5 replies to this topic

[clarky]

This may seem like a simple question but trying to find out for a school project...

Is there an easy way to calculate the power rating of a tubular heat exchanger coil in a storage water heater? If I know the temperature of the primary water into the coil, the temperature drop across the coil and the surface area of the coil can this be worked out? Do I need to know the quantity of water being heated, or the temperature rise needed in the stored water (or anything else for that matter)? Incidentally the stored water is static.

Any help would be appreciated

[djack77494]

Is there an easy way to calculate the power rating of a tubular heat exchanger coil

Power rating = Duty (or energy exchanged)??

If I know the temperature of the primary water into the coil

Neither power rating nor primary water are common terms, so I'm guessing a bit as to what you mean. It sounds as if you are using a hot water utility to heat the water in this storage tank. If you have enough information, you can do an energy balance with the heat lost by your hot water utility = heat gained by the water in the storage tank. You could solve for one variable, but would need flowrates and temperatures and heat capacity of both fluids in and out. If there is no flow into/out of the storage tank (which sounds like your situation), then you have an unsteady state system and the heat being transfered will vary moment to moment. In any of these cases, you will need to know flowrates or quantities of both fluids, and probably all but one of the temperatures. Refer to your basic Chem E book for USS calculations (at the back of the book if you used Himmelblau).

[clarky]

Thanks djack77494 for your reply.

Pardon the use of non standard terms, but I am not from a chemical engineering background. This is a task that has been set for a school project, judging by the complexity of the variables that seem to be needed it would seem a tough assignment!

Here in the UK "primary" water is the term given to the water heated by a boiler and circulated around a heating system which usually includes a storage vessel with an internal coil in a domestic household. Usually the flow temperature from the boiler is around 80 deg C at a flow rate of 15 litres/minute. For the purposes of the investigation we have been told the stored water in the vessel starts at 15 deg C and needs to be heated to 60 deg C. There is no flow in the stored water vessel, only through the heat exchanger coil. Both fluids are water. The vessel capacity (amount to be heated) is 150 litres.

[JEBradley]

Q=UAdT
Where Q is the duty, A is the surface area and T is the temperature differance. However, you need to know 'U' the overall heat transfer co-efficient

You can find U in tabulated data from various sources -
150-300 Btu/h ft^2 F or 900-1700 W/m^2 K

Or you can calculate it yourself - Just look in any heat transfer book.

This question begs the question 'how accurate do you need to be?' because there are a few other considerations.

[clarky]

Thanks Mr Bradley

I don't think my answer needs to be too accurate, but what would those "other considerations" be. The data I have put in my previous posts is all I have, is this too simplistic? Can you recommend any good basic text books on heat transfer that would give me the data for the overall heat transfer co-efficient?

When you say the temperature difference do you mean in the water being heated or the primary water heating flow?

I appreciate your help

[Andree]

overall heat transfer coefficient depends on hydrodynamics of fluids exchanging energy as well as on their physico-chemical properties...
my guess: U in the range 900-1700W/m2*K seems to be too high for natural convection on one side (static water)...

temperature difference in eqn Q=UAdT is driving force for heat transfer, i.e. temperature difference of fluids exchanging heat... it is often a logarythimc avg.