Ahu coil Pump 1

[saeedplc]

Dear experts,

I have seen some air handling units that their coil are equipped with freez protection pumps along with a check valve. These pumps not only run when the outside air temperature come below 5'c but also whenever coil control valve is opened. these pumps are designed at full flow of the coil.
I have heard the pump create even temperature throughout the coil and should be used when the coil has large surface.
Is using the pump obligatory for large coils and what are benefit of the using pump irrespective of freeze protection?

Regards,

 

[EnergyProfessional]

You don't need them when no freezing. Coils have multiple rows anyway. Yes, there is probably some uneven heating, but does it matter? you have a fan and duct system where air will mix.

In addition they are a poor freezing control, IMHO. What if the pump fails? and pumped water also will freeze (i.e. if hot water supply is stopped). Just a bit later. Rivers freeze too. If freezing is an issue, you need a freeze-stat as a safeguard. And pre-condition air with an ERV. If you can't have an ERV, use glycol.

It is probably more important that incoming outside air and return air mix properly before entering the coil. Especially if OA is not conditioned by an ERV.

 

[saeedplc]

Thanks a lot for the reply.

Can the pump cause low delta t syndrom in chilled water? I say this because the coil is supplied with full nominal flow and temperature difference would be lower but without pump the coil flow will be controlled with a control valve( mostly pressure independent valves)and delta t would be higher and the chiller get higher return water temperature and will have better efficiency.

Am i right?

 

[EnergyProfessional]

No. The dT of the system loop only depends on the system flowrate and the coil capacity (which depends on the coil, air flow and fluid temperatures). Look at an energy balance... a coil dissipating 50K Btuh, must take that heat from the water. With a given heat capacity of water, the flowrate of that water will determine dT. The coil loop being 10 times the flowrate will change nothing, it doesn't put in or extract energy outside the coil (if we ignore pump heat from friction and pump inefficiency).

the coil loop could increase coil capacity by a tiny bit since you have more turbulent flow and more even temeprature distribution. but if you control based on discharge air temperature, the control valve will just throttle a bit more.

 

[BronYrAur]

EnergyProfessional, I respectfully disagree. If your coil flow is larger (especially much larger) than your primary flow, you will dilute the temperature hitting the coil. Heat transfer from the coil will be less, and your delta-T back to the primary loop will be less. In a cooling application, you will also lose dehumidification capacity due to the diluted water temp.

 

[EnergyProfessional]

At very high coil flow you probably even will have slightly higher energy transfer to the airstream due to more turbulent water flow (heat transfer from fluid to coil tube).

Do an energy balance around the mixing valve and consider the first law of thermodynamics.

 

[lilliput1]

Don't mess with it if it is for preheat coil freeze protection using preheat heating water. It works fine.

 

[saeedplc]

I think because of existence of the pump in the loop the deltaT become lower since pump supply the coil with nominal flow.
I mean without pump not only flow but also deltaT should be increased.
In both cases suppose we consider the same amount of btu/h

However i am not completly sure of my thought.