VFD for AHU

• Easily variable flowrates
• Lower starting currents may be of economic advantage if the equipment is on a peak demand tariff.

Variable speed fan drives are generally used in conjunction with a VAV (Variable Air Volume)airconditioning system. VAV systems are popular where individual space temperature control is required.

Hope this helps

Phil

The first post pretty well covers it. On large units some method must be used to control the static pressure of the system, nowadays the variable frequency drive has replaced inlet vanes.

So economy of operation, and static pressure control are the primary uses. Although most large units incorporate zone controlling with its own flow controls some older systems were basically non-flow controled, constant volume systems.

Retrofitting a variable frequency drive to the systems can aid in economical operation an adds flexibility.

With dramatic price drops in the last 20 years and a much easier setup of newer VFD'S , it is almost a must for a larger system.

On large units some method must be used to control the static pressure of the system, nowadays the variable frequency drive has replaced inlet vanes

This is the primary correct answer. GA.

also that VFD's permit programming of energy saving strategies.

Chris

Due to poor energy characteristics of the constant volume air system particularly during the light cooling load in an a.c system VAV systems are preferred.There are number of variations of VAV systems and also a number of possible combinations of VAV with other systems. Cooling only,VAV reheat and VAV dual duct are some of the configurations.In the cooling only system a single stream of cool air serves all the zones of area to be air-conditioned and a thermostat in each zone regulates a damper to control the flow rate of the air to the zone.VAV reheat system is better than cooling only system.VAV reheat and dual duct systems provide all flexibility . VFD is always associated with VAV systems. Regards, Dr.Ram

Thank you folks, especially to Roman Kabayan.

Having had this done to our unit that controls the zoned area of our offices, I have to say it was a home run. The system is now more controllable and has saved a great deal on our energy costs for running it.

I can also maintain a specific static pressure not only in the ducts but in the office space as well. We installed 2 VFD's in this particular unit. One on the return and one on the supply. They are interlocked to monitor each other as well as themselves.

We chose ABB for this installation and they have been problem free for about 5 years now.

We us it on a 1986 Mamoth 35 ton cooling unit.

air handling unit is a basic part of the ir conditioning unit in which air air is filtered ,humidified or dehumididfied,check for temp ranges and then put bact in the rooms for air conditioning ,the varible frequncy drive gives a variable aspects to modify the controlling of these parameters easily and more effetively for better and simplfied manner,

Roman has covered the basics very well - the main ones being energy consumed dropping by a cube, and lower operating speed allowing motors, bearings, belts, basically all of the mechanical side of things last longer, for the most part.

I'll give you a couple of other considerations to be aware of.

In a commercial application, an air handler with a VFD controlling the supply fan (and possibly on the exhaust fan as well) may have a refrigerant/compressor based cooling section, if it is not served by a central chilled water plant. In that case, there are a few things to file away that you will need to be sure to look at that are not immediately obvious sitting at your desk dreaming of the energy savings to the system when you slow down that fan.

There will be a large portion of the operating season where minimum ventilation rates must still be met by Code, but very little 'load' for cooling is present. So there is a low-end limit for the airflow, when you reach the volume that equals the sum of your minimum ventilation rates for all spaces served. At that airflow, your outside air damper must be at 100%, with all return air exhausted, so you have to have control of that as well, either by dampers or, in larger systems, by controlling an exhaust fan.

Now...as for that "load" that is present in your 100% outside air, which must also be dehumidified as well as brought down to your supply air setpoint temperature. An Energy Recovery Wheel (ERV wheel) will immediately help both with the load as well as energy savings, where the equipment allows for one. Alternatively, a Heat Recovery (HRV) section using a core or other technology where suitable (mostly where exhausted air conditions require zero crossover contamination, or loaded with particulates that would load up a wheel, etc.).

Then, you must consider the refrigerant evaporation coils and system design. Many larger AHU's have a coil for each of several "stages" of cooling, tied to one or a pair of compressors for each coil. IF you have a system where each coil only passes a portion of the full cross-sectional area, you have another problem to address. For instance, a 6-compressor unit with 3 coils may have each of the 3 coils only the size of 1/3 of the area...like a square with a "T" drawn inside of it. A coil at top, one below left and one below right.

Each coil requires a minimum airflow to allow the refrigerant circuit to work efficiently, so a VFD reducing airflow for the system may quickly drop the portion flowing over the active coil below it's stable minimum airflow. e.g. a system designed for 10,000 cfm with 6 stages using 3 coils may need a minimum of 2,000 cfm over any coil in use, but only 1/3 of the airflow goes through any coil, so your minimum is 6,000 cfm. Do NOT install a VFD in such an AHU without also installing some control, like an actuator/damper, to block the reduced airflow from escaping through coils that are not being used, or you will have liquid trying to get back to your active compressor(s).

For a new system, there is "Hot Gas Reheat" to keep the first stage compressor running by false-loading, sending the refrigerant through a reheat coil before coming back to the compressor. This doesn't save as much energy, but keeps your system draining humidity out of the air, reheating it, and avoids short-cycling the compressor.

For the newest in design flexibility, some manufacturers have integrated the new "digital scroll" compressors into their rooftop units.

This allows for a fully variable system, including supply fan, exhaust fan, modulating ventilation air control, an ERV, modulating hot gas reheat, and have modulating exact control of first stage cooling with a digital compressor, to meet your supply air setpoints for both temperature and humidity. This is perfect for high ventilation load applications like schools, especially with loads that move around from zone to zone, or as a fresh-air unit serving outside air tempered to terminal units like geo-thermal heat pump units at each zone.

*Heads-up for what could be considered a referral but is only meant as a reference: I'm just an engineer, not a rep, but it seems apparent that with many bids over the last few years for my recent designs using this approach have been awarded to contractors packaging equipment with the supplier in our area for "Aaon"s RM and RN series, as they integrate all of the above into a packaged system at an efficient enough price point for them to beat local commercial competitors in our market. They have a larger RL series, but I do not design for buildings in that larger size market, just use multiple smaller units instead.

we're running 3 water chillers @ 300tons capacity, 3 chilled water circulation pumps, and 3 condenser water circulating pump, can these be included into such cosideration to vary the speed of all these pumps? i've actually observered that running 2 chilled water circulating pumps with all 3 water chillers, leaving water temperature @ the evaporator is much cooler than running all 3 chilled water circulating pump. WHAT IS THE REASON BEHIND? is the flow rate of the water might be the reason? how it will affects?

i appreciate for your techtips.

By The Way:

For anyone wishing to add the features of variable speed airflow in their home system using a standard electric heat pump system, I have a blatant plug for a company I do not have any connection to.

I found them kind of by accident, back in the pre-Google days, but can highly recommend this controller product, IF you have a motor in your home split-system that was not mass-stamped somewhere south of the border. (Fan with a good quality motor).

I was just looking for a way of putting a relay in the mix, to watch the difference in temperature between supply and return ducts, to keep my fan running on the "low tap" all the time but switch to medium or high taps when needed.

I wanted my electronic filter to stay active 24/7, and knew that the low tap - at about 25% airflow, would dramatically reduce energy use and keep the house air from stratification - keep it mixed - for better comfort.

Remembering the name of the perfect magic packaged electronic controller that I found, I can now let you in on the fact that a quick search tells me they have a little website now set up at FanHandler.

Looks like he's doing a few more things now as well, like using his black box for pump control. Now I have to take another look and see if I can use their stuff for any of my residential designs. We'll see.

Also looks like he won't sell direct to consumers, but only to "contractors." I'd forgotten that - It's still cheaper as a retrofit than the "variable speed fan" option direct from manufacturers...hmmm
...Have fun sleeping after you read his site - you'll likely be thinking up new applications like I am now...;)

Your right that Fanhandler has its place and works well. But is kind of expensive for what you get.

ICM makes one called the CC750 that cost much less and is a much smarter controller. I discovered it after I found the Fanhandler.

The Fanhandler controls motor speed by chopping the sine wave to lower voltage thus lowering RPM. The CC750 is actually an AC Drive that not only controls voltage but the frequency of the motor and is more efficient.

For this reason it is odd that that Fanhandler charges so much more for a product that is much simpler in design and should cost less to make. Of course the CC750 has not been on the market as long; I believe it came out in 2006.

It has a digital display and several parameters that can be adjusted. including skipped frequencies. Like the Fanhandler it monitors return air and the indoor coil temperature. It is advertised mostly for keeping indoor humidity down by lowering the fan speed as needed. It also can ramp down the fan so it runs at low speed 24/7 if desired. It also works well with a Heat pump to keep the supply temperature comfortable when calling for heat. Sells for around $230.00 anyplace ICM products are sold. No this is not a plug either I just really like this product. I will be installing one on my furnace as soon as I get some other projects out of the way.

It is worth checking out