VFD on pump motors in a dual duct system

[yyj10]

Hi guys,

I am working on energy conservation project. This is a dual duct system, the supply fan and return fan motors are all equipped with VFD already. Is it necessary to install VFD on the hot water pumps and chilled water pumps? I do not think it is necessary since the air flow is adjusted already and heat load of the coils is reduced already. Is that correct? Thanks!

 

[DRWeig]

As your load (thus airflow) decreases, you'll find that the water side delta-t across your coil drops. You're pumping more water than what is needed to do the same job of cooling/heating.

Investigate how much pumping energy can be saved by converting to variable flow, and weigh the cost savings against the cost of the conversion.

Most often, in my experience, conversion of constant-flow water to variable-flow water is a good investment, as long as the load varies substantially.

Don't forget to evaluate the control valving -- you may have existing 3-way valves that simply bypass the unneeded water. You'll have to convert to 2-way valves to get the variable-flow benefit. But don't let the flow go below the minimum for your pump, chiller, boiler...



Good on ya,

Goober Dave

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[yyj10]

Thanks a lot , Dave!
So it is fine to control both air and water flowrates. My concern is whether we would make the control strategy too complicated. We know four types of control methods in this kind of system:
1. change air flowrate
2. change air temperature
3. change water flowrate
4. change water temperature
If I do control on both air and water side, would it be too complicated and cost much more than just control on one side?

Regards
Eugene

 

[marcoh]

Hi Eugene,

Most modern buildings control all 4 items continuously. There are more control loops, but each control loop in itself is fairly simple. If in doubt slow down the system response of each one.

 

[BronYrAur]

In my experience, the air and water rates are controlled by pressure. Air by a static pressure sensor out in the system (say 2/3 of the way down), and water by delta-P (again, say 2/3 of the way down). I would keep air temp at the unit constant and let the boxes trim. Water temp may be a function of outdoor air reset for heating and probably constant for cooling.

 

[317069]

You better check with chiller and boiller manfucturers specification to see thier function and efficiency under different operation barameters.
also ASHRAE 90.1 mandate variable pumping under certain conditions, if you follow ASHRAE ou must check this point too.
and as mentioned above you might have to change control valves, and you have to check exisiting boiler and chiller control loops.
but still the first step is to talk with manufucturer first then start from there.

 

[DRWeig]

Hi again,

You'll be controlling air flow rate to maintain space temperature (modulating the VAV damper).

You can add reset to control supply air temperature if desired (modulating the chilled water valve at the air handler). This may be done to conserve energy, raising supply air temperature during low-load times and lowering it for higher load times. Or, it may be done for better humidity control, lowering supply air temperature for more dehumidification and raising it for less.

You'll be controlling water pressure to maintain a minimimum needed by your worst-case coil. As the two-way valves close with decreasing load, your pump will slow to save energy.

You may add reset to control water supply temperature if desired (modulating the chiller or boiler setpoint). This may be done to conserve energy or as a complement to supply air temperature reset for humidity control.

Each loop adds some cost, but in general the payback is very good for each (depending on location, electric/fuel costs, etc...).

Most non-residential jobs I encounter include at least the VAV, variable-water-flow, and hot water supply temperature reset controls (I think the hot water reset is mandated in the 90.1 energy code by ASHRAE, as noted above). The supply air temperature reset is a bit less common -- more often, it's employed for humidity control rather than energy savings. But both advantages are available.

Since each loop is handled independently, complexity isn't as serious as you'd think. It may seem daunting, but building automation contractors nowadays do this stuff in their sleep.

Good on ya,

Goober Dave

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