Calculate ROI for a custom Unvented Indirect Heat Pump Cylinder???

[myatix]

@Notch7 You are soooo right about that...

Funnily enough I would say that Denmark has it's fair share of plumbers that oversize ASHP systems as they can't be bothered to take the time to do proper heat loss calculations. While also zoning heating systems so that they don't function optimally rather than running them at lower steady constant temperatures.

 

[MNW67]

lower flow rate therefore optimizing's the energy transfer

To optimise the SCOP you want to keep the temperature as low as possible with as small a DT as possible so by using a larger coil your slowing the flow rate down and improving the thermal transfer

I am finding this all very interesting but am struggling with these bits. I had always thought that a lower flow rate meant both a larger dT and lower thermal transfer overall? When you look at cylinder specifications, if you increase the flow rate, then the output is increased and therefore the reheat time is reduced. So, you could end up with the worst of all worlds. A dT which is too large and causes your heat pump to fault, and a slow reheat time.

 

[MNW67]

I have had a few companies provide a quote for this type of cylinder but I would like to calculate the return of investment to see if it is financially viable over a 5-10 year period.

But to try to answer your original question. This is all on the basis that your theory is actually sound which, as you will see above, I am not sure I agree with!

Say your cylinder needs 10kWh of heat each day. And doing it your way, means you can run the flow temperature 5 degrees lower. Say that increases the COP from 2.5 to 3.0. At the higher flow temperature, you would need to burn 4kWh of electricity to get get 10kWh of heating. At the lower flow temperature you would need to burn 3.33 kWh of electricity instead. So, each day you would be saving 0.67kWh of electricity. Electricity costs 25p per kWh. So, that is a saving of 17p per day. If your new cylinder costs £1500 more, it will take 8824 days to get your money back, which is 24 years.

These are invented figures! I am just giving you a framework so you can plug in the right numbers.

 

[myatix]

@MNW67 You are right about the flow increased flow rate heating up the cylinder faster. The issue is that you don’t want to run flow rates too high as it can cause noise and erode on the pipe.

If you use the Mass Flow Rate calculation for say a 7kw heat pump and a dT 5 C then flow is around 0.34 l/s and ideal for 28mm coil. A 22mm coil would have a flow rate of 1.2 l/s which is getting a bit fast and can cause noise and erosion to pipe work. This keeps the Flow rate acceptable and the dT low.

At least that’s what I have understood!

Thanks for the explanation on the calculations I will run the numbers and see what comes out… doesn’t sound like the HG cylinder is a great investment however.

 

[MNW67]

@myatix

In my opinion, calling it Mass Flow Rate just confuses things and makes it sound a lot more complicated that it needs to be. The density of water is 1 so mass really becomes irrelevant. Usually we just call it flow rate. It can be a pretty easy thing to work out. I'm just trying to simplify things a bit.

 

[MNW67]

If you use the Mass Flow Rate calculation for say a 7kw heat pump and a dT 5 C then flow is around 0.34 l/s and ideal for 28mm coil. A 22mm coil would have a flow rate of 1.2 l/s which is getting a bit fast and can cause noise and erosion to pipe work. This keeps the Flow rate acceptable and the dT low.

At least that’s what I have understood!

I am not sure what you are wanting to calculate here. I would say your value of 0.34 litres per second is the correct flow rate for 7kW heat pump at dt5. But surely the required flow rate will be the same whether you use 28mm or 22mm pipe? Where does the figure of 1.2 litres per second come from? I'm happy to go through it with you as I am finding it interesting!

 

[MNW67]

I think it has clicked for me now. Is the 1.2 litres per second value you are trying to calculate actually supposed to be a velocity, rather than a flow rate. Should that be 1.2 metres per second. How have you calculated that figure?

 

[myatix]

I think it has clicked for me now. Is the 1.2 litres per second value you are trying to calculate actually supposed to be a velocity, rather than a flow rate. Should that be 1.2 metres per second. How have you calculated that figure?

I used the IMI HyTools app that does a mass flow rate calculation and shows suitable pipe sizes based on pressure and flow rates.

 

[MNW67]

I used the IMI HyTools app that does a mass flow rate calculation and shows suitable pipe sizes based on pressure and flow rates.

I am hoping this won't sound patronising. But the problem sometimes with using tools is that they don't always explain the output.

As I suggested above, you may be confusing flow rate with velocity.

Also, do you realise that a flow rate in litres per second is not a mass flow rate. It is a volumetric flow rate. A mass flow rate would be expressed in kilograms per second.

 

[Johntheo5]

If the heat pump is oututting 7kw at a dT of 5C, the flowrate is 7.0*860/60/5, 20.066LPM or, 20.066/60, 0.334L/sec.

 

[old duffer]

Rinnai continuos water heaters have 13:1 modulation, can detect incoming water temp and modulate accordingly so the gas/lpg is the boost.

 

[old duffer]

I am hoping this won't sound patronising. But the problem sometimes with using tools is that they don't always explain the output.

As I suggested above, you may be confusing flow rate with velocity.

Also, do you realise that a flow rate in litres per second is not a mass flow rate. It is a volumetric flow rate. A mass flow rate would be expressed in kilograms per second.

but a given volume has a given mass at a given temperature so refer to whatever you want, there is a fixed correlation play with this chart to see

Water - Density, Specific Weight and Thermal Expansion Coefficients

Definitions, online calculator and figures and tables with water properties like density, specific weight and thermal expansion coefficient of liquid water at temperatures ranging 0 to 360°C (32 to 680°F).

www.engineeringtoolbox.com

 

[MNW67]

If the heat pump is oututting 7kw at a dT of 5C, the flowrate is 7.0*860/60/5, 20.066LPM or, 20.066/60, 0.334L/sec.

Exactly. But the OP then goes on to say that if the pipe is reduced to 22mm then the flow rate would need to be 1.2L/sec. Which obviously can't be right. I was suggesting that the OP might be thinking of the velocity instead. Which would be measured in metres per second.

 

[MNW67]

but a given volume has a given mass at a given temperature

Obviously, but it's irrelevant. Mass flow rate is a specific and well understood technical term and its SI unit is kilograms per second (kg/s). The volume of my kettle is 1.7 litres. The mass of that water is 1.7kg at 4C at sea level. But the volume of my kettle will never be 1.7kg.

Anyway, the clue is sort of in the title

If people want to use scientific terminology, then use it properly. Which was why I suggested above that simply using "flow rate" would be much less confusing. Then it can mean whichever you want.

 

[myatix]

Also, do you realise that a flow rate in litres per second is not a mass flow rate. It is a volumetric flow rate. A mass flow rate would be expressed in kilograms per second.

You could be write about that but it would appear most people use l/s for mass flow rate.

I can see now that the IMI HyTools shows the maximum size based on m/s. But the maximum recommended velocity is around 1 m/s if I understand the video I referee to correctly which is still a 28mm pipe size?