Heat exchanger efficiency

[Science266677]

Hi,

 

I am trying to calculate the heat exchanger efficiency of a system heating a sludge using hot water.

 

The boilers on the system heat the water to a set point of 70C. The water flow is constant because the water is low in solids and the incoming water temperature is read at 68C consistently.

 

I cleaned the heat exchanger today and the sludge was incoming at 30C and outputting at 36C, this varies though from 15C input up to 45C input. It is a spiral heat exchanger.

 

The hot water pump flow rate is 30m3/hr or 8.4kg/s

The sludge pump flow is also 30m3/hr or 8.4kg/s (No flow meter fitted but assuming this value today after cleaning not including pump wear)

 

The sludge is contaminated with lots of heavy fibourous solids. The heat exchanger is blocking over time, gradually until it gets so bad the sludge pump blows its mechanical seals. I clean the heat exchanger after about a week to stop the pump damage. As the sludge circuit of the heat exchanger blocks the flow rate decreases and the heat transfered to the sludge drops.

 

The rate of blocking varies between 2 days and one month depending on the upstream sludge processing which impact the sizes of the fibourous solids coming through the exchanger.

 

The temperature readings are measured on screen in and out on both circuits of the heat exchanger. Ideally I would like to have a value I can set myself where I will clean the heat exchanger (maybe when I lose 40% of the transfer efficiency)

 

I know this might be quite simple but my maths isnt all that good to work this out.

 

Thanks for your help in advance,

 

Rob

[insane_alien]

okay, find out the heat capacity of the sludge it'll probably be less than water but on the same order of magnitude. then work out how much energy the water is dumping in the heat exchanger and how much energy the sludge picks up while in the heat exchanger. divide the energy picked up by the sludge by the energy lost by the water and you'll get your baseline efficiency(do this after a cleaning).

[Science266677]

Thanks

 

So if I assume that the sludge has a heat capacity of 3.99 kJ/Kg.

 

How do I do the energy calculations based on my given temperature readings before?

 

Lets say my temperatures were -

 

Input hot water 68C

Output hot water 61C

Input Sludge 30C

Output sludge 36C

 

Hot Water Inlet Temp 68 T1

Hot Water Outlet Temp 61 T2

Sludge Inlet Temp 30 t1

Sludge Outlet Temp 36 t2

 

Temp 1 32 T1-t2

Temp 2 31 T2-t1

δt 7 T1-T2

 

This is where I am getting lost!!!

 

Thanks

 

Rob

[insane_alien]

okay there is a basic equation : dH = C_p *M *dT

 

dH = change in heat(energy)

C_p= heat capacity of the material

M = mass of material

dT = change in temperature.

 

dT for the water is 7*C and for the sludge is 6*C

 

none of these calculaions will mean anything until you get an accurate heat capacity of the sludge.

[Science266677]

Thanks, I have found out the heat capacity of the sludge. it is 3.10.

Please can you sense check my calculations below. I am sure they are right.

 

 

Heat Exchanger 1 Heat Exchanger 2

 

Hot Water Inlet Temp 68.23 Hot Water Inlet Temp 68.23

Hot Water Outlet Temp 61.49 Hot Water Outlet Temp 60

Sludge Inlet Temp 30.8 Sludge Inlet Temp 30

Sludge Outlet Temp 35.16 Sludge Outlet Temp 36

 

HW Temp Diff 6.74 HW Temp Diff 8.23

HW Temp Diff 4.36 Sludge Temp Diff 6

 

Water Water

4.18 kj/Kg 4.18 kj/Kg

92 litres 92 litres

6.74 change 8.23 change

2591.9344 Kj 3164.9288 Kj

 

Sludge Sludge

3.1 kj/Kg 3.1 kj/Kg

100 litres 100 litres

4.36 change 6 change

1351.6 Kj 1860 Kj

 

52.14638148 EFFICIENCY 58.76909458 EFFICIENCY

 

Water flow rate 30m3/hr or 8.4kg/s

Sludge flow rate 30m3/hr or 8.4kg/s

[Fella]

May be this could help you

http://www.kamlex.com/index.php?option=com_content&task=view&id=13&Itemid=28