Large Preheat Coil requires recirc pump? 2

[Thomas C.]

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?

 

[lilliput1]

Because of the relatively light preheat coil duty and to avoid laminar flow the recirculating pump would be required. Use the coil manufacturer coil selection program to see corresponding entering heating water flow temperature and gpm would be required at various duties of the coil including at minimum supply air CFM to see what I mean. The heating water temperature must reduce for the flow to be above laminar.

 

[Thomas C.]

That makes sense, but if you reduce the entering hot water temperature based on outside air temp then you would need more hot water flow through the coil to maintain discharge air temperature. Would you still need the recirculation pump to run during this time? Also when load does pick up and you have an outside air temp of 24 degF but the recirculation pump is still running doesn't that hurt the performance?

 

[willard3]

It really depends upon the coils and their configuration.

If the flow is laminar, running the pump will increase the heat transfer by decreasing the film coefficient and making turbulent flow.

 

[lilliput1]

You don't need to reset the entering heatring water temperature. The control valve will modulate the primary gpm to mix with the recirc gpm and reduce the water temperature to that required for the given total flow and preheat coil duty.

 

[Thomas C.]

Ok. That makes sense.

 

[lilliput1]

You have to use the coil selection program to determine the mixed heating water temperature (mix of primary and return from preheat coil discharge)and total gpm to determine the primary heating gpm and size the control valve. Your primary pipe and valve size will be smaller than the size of the total recirc pipe loop from the preheat pump to & from the preheat coil.

 

[Thomas C.]

Thanks for the response!

 

[Thomas C.]

I did have one more question. Not sure why I didn't think of it before. Why don't you need a recirc pump on the chilled water coil? It is much larger than the preheat coil on every job yet it never has a recirc pump.

 

[lilliput1]

Cooling is not as critical as heating. Also on low cooling load the AHU should index to outdoor air economizer cooling.

 

[BronYrAur]

I think whoever wrote the sequence was copying something from a northern climate. Why would the coil be so large as to result in laminar flow in the first place? If demand is that low, then there is little heat needed and the coil should be smaller. I am in the Chicago area and only see these pumps for freeze-protection.

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.

 

[Thomas C.]

^That makes sense.

 

[ChasBean1]

A freezestat has an averaging element that snakes across the coil it protects. If any small segment of the freezestat senses a temperature below its set point, the AHU trip is initiated.

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.

 

[Thomas C.]

I see your point but do you select the recirc pump for full coil gpm? Half gpm? Also, have you experienced this actually happening? You verified the coil was properly balanced to design gpm, the outside air comes in at 25 deg F with the preheat coil maintaining 50 deg F average discharge air and the freeze stat trips @ 38 deg F?

 

[ChasBean1]

Full coil gpm. Yes, you are thinking about this wrong, to answer your original question. Full coil gpm, yes. Not half gpm. Yes, I would not have posted if I had not seen this.

 

[Drazen]

That's not only Chas' point, but that is also a knowledge that stands behind preheater coils pumps, which is explained by Chas excellently. The sole point of freeze-sensor is to sense temperature all over the heater area, because it is simply known that temperature fifference allways exist over the area.