Freeze protection pump on a HW coil 3

[tbarkerjr]

I have an application that uses a 100% outside air intake with a HW coil in the duct. The application calls for freeze protection with a pump. Problem is, there is no schematic on this. I've searched high and low for something with no results. Can someone provide me either a schematic or explanation of what is involved in such a case? Assuming the main loop pump cannot circulate the necessary flows needed for freeze protection.

Namely:

Should the pump run at all times?

If not, what are the controls(automated valves, etc.) needed?​

I'm assuming this loop will need check valves?

I appreciate any advice offered and hopefully with gained experience I can contribute more than just questions to the forum.

 

[Drazen]

cry, you have to mix water for pre-heater to allow this system to work, three-way valves or PICV + bypass do the same function, the difference is in supplying loop, which can be variable-flow designed in both cases.

solution with glycol is customary used for lower outdoor design tempereture - it adds the cost in all imaginable ways, pump-protection is actually devised to avoid glycol in climates with temperate outdoor design temperatures, or where temporary shutting fresh air off is not critical.

 

[ChasBean1]

317069 – great question and you got me on that. I thought about your question a bit and ran through some math. You’re right, if you run through calcs (Q = 1.08*cfm*dT for air and Q=rho*Vdot*Cp*dT for water), figure out part loads, redo dT at part load, the air dT should still be 30 damn degrees if the original condition was 180 in and 150 out.

I've been conditioned by reality at such a pace that I've not hearkened back to engineering science, and I appreciate you bringing me back!

Somehow, however, it really is true. Very large water delta Ts are seen at part load and coil stratification is a definite reality. Very low leaving coil air temperatures and freeze trips have been seen by every facility manager plus their grandmothers and a few cousins once-removed, but dealt with by the methods I described prior, rather than trying to determine why and correcting the real issue…

And Wilbur, YES, that is the detail. Thank you.

 

[Drazen]

simple math will hardly help as among three variables: delta t, flow and capacity, only two can be controlled by exchanger, the third is "as is" according to construction of heat exchanger that is presented through performance curves or, more and more often, by manufacturer's proprietary software.

more often than not heating load of pre-heater is not directly proportional to outdoor temperature due to internal heating sources, air mixing-schemes, occupancy schemes etc.

that means temperature control will often try to lower flow through coil to match load even when outdoor temperature is withing freeze danger range. inevitable stratification makes things worse, and that is why this protective loop is needed, to impose full flow, overriding temperature control. shutting outside louvers down is not sufficient in such circumstances.

 

[317069]

Chesbean1
how can we arrange between equation Q=500GPMdT and freeze trips in reality, there should be a theory base of what is happening in case of freeze trip.

Drazen:
why do we use a temperature control to reduce the flow, then we use a freeze protection to increase this reduced flow again.

 

[Wilbur55]

I would make a few more changes to this coil detail before using it. Yet another file is linked to illustrate the results.

The balancing valve was moved to the vertical line. Why hinder the main system pumps with the pressure drop through this balancing valve if the in-line pump fails?

If there is a manual bypass around the control valve, there should be isolation valves that allow repairing the control valve while the AHU remains in service.

A bypass around the control valve is probably needed on an AHU serving a surgical suite in a hospital, but it will probably be value engineered out of many applications. Valves that may be appropriate for critical applications are shown in blue; they might be eliminated in most systems to save cost and space.

A strainer (with a blow-off valve) was added for protection of the control valve. The blow-off valve is shown in blue because some might omit it through value engineering.

There would be parallel flow through the pump and pump-bypass if the flow through the balancing valve in the return exceeds the pump flow rate.

The pump motor size must be selected to be non-overloading because there may be parallel flow through both the pump and the pump-bypass. In such a case, the pump will run near the end of the pump curve. An under-sized motor would trip the breaker and the coil would be without freeze protection.

The location of the tee from the pump-bypass line was moved to keep the thermometer in the flow even if an isolation valve by the pump is closed.

Thanks and credit to cdxx139, who provided the link to the VA diagram and mentioned using a check valve instead of the manual bypass valve in the pump-bypass line.

For 317069, a link to a document evaded my search. The formulas ignore local variations in properties. When air is stratified such that freezing-cold air is flowing only at the bottom of the coil, the heating load at the bottom of the coil (in bth/square-foot) could exceed the coil capacity. The tube wall temperature may go below freezing for a short length of tube. A small amount of ice may form on one side of the tube. If the flow of cold air persists at that location, the extent of freezing may proceed to the point where the water flow rate is reduced. Then the overall values in the equations need to be adjusted. Rinse and repeat until the tube is completely frozen and possibly ruptured at the location where the freezing started.

 

[Drazen]

317, first sentence in my -2 post gives tip on it.

flow reduction as a means of temperature control is generally prevailing for energy efficiency purposes. this is in contradiction with need to maintain maximum flow, so either 3-way valve scheme or 2-way PICV valve scheme with bypass is needed.

with 3-way valve scheme, one more bypass is needed to fully separate primary from secondary, otherwise variations in primary flow would both disturb fixed balancing and authority of 3-way valve.

that gives comparison:

- 3-way valve scheme needs one 3-way valve, three fixed balancing valves and two bypasses
- 2-way valve scheme needs one PICV valve and one bypass

ordinary shut-off valves are not counted, but pricing of the mentioned can give good clue on which scheme is more cost efficient.

 

[Wilbur55]

Check valve on pump-bypass found in the wild:

http://www.fs.illinois.edu/docs/def...ing-coil-detail---single-coil4357421E6DA6.pdf

The arrows at the check valves are clearer in the CAD file.

The balancing valve near the coil seems out of place; the pump sees two balancing valves in series. I would put this balancing valve downstream of the control valve and upstream of the shut-off valve in the HWR line.

The pump would need to run only when there is a danger of freezing. The needed water flow rate through the coil should be much less than the maximum design value when there is no danger of freezing.

The main system pumps would need enough pressure to provide adequate flow through the coil and the check valve in the pump-bypass if the pump fails or has been turned off in mild weather. A balancing valve near the control valve would be set absorb the full pressure difference between the mains at design flow.