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Freeze protection pump on a HW coil 3

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tbarkerjr

Mechanical
Oct 31, 2008
20
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.
 
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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
 
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.
 
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
 
...and not ONLY when there is a chance for freezing...

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

 
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
 
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?
 
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.
 
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.

 
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.
 
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.
 
 http://files.engineering.com/getfile.aspx?folder=e779718c-263b-4514-a676-05a8bdc5819d&file=modified_coil_piping_detail-fixed.png
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
 
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