energy saving tips

[MeanMack]

Hi, MeanMack.
Welcome back.
This is some information about hot gas defrosting and about optimized suction pressure.
1. Reduce system lift. Don't take it personally, but this is old simplified approach. Suction pressure up, head pressure down. This is major misconception in energy savings in industrial refrigeration. Look, you have two contradicting staments. Floating condensing pressure save condenser energy. It doesn't save compressor energy. To save compressor energy we have to run all condensers at full capacity. What should we do? I think that head pressure and suction pressure should be optimum to minimize plant energy use. This is an idea of optimization.
2. 95% of capacity. I suggest you to read Doug Reindl(IRC) book "Industrial Refrigerating Energy Efficiency Guidebook" page 6-168. Couple years ago Frick and FES had presentations at IIAR conference and they gave similar information about pert load energy use. Not all industrial compressors have poor part load performance. Reciprocate compressor have good part load efficiency.
3. 40-60% of energy savings sounds like fairytale. It can happen at two scenarios.
- New plant. Somebody done estimation of potential energy use based on false assumptions.
- Existing plant. Somebody started up all compressors, set head pressure to 180psig, defrost to 6 times per day and let it run. I didn't see such plants. Typically 10-20% can be saved.
Optimized suction pressure(1) October 2005.doc

Optimized suction pressure(2) November 2005.doc

Optimized hot gas defrosting(1) January 2006.doc

Optimized hot gas defrosting(2) February 2006.doc

Thanks for the welcome back
1. Reduce system lift.
Things that may keep you from lowering head: Hot Gas Defrost, Heated Zones (40-55 degF rooms), Oil Separators, Hot Gas Unloaders, Flooded system delivery, Liquid injection cooling, Water defrost Tanks, Under floor heating, Etc. All have viable solutions.
Misconseptions: Reducing condensing pressure increases fan/pump power. This is true but compressor energy savings will almost always exceed the increase in condenser power. Wet bulb should always keep this in line.

I think this is your optimization thought. There can be a conflict between increasing suction for compressor savings and keeping a good enough Coil TD so Fan Cycling or VFD control can be effective. This balance is different with each application. It is very sensitive to the ratio between compressor and fan power especially because of the non-linear nature of VFD fan energy savings. The evap fan HP can be low enough that operating the compressors at the highest possible suction and operating the fans at full capacity is most efficient. If dealing with large HP fans the VFD might make fan savings a higher priority than compressor. It gets complicated with different process loads on the same compressor header, here more detailed analysis is the only way to optimize. If a plant has one small -20 room and 12) -10 zones the -20 is dictating the evaporating temp. We may find here the savings may be in dedicating a compressor to the -20 room and letting the rest of the plant run a lower temp off the main compressors. Done this several times. In other words the goal is always the same that is the lowest system energy use. Some systems may not like the higher speed of the refrigerant at higher suctions, current limiting may need to be added, lots of issues may arrise. Generally speaking most plants can run at a higher suction and lower (Floating) discharge.

2. I totally agree that recip compressors have a nice linear power to capacity ratio when unloading but not Screws. Their own numbers show it. Mycom, FES, Vilter and Frick. That's why we put VFD's on Screws and not recips, because of the part load performance (we actually have put a few on recips but for smaller load reasons). We can run a compressor speen down to 40-50% and the power to tonnage in a pretty equal linear slope, not a curve like when you start unloading the Slide Valve.

3. 40-60% of energy savings sounds like fairytale.
I think I said 25 to 50% this most being closer to 50.
I don't think I have ever seen below 25% and you haven't seen above 20. One of us needs to get out more.
BTW all the numbers I supplied here are verified by 3rd party audits through major utilities like Boenneville Power, Pacific Power, Puget Power, Seattle City light. These are extensive verifications that can have several months of data. Like the saying goes the proof is in the pudding.

A) Installation Cost - 148,800
Incentive rebates -
documented energy savings before/after 50+% reduction (722,000 kWh/yr) or 26,900 per year. 5.5 year payback

A) Installation Cost - 225,317
Incentive rebates -
documented energy savings before/after 48% reduction (897,000 kWh/yr) or 32,600 per year in cheap Pacific NW power.

Probably 2/3 of the projects I deal with are retrofit. Those are obviously the only ones you can get before/after numbers on. I think we probably agree on more than we might think we may be loosing a few things in translation. (Except the part-load SV and the 20%)

[US Iceman]

The efficiency of "all" industrial compressors degrades as they operate at a fraction of full load capacity.

For screws this is true, for recip.'s not so much. You should not categorize all compressors as the same. Now if you said it's a general tendency for all screw compressors to behave this way (some have different variations of what you described) then I would agree.

Another obvious attempt at low defrost pressure is to find the minimum pressure needed for adequate defrosts and then use a dedicated compressor to supply the hot gas for defrosts. That way you only penalize the system power for a much smaller compressor instead of the whole system.

[Segei]

For screws this is true, for recip.'s not so much. You should not categorize all compressors as the same. Now if you said it's a general tendency for all screw compressors to behave this way (some have different variations of what you described) then I would agree.

Another obvious attempt at low defrost pressure is to find the minimum pressure needed for adequate defrosts and then use a dedicated compressor to supply the hot gas for defrosts. That way you only penalize the system power for a much smaller compressor instead of the whole system.

I agree that this is one option. However, it requires capital investment for re-piping.

[Segei]

This is the artical.
Wet bulb approach April 2007.doc

[Segei]

Part load performance of a screw compressor(Frick)
Energy Efficiency and Enhanced Performance by Applying Variable Speed Drives.doc

[unilevermike]

[QUOTE=Segei;166356]This is the artical.Hi I am interested in learning more on your aproach to this our plant has installed the hench system and i would like to learn to optimize for our conditions . what type of info do i need to get this accomplished. Thank you Mike. We are a production facility using ammonia and are located in maryland

[Segei]

Thanks for the welcome back
1. Reduce system lift.
Things that may keep you from lowering head: Hot Gas Defrost, Heated Zones (40-55 degF rooms), Oil Separators, Hot Gas Unloaders, Flooded system delivery, Liquid injection cooling, Water defrost Tanks, Under floor heating, Etc. All have viable solutions.
Misconseptions: Reducing condensing pressure increases fan/pump power. This is true but compressor energy savings will almost always exceed the increase in condenser power. Wet bulb should always keep this in line.

I think this is your optimization thought. There can be a conflict between increasing suction for compressor savings and keeping a good enough Coil TD so Fan Cycling or VFD control can be effective. This balance is different with each application. It is very sensitive to the ratio between compressor and fan power especially because of the non-linear nature of VFD fan energy savings. The evap fan HP can be low enough that operating the compressors at the highest possible suction and operating the fans at full capacity is most efficient. If dealing with large HP fans the VFD might make fan savings a higher priority than compressor. It gets complicated with different process loads on the same compressor header, here more detailed analysis is the only way to optimize. If a plant has one small -20 room and 12) -10 zones the -20 is dictating the evaporating temp. We may find here the savings may be in dedicating a compressor to the -20 room and letting the rest of the plant run a lower temp off the main compressors. Done this several times. In other words the goal is always the same that is the lowest system energy use. Some systems may not like the higher speed of the refrigerant at higher suctions, current limiting may need to be added, lots of issues may arrise. Generally speaking most plants can run at a higher suction and lower (Floating) discharge.

2. I totally agree that recip compressors have a nice linear power to capacity ratio when unloading but not Screws. Their own numbers show it. Mycom, FES, Vilter and Frick. That's why we put VFD's on Screws and not recips, because of the part load performance (we actually have put a few on recips but for smaller load reasons). We can run a compressor speen down to 40-50% and the power to tonnage in a pretty equal linear slope, not a curve like when you start unloading the Slide Valve.

3. 40-60% of energy savings sounds like fairytale.
I think I said 25 to 50% this most being closer to 50.
I don't think I have ever seen below 25% and you haven't seen above 20. One of us needs to get out more.
BTW all the numbers I supplied here are verified by 3rd party audits through major utilities like Boenneville Power, Pacific Power, Puget Power, Seattle City light. These are extensive verifications that can have several months of data. Like the saying goes the proof is in the pudding.

A) Installation Cost - 148,800
Incentive rebates -
documented energy savings before/after 50+% reduction (722,000 kWh/yr) or 26,900 per year. 5.5 year payback

A) Installation Cost - 225,317
Incentive rebates -
documented energy savings before/after 48% reduction (897,000 kWh/yr) or 32,600 per year in cheap Pacific NW power.

Probably 2/3 of the projects I deal with are retrofit. Those are obviously the only ones you can get before/after numbers on. I think we probably agree on more than we might think we may be loosing a few things in translation. (Except the part-load SV and the 20%)

1. Reduce system lift.
I give one more article to show the difference between minimum condensing pressure and floating cond. pressure. I think that it is clear that this pressure should be floated down to minimum allowable pressure. Today almost everybody float condensing pressures. "We float head pressure" is very popular expression. However, 99% people have little information about this issue. Why to float? How to float? When to float? Head pressure can float itself when wet bulb temperature and/or load change. To save energy, it should be floated at optimum level. What are the benefits of floating? Actually the benefits are not huge. What can we save? I found that good floating can give savings of 15-20% of condenser power because floating saves only condenser power. This is 1-2% of plant power. Not huge savings. How long can we get these savings. Typically in summer all condensers run at full capacity. In winter plants run at minimum allowable pressure(no floating). Spring and fall are seasons to float. Actually I think that lowering minimum allowable head pressure by 5 psig will give savings similar to the floating head pressure.
About suction pressure. I mentioned in my articles that this for single speed evaporator fans. I agree that with evaporator fan VFDs it will be more complicated. However, suction pressure should be optimum based on power use of evaporator fans.
2. Compressor VFDs. These VFDs are expensive. They use additional energy( around 3.5%). Let's compare efficiencies of regular compressor with slide valve and compressor with VFDs. From 100% to 85% regular compressor more efficient because of additional 3.5% for VFD. Efficiencies will be equal around 85%. From 85% to 70% compressor with VFD is more efficient. So if we can run compressor in the range 70-100% efficiencies compressors with and without VFD will be equal. It can be done by choosing the right combination of different compressors. Good designed and good operated refrigeration plant without VFDs have efficiency similar to the plant with VFDs.
3. Percentage of energy savings.
You gave 3 example: 38, 40 58%. This is the range of 40-60%.
You gave 2 additional example with savings around 50%. Can you give us more information about one project? For example. Condensing pressure was lowered from 140psig to 110psig and savings 20%. Defrost changed from...to.... and savings 15%. Suction pressure changed from... to .. and savings 10%............ Total is 50%. It will look more like technical discussion but not sale pitch.