (80-75) = 4.05 kW.

So, in one hour, the heat loss will be 4.05 kWh.

I have assumed that the density of water is 1 kg/litre.

You are an absolute star, many thanks

 

[darkart]

Sorry to be a pain but im having a thick moment. I am trying to set up an Excel spread sheet where I can enter formulas into it and give me total cost losses. I have tried to alter (80-75) in my formula to (80-78), but it seems to mess up the calculation and give a bigger loss rather than less?

 

[D_Hailsham]

Yes, the loss should be smaller as you have only a two degree temperature differential, not a five degree.

To say where you have gone wrong, I need to know how you have laid out the spreadsheet and the formula you have created.

I presume the circulation is by gravity, not pumped, so the flow rate will change with temperature. You therefore can't assume it will be a constant 700 litres/hour.

 

[darkart]

Thanks for getting back to me and offering to help out.

The plate heat exchangers are installed on a district heating system and yes the flow rates do fluctuate but i am using average flow rates for my calculations.
Would it be possible to contact you by email, I could then send you my spreadsheet, containing formulas.

For what ever reason it does not give the end calculations correctly in its made up format.

Thanks

 

[D_Hailsham]

Sorry, but I never divulge my email address on a forum. Maybe this will help:

Assuming:

A1 = Flow temp (deg C)
A2 = Return temp (deg C)
A3 = Flow rate (litres per hour)
A4 = heat lost (kW)

Then the formula in A4 should be: =(A1-A2)*4.18*A3/3600