VFD fan savings calculation 1

[garyefficiencycnslt]

Does anyone know how to answer this? I am getting widely varying answers from manufacturers reps trying to sell me results.
A warehouse freezer (-20°F) is served by four (4) evaporator coils. The full refrigeration load in the freezer is 50 tons. Each coil has four (4) fans, each driven by a 5-horsepower motor running at full speed. I am considering retrofitting the fan motors with variable frequency drives (VFDs), with a total project cost of $72,000. The refrigeration compressors serve other loads and operate at 1.8 BHP/ton. The customer’s marginal cost of electricity is $0.085 per kWh. The load profile in the freezer is as follows:


Percent of hours in year Percent of full refrigeration load
2% 100%
10% 95%
12% 90%
15% 85%
20% 80%
15% 75%
13% 70%
10% 65%
3% 60%

The facility operates continuously, except for a one-week shutdown each year.
Each coil is defrosted four (4) times per day for 30 minutes per defrost cycle.
The VFD efficiency is 96% across the range of expected operation. The fan motors are three-phase NEMA premium efficient motors with an efficiency of 90%. At full speed in this application the motors operate at a load factor of 85%. 1.What are theannual energy savings for this project (kilowatt-hours and dollars). 2. Calculate the motor speed for each part-load bin.

 

[waross]

Is this homework??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[garyefficiencycnslt]

No. I am a general contractor. Trying to teach myself energy efficiency retrofitting. Gary gerden. Gerdenroofingandsiding.com there seems to be some uncertainty in the industry about how much savings can be achieved w vfd retrofits. The mfgs reps claim big savings. I am trying to figure out how to calculate savings myself i got this from a teaching manual. I thought if i could get the formula, i could creat a spreadshhet. But im thrown off by the table. I cant figure out how to use the differing loads in the formula i have. Its a summation formula. I need to understand the theory behind my calcs. Im using calculators i get off line im getting different answers. The reps can only give me rough estimtes w their specific eqpt.

 

[waross]

Is this a freezer or a storage facility?
For a storage facility at -20, the heat load may be primarily the heat gain through the warehouse insulation.
For a freezer at light loads the heat loss may still predominate.
Trying to control the heat load with the speed of the fans may lead to unintended consequences.
Consider cycling off one or more evaporators with a simple relay. Cost less than $1000
The cost of running your fans 24/7 looks like a little under $50,000.
You will need a considerable percentage increase in efficiency to return the investment on $72,000 in a reasonable time.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Reducing the fan speeds of the evaporator is probably going to cause problems.

The fans are what couple the air in the freezer to evaporator coils. If you slow them down the expansion valve becomes the wrong one for the evaporator because the evaporator's capacity will be reduced, yet the expansion valve's capacity doesn't change. This kind of thing results in liquid refrigerant returning to the compressor. That's one of the two most destructive things that can happen to a refrigeration system - liquid coming back to the compressor. It can't compress liquid and "slugs" the compressor - refer speak for broken crankshafts, bent connecting rods, and exploded pistons.

Unless the process brings heat into the freezer insulation improvement would be the most energy saving route you could take.

If the load actually reduces, putting a VFD on the compressors themselves would save a lot more energy.

If you want to cut down on an evaporator and you ARE SURE all the product in the room will continue to be cooled you could turn the fans all off on an evaporator as long as you use a solenoid to cut off the refrigerant to that coils also.

Another large saving is reducing the defrost cycles down to exactly what is needed. Every watt-second that is dumped into the space by those Cal Rods has to be yanked back out with the compressors. Using a more refined defrost termination method can save a lot of energy over straight time termination.

Lastly, most places that are doing any kind of production need hot water. Lot and lots of expensive hot water. Instead of dumping a titanic amount of hot air out of condenser units a lot of facilities now install a heat exchanger ahead of the condenser unit and extract the heat into water that they use elsewhere in their process.

I hate to say it, but in a system this large you should have a skilled refrigeration company in the loop or You could get a lot of financial blowback from an unintended consequence.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricuwe]

I fully agree to the statement from itsmoked:

"If the load actually reduces, putting a VFD on the compressors themselves would save a lot more energy."

Putting VFDs on the fans maybe additionally usefull for other resons, e.g. reducing noise emission.

 

[MiketheEngineer]

More insulation??

 

[Strong]

Gary,

Here is a link to an alternative to a VFD and a calculator for variable speed fan:

http://www.fluxdrive.com/technology.html

Select "ASD calculator from Bonaville Power."

Walt

 

[waross]

And beware of running a compressor too slow. If you loose the flywheel effect you may have large current pulses.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

"VFD's save energy." This is pushed at everyone so often you begin to believe it's a universal truth. And what a simple solution - stick a VFD on a motor and sit back and reap the savings. To bad it's not that simple.

You need to start with the basics first. Insulation, defrost cycle, turning unneeded compressors and fans off, and the possible use of the waste heat are good places to start before you even talk about a VFD.

You need more help than a forum can provide. Most manufacturers reps don't have the skills for this project. Heck, many refrigeration companies won't either. You need to find someone with experience, and not who's just holding some nice literature either. Somone who's willing to show you the data from a similar project and willing to take you to the site where you could talk to the operators and see it working.

It would be fairly simple to co-relate the loading to the fan speed and then calculate savings based on the reduced fan speeds. I would suspect most reps would do something like this. However, that approach is overly simple since it's ignoring the compressor cycle requirements.

 

[hvacfanatic]

Was anyone able to ever figure out the answer to the original question? Im curious as well!

 

[zdas04]

hvacfanatic,
On eng-tips.com, the "common wisdom" is often shown to be more "effective marketing" than any sort of wisdom. Several of the guys who've contributed to this thread are at the very top of this field and from the tone of their comments I suspect that they are getting tired of hearing VFD being espoused as the universal answer to all problems.

Keith's thermodynamic approach is absolutely the only way to develop a systematic solution that really does minimize costs with acceptable unintended consequences.

The comment above that to get real savings from a VFD you should put it on the compressor instead of the fans did confuse me. I understood that on dynamic loads (like fans) power consumption dropped as the square of the speed reduction (i.e., cut the speed in half and the power goes to 1/4), and that PD loads (like compressors) were closer to 1:1. Consequently, I've been more excited about varying the speed on dynamic loads than on PD loads. Was the statement an indication that my understanding was too basic or just an observation that the compressor load is so much larger than the fan load?

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[itsmoked]

David; You're quite right about the ratio of power reduction in 'fans' verses 'compressors' but when it comes to a refrigerated space it's NOT about how much fan energy you can save.

How much fan is needed to PROTECT the product has been figured out over the last century. Have you ever been in a big refrigerated space? They can be poorly stacked causing really poor airflow and bad coupling to the evaporator. Turning the fans down borders on moronic. It means the space ceases to be homogenized and flat stops heat extraction from the product in the outer perimeter areas. This can result in poor product quality, spoilage, shortened shelf life, loss, and flat-out mass food poisoning.

Where does it end? Reduce the air flow and you just think you're saving money.

Keeping the product coupled is essential. After that you can reduce the HP dumped into the compressor. Throttle it back as far as you can without violating its oil pressure and you can safely do that. Also the compressor hp will be far larger than the fan hp.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[zdas04]

That is as I thought, but I wanted to make sure that my understanding of power vs. speed for dynamic vs. pd loads was in the ball park.

I've not only been in those spaces, I've help design fixes for the problems you mentioned. I never did consider reducing fan speed, but I did look at changing from air-cooled to water cooled (too big a philosophical change for the owners) and the arithmetic looked very good. Mostly we saw the biggest bang for the bucks being keeping coils clean (i.e., de-insulating) and reducing heat gain from the environment (insulating). After getting those two things right everything else was pissant.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[hvacfanatic]

I completely agree, these were fantastic and interesting answers. Im relatively new to HVAC systems so I was refering to the actual calculations used to come up with the solution to gary's original questions:
"1) annual energy savings for this project (kilowatt-hours and dollars)."
"2. Calculate the motor speed for each part-load bin."

It would be very helpful to know how to calculate the numbers so I can make the most ideal decisions for similar situations.
Thanks!

 

[itsmoked]

hvacfanatic; It would be nice to have tidy equations for those, I agree. Ain't gonna happen as it would take some serious work on-site to come up with ones that actually reflect reality.


David; Another point on turning down the fans. When you do that you actually physically thermodynamically make the refrigerated space smaller. This at the expense of the general perimeter. This does several things to the system. One is the evap coils run colder since less heat is brought to them. In refrigeration, spaces as apposed to freezer spaces, that causes rapid icing of the evap coils. This desiccates the product, requires, dumping more heat into the space because now you need more defrost cycles and has your compressors running more lightly loaded, which is wasteful.

An alternative invited disaster is that if the temp control is out on a peripheral wall where it is no longer well coupled to the air in the space, slowed fans can cost you a lot more energy because the cold air just hangs around the evap local. Now, everything near the evap gets super chilled before it finally gets cold by the sensor. You have wastefully cold stuff in one area and inadequately cooled stuff out on the perimeter.

If you have a case like the OPs it would probably be better to have multiple evap coils scattered all around the space and in the low stock period keep the stock in one area and run only the evaps there. Possibly even partition the space.


On the energy reclamation angle seen a few micro-breweries switch to the condenser heat exchangers for hot water. They got two year paybacks, which is always a good deal.

Keith Cress
kcress -

http://www.flaminsystems.com