Large Preheat Coil requires recirc pump?
Large Preheat Coil requires recirc pump?
(OP)
I came across a job in Texas where the engineer says the preheat coil for a 75%OA and 25%RA VAV AHU must have a recirculation pump installed at a large stacked parallel piped preheat coil and is to run anytime preheat is required, discharge air setpoint is 50 deg F. Usually when I see a recirc pump installed at a preheat coil it will only run when the freezestat is tripped. So my question is, Is this the general installation and sequence of operation in the Northern States which have much colder weather than what we deal with in Texas? I thought that when the recirc pump runs anytime the coil is heating then the performance of the coil will decrease. It will take return water and feed it back to the supply making supply water colder when entering the coil. My thinking has always been that a recirc pump is the last ditch effort to stop the coil from freezing and should only run when the coil cannot keep up with the cold air. Am I thinking about this wrong?





RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
If the flow is laminar, running the pump will increase the heat transfer by decreasing the film coefficient and making turbulent flow.
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
Chilled water coils should only have them again for freeze protection. Basically just recirculate the coil in the wintertime or based on a freeze-stat. Energy waster, but it is a safeguard if you don't have glycol. You would never run them during cooling mode because any mixing of chilled water return with supply will diminish dehumidification.
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
A typical coil might be designed for a winter condition with a DAT of 55°F with 180°F water. For most conditions including intermediate seasons, where outdoor air temperatures are 25-30°F for example, supplying the design heating water temperature tends to result in a coil that does all its heating at the entering point (low, on the entering water/leaving air side) and is near outdoor air ambient (high, on the leaving water/entering air side).
Your freezestat element gets a stream of excessively cold air and the unit trips, because you’ve done all of your heating in the bottom ¼ of your coil, while the average leaving air temperature is at its set point (say, 55-60°F) but the top of the coil has a stream that is below 38°F.
The “freeze pump” is not the best term for what this pump is. What the pump does, with a check and a “deny” valve, is to maintain maximum flow with the minimum temperature. This creates the most even leaving temperature condition across the coil. It runs when heating is needed, not just when a freezestat is tripped.
With the minimum temperature and maximum flow, the coil leaving air temperature is more uniform.
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?
RE: Large Preheat Coil requires recirc pump?