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Freeze protection pump on a HW coil
3

Freeze protection pump on a HW coil

Freeze protection pump on a HW coil

(OP)
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.

RE: Freeze protection pump on a HW coil

You would program it to run when OAT is near freezing AND the fan is on.

RE: Freeze protection pump on a HW coil

The pump should rat race the the coil gpm requirement in and out of the coil, and the control valve will bleed hot water in to maintain HW setpoint.

I suggest the pump be controlled to run at ALL times the temp is close to freexzing (38 degF)

Coil should have been sized to maintain a minimum of 3 fps.



knowledge is power

RE: Freeze protection pump on a HW coil

there are several setups, circulation pump can be installed in dedicated bypass or in main line, but in general it is expected to run all the time.

hydraulic scheme has to enable that maximum flow is ensured no matter how load regulation is planned.

RE: Freeze protection pump on a HW coil

Why would the pump be expected to run at all times, and not when there is a chance for freezing?

Is there no BMS or is having the pump run at all times more "full proof", and worth the waste of energy?

knowledge is power

RE: Freeze protection pump on a HW coil

...and not ONLY when there is a chance for freezing...

knowledge is power

RE: Freeze protection pump on a HW coil

if pump is installed in main line, it is either installed in series with main branch circulation pump or it fulfills function of main branch pump. i both cases it has to run so that main pump or differential pressure does not "push" flow through pump that is switched off, which can make damage in many constructions. that imposes need to run all time when main circulation runs or when main circulation is off and there is freezing danger.

you are right it is not ABSOLETELY all the time, but given the fact that most of such systems run year-round, that's quite close to all the time, and I am a bit lazy to elaborate such things all the time. wink

RE: Freeze protection pump on a HW coil

Understood thanks.

So your installation is in series and the freeze pump must overcome the coil DP (so you could technically and potentially remove that coil head requirements from the system pump).

An alternative to running the freeze pump 24/7 could be to pipe a full size bypass around the freeze pump with a check valve in the direction of the coil. This would allow flow to bypass the freeze pump when the OA is not near freezing and pump not on, would allow flow to the coil, if the freeze pump would fail; and would not allow the coil pump to rat race when energized.

I assume the bypass with check valve is of lower PD and would see most of the flow, when freeze pump is off, but could a small amopunt of flow still push through the freeze pump as well when it doesnt run? If so, would that also damage the pump?

knowledge is power

RE: Freeze protection pump on a HW coil

Sounds very much like the VA standard detail for hot water preheat. PECI has some good schematics/sequence for it as well.

RE: Freeze protection pump on a HW coil

you said "Assuming the main loop pump cannot circulate the necessary flows needed for freeze protection"
- Can you explain it.
- is this application still in design stage or is already has been constructed?
- who calls for freeze protection.
- did you talk to the application designer.
- when do you expect you need freeze protection?

RE: Freeze protection pump on a HW coil

The “freeze pump” moves the coil design flow whenever OA temperature is less than about 40°F. They are often fractional-horsepower as they only need to overcome the coil pressure drop and a couple of bends.

The purpose of the pump is to maximize flow through the coil while minimizing inlet/outlet temperature (or delta-T). The bridge/deny/check assembly is common for these assemblies. Back to the basics (sorry if I’m preaching to the choir)—Q (heat transfer) is proportional to flow times delta-T. Raising flow minimizes delta-T for the same Q.

These assemblies are necessary because coils that are designed for very low winter design conditions (e.g., 0°F) perform very poorly during outdoor air temperatures of 20-50°F. Without the freeze pump/bridge/deny assembly, a 0°F designed coil might transfer all necessary heat to keep the bulk leaving air temperature of 55-60°F in the bottom one-quarter or one-third of the coil. So, near the coil return connection (meaning at the top or leaving water point) the water temperature would be nearly equal to the inlet air temperature. Freeze-stats, which protect the downstream cooling coil or other components, only need to sense a portion of the air below about 38°F to trip and shut down the unit.

The freeze pump assemblies ensure there is the maximum flow through the coil with minimum delta-T, so that the coil leaving air temperature is more uniform. I am an advocate for these assemblies, plus air blenders (which cost minor air pressure loss) after several years of seeing freeze trip issues.

CB

RE: Freeze protection pump on a HW coil

Chasbean1
Do you have a specific coil product data book that support your explanation,
Coil is a hydronic component, and as you know delta T in hydronic systems usually is designed between 10 to 20, hot water is around 160-180F.
you say:( So, near the coil return connection (meaning at the top or leaving water point) the water temperature would be nearly equal to the inlet air temperature).
According to the above, the detla T would be in range of 100-140F. is this right?

RE: Freeze protection pump on a HW coil

Yes, the attachment is one practical way to achieve that.

In my area it was customary to use three way valve instead of presented modulating valve + bypass, but this seems to be better setup when using PICV valves.

317069, the subject falls within generic knowledge of hydronics, it is not some detail bound to specific manufacturer. you also need to differ design delta t from actual delta t: even at design conditions, actual delta to will likely be somewhat different except if coil as manufactured to design spec, which is rare in common hvac field (designer rather selects nearest-matching coil). in freeze danger conditions, scenario is as elaborated by chas.

RE: Freeze protection pump on a HW coil

Drazen
- my point was about how come a hot water inside (160-180F) a coil will reach the inlet air temperature (0-30F).Can you help to explain it?
- it is about a specific product which it is a hot water coil performance in our case.
- actual delta T could differ from calculated delta T by a limited ratio, but not from 20F design delta T to 140F actual delta T if the hot water inside a coil will reach inlet air temperature.

RE: Freeze protection pump on a HW coil

About the VA detail in the link:

1. Why not put a check valve in the bypass across the pump? With a check valve in the bypass, the pump could be off when there is no danger of freezing the coil. When the pump is on, it would hold the check valve closed.

2. The location of supply connection to the coil does not produce counter-flow heat transfer.

RE: Freeze protection pump on a HW coil

The new balancing valve was on the wrong side of the vertical check valve; there were two balancing valves in the pump loop. The image has been fixed to show this more correctly - one balancing valve in the pump loop, and one balancing valve for the entire assembly to limit the flow when the control valve is fully open and the pump is running.

When the pump is not running due to warmer outside temperature, the coil load will be less and the flow through the bypass around the pump can be reduced to less than the full design flow.

The "Manual Bypass Valve" in line with the new check valve should now be a shut-off valve for isolation to repair the check valve. There should be a shut-off valve on the other side of the check valve for this purpose, too, but the image was not updated to show these.

RE: Freeze protection pump on a HW coil

The VA actually went away from this arrangement of protection of pre-heat. Well, depends on who you talk to at the VA I guess.
They typically want a glycol solution for pre-heat. i.e. use a heat exchanger and run glycol.
the heat exchanger is cheap in the scheme of things, and it is full proof. Simple controls and no need for the freeze protection pumps all over the place, no air blenders neither.

Piece of mind........ priceless.

PS:
Drazen: you should stay away from those three-way valves with today's IECC and ASHRAE 90.1

RE: Freeze protection pump on a HW coil

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.

RE: Freeze protection pump on a HW coil

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.

RE: Freeze protection pump on a HW coil

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.

RE: Freeze protection pump on a HW coil

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.

RE: Freeze protection pump on a HW coil

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.

RE: Freeze protection pump on a HW coil

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.

RE: Freeze protection pump on a HW coil

Check valve on pump-bypass found in the wild:

http://www.fs.illinois.edu/docs/default-source/fac...

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.

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