What is a typical value for the thermal coefficient of a typical heating coil (water side) in the equation Q = U x A x LMTD.
What is its relationship to water flow rate given that the U value goes down if the flow rate goes down, anybody got a formula governing this.
We use manufacturer's coil selection programs or tabulated performance data instead of using the formula because actual coils have numerous combinations of circuiting, number of rows, fins, material, tube enhancements etc. If you have the CFM, EWT, desired LWT or gpm and maximum allowable air velocity, water pressure drop and air pressure drop, maximum psig pressure, manufacturers can typically provide a coil.
Lilliput1 is right. Normally, we end up doing what I call an "application engineer's" role, wherein we try and design a system based on inputs from various product manufacturers and standard acceptable design standards and codes as per ASHRAE. The detailed design and engineering of the individual components are generally done by the engineering guys of the manufacturers, which in turn is converted as a software programe to be used by the sales engineers of these companies. There is no need to reinvent the wheel and hence we probably don't that amount of detailed engineering on individual products and leave it to the specialists.
Whenever one is trying to improve the "U" (as in the formula) of a given heat exchanging unit, one has to improve the HTC on the side of the dominant resistance.
If, in this case, it is air, negligible improvement could be obtained by increasing the linear velocity of the water while already keeping it in the usually turbulent flow regime.
This is because U is always lower than the lowest HTC values in the heat transfer equipment. Kindly recall that by definition
Try to get your hands on ARI Standard(ARI 410-2001) that deals with forced circulation air cooling and air heating coils. You can download it for free from
along with other good standards. There is good amount of data and performance prediction with respect to air side parameter changes but one graph discusses the performance of coil with respect to water velocity.
