variable volume AHU 4

[Herman2]

We have 29 air handling units on the roof of a machine shop that covers about 25 acres. Chilled water and steam are used to maintain space conditions, although we cool for most of the year because of internally-generated heat.

We modulate air dampers to control return, relief and make-up air while maintaining a discharge temperature set point.

There are no return fans. The supply fans are centrifugal with 75 hp drive motors.

My boss thinks we should consider a variable volume system with speed controls on the supply fans. This (he thinks) should saves us lots of money.

I would appreciate some reaction from the experts. Is this something worth pursuing?

Thanks!

 

[remp]

Why dont you put a variable speed drive (frequency inveretor) on your AHU motors which would ramp up and down using PI control to maintain suitable space conditions. Tis is a simple form of VAV and requires minimum work. Dont worry about installing VAV boxes unless you want to go full blown VAV. This is fine if each AHU is serving open spaces, so basicly the AHU becomes the VAV box.

Maintain a minimum off coil temperature until the VSD has ramped down to minimum then start to re schedule the temperature up.

Make sure the diffuser types you are using are suitable for low air flow rate and you will still get the throw required to reach the occupied zone. The VSD should be able to ramp down to 20 Hz fairly safely. This is worth considering. Talk to VSD suppliers. They would be delightl to help if they knew an order for 29 VSD was possible.

Most VSD these days have their own PI controllers too inbuilt

Hope this helps.

 

[quark]

You can do it and it will give you energy saving if you choose suitable control scheme.

I have been using VSDs for AHUs with single fan arrangement. But my prime motive is to take care of extra air during clean filter condition(because pharmaceutical AHUs are filled up with many of them:-().

Suggestion given by billyq seems to be ok except that low air flow in addition to low chilled water temperature may cause icing at the cooling coils and your fan may have to speed up due to fualty reason.

If you already have return air control by temperature, you can operate your variable speed fans by taking a pressure signal just before return air control dampers.

Regards,

 

[ChasBean1]

Herman2 - there could be pressurization issues. The minimum outdoor air delivered by the AHUs should be equal to or greater than the sum of all exhaust. Without return fans, your building pressure could be very positive during full outside air and very negative during full return air. You'd have a full return (or minimum OA) condition when it's hot outside. During this time, while make-up air is low and exhaust continues at design, you'll experience significant infiltration of hot/humid air which could lead to condensation points throughout.

You're already saving money by not having return fans, but at the expense of building pressure. Further reduction in supply fan speed could worsen infiltration and overall IAQ.

A typical building might be 2-4 W/ft2 in supply/return fan power, but 7-12 W/ft2 in air-conditioning. Your fans are 1½ W/ft2. Just some thoughts. I would lean away from VAV control unless you have a handling on minimum OA and total plant exhaust.

Best regards, -CB

 

[wilg]

VAV should only be applied to buildings
or "zones" that generate diverse heat loads
during differing times of the day.
An example of good application of
VAV is a high rise office building
with sun exposure to all sides and a
lot of glass, an eastern exposure
would require a high amount of CFM
in the early morning hours, the western side
would require a reduced amount of cooling
(less air) in the AM, however in the afternoon the
situation is reversed.
If different plant zones have large process
diversity, such as higher cooling requirements
at various times of the day then VAV makes
sense. A bad application example is a building
with zones that generate the same amount of heat
all day long, such as a building with very little
glass that has the same amount of equipment
heat production and same occupancy all day long.
Care should be taken not to apply VAV just because
it sounds high tech, good heat load estimations
should be made for differing zones to assure
a decent (ROI) return on investment period.
Don't waste money on something that cannot
be justified.
wilg5404@aol.com

 

[Rhonda2]

Herman2.

Do you know the supply and return water temperature performance across the cooling coils on a typical cooling day? What is the leaving air temperature setpoint? Is it stable and consistently achieved in operation?

I typically like to explore these issues before considering VAV. We have found many cases where VFDs continue to run at full speed due to poor chilled water distribution system performance. In some cases, high chilled water supply temperatures prevent a low leaving air temperature setpoint from being achieved and can lead to humidity and excess air flow issues.

 

[rogzog]

If you consider using VFDs (Variable Frequency Drive) to modulate the fan speeds be sure to consider that you may need to replace the motors with VFD compatible motors. Not all motors will survive running at lower than design speeds.

 

[lilliput1]

Using VFD usually pays. Motor needs to be inverter duty type. VFD should have manual bypass & line reactor to prevent harmonics. Harmonic filter transformers may also be required if VFD is 50% or greater the switchgear capacity. Determine energy saving using bin data of temperatures & hours of occurance. Estimate internal load profiles for weekday & weekend/holiday. Estimate CFM for each temperature/hour of occurance bin (a day is typically broken down in (3) 8 hour groups). The NAVFAC publishes bin temperature data for various military sites around the world. Come up with kwh usage for both case & difference is energy savings. Use average $/kwh to get cost saving.
Or do a quick estimate of operating hrs x average kw (say 50%) vs x peak kw (100%).
For example at average 50% kw savings, 2550 operating hours/yr, 70 bhp & 90% motor efficiency:
$/yr savings = 70 bhp x .746 kw/hp x 1/.90 eff x .50 saved x 2550 hrs x $0.08/kwh = $ 5918 per year.
Say 75 hp x .746kw/hp x 29 fans x .50 savings x 8760 hrs x $.08/kwh =