Surface temperature of cooling coil
Surface temperature of cooling coil
(OP)
Hallo to everybody !
I am posting also to this forum following question:
I am trying to calculate a cooling coil (it's fin and tube heat exchanger).
I know inlet conditions, water inlet and outlet conditions (or, alernatively, water mass flow), but I am looking for a formula to get the fin temperature.
This temperature is what I need to get the working line of the coil.
Please, can anybody give me a formula to calculate this temperature (I suppose it would be function of fin pattern, air inlet temperature and somehow linked to fluid temperature), or suggest me a book or some literature where to find this formula ?
Thanks in advance
I am posting also to this forum following question:
I am trying to calculate a cooling coil (it's fin and tube heat exchanger).
I know inlet conditions, water inlet and outlet conditions (or, alernatively, water mass flow), but I am looking for a formula to get the fin temperature.
This temperature is what I need to get the working line of the coil.
Please, can anybody give me a formula to calculate this temperature (I suppose it would be function of fin pattern, air inlet temperature and somehow linked to fluid temperature), or suggest me a book or some literature where to find this formula ?
Thanks in advance





RE: Surface temperature of cooling coil
TTFN
RE: Surface temperature of cooling coil
RE: Surface temperature of cooling coil
Thanks in advance
RE: Surface temperature of cooling coil
Whenever a heat transfer process has a sensible heat ratio that is less then one, it indicates that some of the heat absorbed by the refrigerant (water) is used to condense some of the moisture out of the air. So when he temperature of the air drops below the water vapor mixed with the air condensation will occur. This requires latent heat removal and is known as the dew point.
I.m not sure I answered your question But if your looking for the rule of thumb for an air conditioning evaporator TD
it's usually about 16-22*F Remember, Evaporator TD is the the mathematical difference betwen the air entering the evaporator and the the saturation temperature of the refrigerant(water) inside the coil. EXample let's say air on =75*F and you have a 20*F TD, then air off should be about 55-56*F.
RE: Surface temperature of cooling coil
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Surface temperature of cooling coil
u need to refresh your heat Transfer funda to solve this.
Use the following eqn to get the LMTD for the coil,
(Coil capacity)Q=U*A*LMTD.U Heat transfer cofft)and A (heat transfer area)and would need to be read from the coil charts.
After solving for LMTD,insert the value into the LMTD equation.The known values in this equation are air entering and leaving temperatures.Treat the coil temperature as constant thru out the section for simplicity.
The same eqn can be appiled to the water side heat transfer as well.
RE: Surface temperature of cooling coil
Fin temperature will vary along its length.
Try "Elements of Heta Transfer", Jacob and Hawkins, Third Ed, chapt 9.
RE: Surface temperature of cooling coil
For a chilled water cooling coil, I can only see dealing with the senible component on the air side but how is the latent heat dealt with? Change in mass flow rate on the air side.
With a DX coil you have some superheat, negigible compared to the overall heat transfer, but a surface temperature change no the less, and you also have phase change on the air side.
So to me when it comes to cooling with dehumidifacation, the best use of LMTD is to explain why it is important to have the return water header upstream of the supply water header with respect to air flow.
Take the "V" out of HVAC and you are left with a HAC(k) job.
RE: Surface temperature of cooling coil
Because the analytical model is so multifaceted, the commercial manufacturers of such coils have atmospheric chambers and similar lab setups.
The mechanisms are all treated in heat transfer books, and if you know a whole lot about the coil geometry, and have a lot of available computer time, the outcome might be significant.
RE: Surface temperature of cooling coil
Mr. Threlkeld's approach seems to be to first of all to calculate the resitances to heat flow. These would include the reciprocal of the inside of pipe convective heat transfer coefficient times the incremental area of pipe wall. There also should be a fouling factor resistance on the inside of the pipe. Then you have the resistance of the pipe wall, a simple calculation involving the conductivity of the pipe, then fin resistance, then the outside film heat transfer coefficient which is in the case of dehumidification related to the dry heat transfer coefficient by the relationship hw/hd=0.626(face velocity)**0.101 .
Once you have all the resistances added up take the reciprocal of that and that gives you a U value. Since we are assuming steady state heat transfer then the same amount of heat transfers across each element of resistance. Therefore the temperature differences across each resistance will be proportional to the amount of each resistance. At that point the solution to the fin temperature questions will be a matter of simple algebra.
Of course this would apply primarily to a small increment of tube and fin but Mr. Threkeld uses the logarithmic means of air enthalpies, some tables and some iteration to encompass real world situations. If you don't have his book in your library you really should get a copy. I have tried to condense over thiry pages into this post.
Most coil manufacturers can give you a heap of empirical data on their products, this would be very beneficial.
Hope this helps.
RE: Surface temperature of cooling coil
I'll keep care of every suggestion.
Please, Mr. Stanlsimon, where can I find the book you are quoting ?
Many thanks
RE: Surface temperature of cooling coil
Also the University of Minnesota Institute of Technology Engineering Library might be able to help you.
RE: Surface temperature of cooling coil
please, do you think that ASRHAE handbook can be useful the same ?
Thanks in advance
RE: Surface temperature of cooling coil