head pressure energy efficiency

[lois]

I work for a food storage warehouse, it is a 384,000 square foot facility, about half of it is cooler and freezer. As most companies are doing these days we are trying to become as efficient as possible. We are in the middle of a refrigeration commissioning and one of their suggestions is to lower the head pressure from 176 to around 140. Has anyone out there had any experience with this.

[US Iceman]

Yes. If you can run at 140 psig and the system works OK, go for it. If you have enough condenser capacity to do this in the summer it would be worth it. Not only does it reduce the kW demand the systems uses it will also decrease the kWh. Both are worth money/savings.

In the colder weather it would be worthwhile to run even lower, again if the system can without creating any operating issues (longer defrosts, increased transfer times on gas powered pumper drums, etc).

The majority of what you can do is either limited by:

1. The overall system design and type of equipment
2. How much money you can spend to lower energy use.

[Drew]

It is beneficial to lower your head pressure from a compressors capacity point of view. make sure your TEXV can handle the lower pressure drop across it.

On a plant of that size it would be worth getting a consultant involved to establish the new balance points and increased capacity which equates to less running time? And how your humidity could be altered.

[Sandro Baptista]

Lois

I think a lower condensing temperature is worthed. depending of the suction pressure of the compressors and the refrigerant you can also get more capacity and less compressor wear, (besides the kWh savings as ICEMAN point it).
For R404A maybe you get around an increase of about 10% » 15% (I would to check to confirm). For NH3 the increase of the capacity would be less but discharge temperatures would be great reduced.
The food storages would get a little more quickly is regimen temperature, the food would cooler a little more faster.

[US Iceman]

make sure your TEXV can handle the lower pressure drop across it.

This may be one of the issues encountered with low discharge pressure operation. Although....140 psig should be OK for NH3 thermostatic expansion valves.

Being able to run at 140 psig may be more involved than simply adjusting controls though.

[D.D.KORANNE]

If it is water cooled condenser ,find out what would be the water flow required to bring down to 140 psi pressure. It will of course need to expend on pump energy , but it could achieve the purpose .Strike a balance between additional energy spend on water flow & saving derived from lowering the head pressure. May be an external cooler can be added to sub-cool the liquid .

[Magoo]

Everyone is now super enery efficient, driven by accountants that want to scrimp on costs/ cash out flows.
Be aware that lowering head pressures will totally screw up system balance, compressor compression ratios change, compressor volume flow rates increase, oil carry over increases, oil separators go out of spec. evaporators control valves start to hunt. Flood backs happen.
For a short period you save money, to the detriment of the system integrity. Maintenance and plant failure cost esculate out the window and the fridge engineer is the culprate.
So be careful, what ever you do.
In this neck of the world we have winter versus summer power rates, add peak load timing at certain times of the day, my general advise to clients is to manage peak loads daily and actually stop plant at particular timings, this saves energy costs and takes out penalty costs from supply co's., when major plants do not need to run.
magoo

[Segei]

Everyone is now super enery efficient, driven by accountants that want to scrimp on costs/ cash out flows.
Be aware that lowering head pressures will totally screw up system balance, compressor compression ratios change, compressor volume flow rates increase, oil carry over increases, oil separators go out of spec. evaporators control valves start to hunt. Flood backs happen.
For a short period you save money, to the detriment of the system integrity. Maintenance and plant failure cost esculate out the window and the fridge engineer is the culprate.
So be careful, what ever you do.
In this neck of the world we have winter versus summer power rates, add peak load timing at certain times of the day, my general advise to clients is to manage peak loads daily and actually stop plant at particular timings, this saves energy costs and takes out penalty costs from supply co's., when major plants do not need to run.
magoo

I think that energy can be saved for every refrigeration plant. Just do it right. Tuning up of the plant should be done step by step. Lower condensing pressure by 10psig and monitor operation of the plant for a week. If no problem, lower additional 10psig.
Certainly, there are barriers to low head pressure, but every barrier has solution. Very often there are several solutions. Choose the right one for your plant. If you understand the reasons of events, it isn't difficult to tune the plant up.

[US Iceman]

...every barrier has solution.

I agree. It is only a matter of finding out what is required to make the system operate well with the minimum amount of energy.

[Magoo]

Like I said be careful, currently have a situation that client has dropped condensing to 28'C, really performs and efficient, but checking compressor ratings tells me the flow rate through screw has increased by nominal 20 %, which explains gradual oil reduction in separator.
magoo

[Segei]

Like I said be careful, currently have a situation that client has dropped condensing to 28'C, really performs and efficient, but checking compressor ratings tells me the flow rate through screw has increased by nominal 20 %, which explains gradual oil reduction in separator.
magoo

As far as I understand you talk about ammonia refrigeration plant.
1. Lowering condensing pressure won't increase mass flow. At low condensing pressure density of the discharged gas will be reduced. At the same mass flow, velocity of discharge gas in oil separator will increase. At certain point this increased velocity will be the reason of oil carry over. Two ways to solve this issue. A. Increase size of oil separator. B. Reduce mass flow.
2. Assume that we loose oil. This oil can be separated in oil pots and drained. At lower condensing pressure may be we can loose $1,000 of oil, but $20,000 of energy can be saved. $19,000 in our pocket.

[US Iceman]

Oil separators are only one of the issues you have to deal with at low discharge pressure. Others will show up also.

[Segei]

As I mention earlier tuning up should be done step by step. This approach will give us opportunity to solve the issues one after another.
Many plants have been operated at minimum head pressure 120-150psig for years. If today we change this setting to 80-100psig, it will be shock for the plant and for the operators. Gradual adjustment is the way to go.

[US Iceman]

If today we change this setting to 80-100psig, it will be shock for the plant and for the operators.

That is true, but it is so much fun to watch their faces.

[Magoo]

Sergie, dropping the condensing temp aka condensing pressure will increase flow rate through compressor, as in lowering the compression ratio.
Reducing effective liquid pressure to expansion devise reduce flow rate through valve, evap capacity will go ballistic, vavs will hunt, super heats at coil and compressor will ramp around, but you will save energy, the system balance will go out the window.
In general false economy.
magoo

[Segei]

Sergie, dropping the condensing temp aka condensing pressure will increase flow rate through compressor, as in lowering the compression ratio.
Reducing effective liquid pressure to expansion devise reduce flow rate through valve, evap capacity will go ballistic, vavs will hunt, super heats at coil and compressor will ramp around, but you will save energy, the system balance will go out the window.
In general false economy.
magoo

1. Mass flow won't change at lower condensing pressure. Compressor Frick RWB-II 134(ammonia) has ref.capacity 111.1TR at suction pressure 3.6psig and head pressure 151.7psig. It will have ref.capacity 114.5TR at suction pressure 3.6psig and head pressure 125.8psig. Useful capacity increased by 3% due to less flash gas at 125.8psig head pressure. Mass flow of the compressor practically won't change.
2. Ammonia refrigeration plants usually have TEVs for high temperature application(dock, high tem. cooler). Major ref. load for this coolers is heat transmission from ambient air. When ambient air is cool condensing pressure can be reduced and capacity of TEVs will be reduced as well. However, temperature in this cooler will be kept because ref. load will be less than at design conditions. At low condensing pressure TEV won't hunt because it undersupply the liquid to evaporator and TEV will be wide open. TEV can hunt when it oversupply the liquid.

[US Iceman]

Mass flow is based on the suction density of the refrigerant and the volume of gas the compressor pumps.

CFM X suction gas density = mass flow.

However, I think you are going in the right direction Magoo. For a fixed mass flow the volume of discharge gas does change drastically when the discharge pressure changes.

If we change the above equation to: mass flow X 1/discharge gas density = discharge CFM, which is volume flow through the separator. This is an oil separator sizing issue, but only one of them!

To finish this off quickly.... If the mass flow remains constant (compressor % load does not change) the lower discharge pressure increases the compressor capacity because the liquid enthalpy is lower (colder). Therefore the net refrigeration effect increases, which raises the actual compressor capacity.

[MeanMack]

The most efficient way to control head pressure is using calculated wetbulb with an approach. There are several NH3 industrial plants who run down as low as 90-100 pounds. If they need higher head pressure for defrosts the head pressure can be increased while the coil is pumping down. The smaller the delta we can achieve between suction and discharge the more efficient the compressors will run. The efficiency of a compressor can increase as much as 2% per DegF of reduction of condensing temperature. During the winter months the temperature can be much lower than in the summer.

Increasing the suction pressure as much as possible is another way to decrease the delta.

[Magoo]

Hi US Iceman. The plant I was referring to was a spiral freezer, not a coldstore situation, so plant goes through pulldown operation daily. I have restricted load rates etc., to ease the situation, but changed the SDT control point higher.
Should have mentioned that at the time of post.
magoo

[Segei]

The most efficient way to control head pressure is using calculated wetbulb with an approach. There are several NH3 industrial plants who run down as low as 90-100 pounds. If they need higher head pressure for defrosts the head pressure can be increased while the coil is pumping down. The smaller the delta we can achieve between suction and discharge the more efficient the compressors will run. The efficiency of a compressor can increase as much as 2% per DegF of reduction of condensing temperature. During the winter months the temperature can be much lower than in the summer.

Increasing the suction pressure as much as possible is another way to decrease the delta.

- It is good idea to float head pressure. What is the approach should we choose?
- I agree about compressor power. However, we all interested to save energy for whole plant. It means that we have to estimate total(compressors+condensers+evaporators) power use and find the ways to minimize it.

[Gary]

1. Mass flow won't change at lower condensing pressure. Compressor Frick RWB-II 134(ammonia) has ref.capacity 111.1TR at suction pressure 3.6psig and head pressure 151.7psig. It will have ref.capacity 114.5TR at suction pressure 3.6psig and head pressure 125.8psig. Useful capacity increased by 3% due to less flash gas at 125.8psig head pressure. Mass flow of the compressor practically won't change.

This assumes that the liquid temp does not increase on the way to the coil. Insulated liquid lines?

[US Iceman]

What is the approach should we choose?

I don't think you can use a single approach temperature. The problem arises if you use more energy driving the condensing temperature down with fan power, than what you might save on the compressors in some situations.

But... I think you already know this because of what you wrote below:

It means that we have to estimate total(compressors+condensers+evaporators) power use and find the ways to minimize it.

[US Iceman]

This assumes that the liquid temp does not increase on the way to the coil. Insulated liquid lines?

No, it assumes the liquid feed temperature is lower at the reduced condensing temperature. That increases the net refrigeration effect per pound or kilogram.

The actual swept volume may increase by a very small margin because of reduced pressure ratio, which increases the volumetric efficiency just a little.

Mass flow is essentially a function of the suction gas density and the swept volume of gas the compressor moves. Compressors do not produce Tons of cooling. The refrigerant properties do.

[MeanMack]

- It is good idea to float head pressure. What is the approach should we choose?
- I agree about compressor power. However, we all interested to save energy for whole plant. It means that we have to estimate total(compressors+condensers+evaporators) power use and find the ways to minimize it.

If you live in a climate with 4 seasons you could use an approach of between 12-14 for the cooler months and during the warmer months maybe 10. If it gets really hot go as low as 8. During ambient temps of above 100 you may want to just put a high limit on the discharge as wetbulb can get quite high. You would also want a low limit on the floating setpoint during the winter, I like 90-100.

You may have to run a couple 20-30 hp condensers fans/pumps a little harder to achieve the lower head but your 200-300+ hp compressor(s) having an easier go of things should more than make up for the smaller motors running harder. You are correct, however, we need to watch and make sure we are not costing ourselves energy but this can be easily checked with our min/max setpoints. All systems, as you know, are different to varying degrees and the operator needs to fine tune to his conditions. The product is the bottom line.

I also like to see drives on the condenser fans as well as the evaporators. This can help smooth out the load (Plus saving tons more energy). A drive on a compressor used as the trim machine is an added bonus.

[Segei]

If you live in a climate with 4 seasons you could use an approach of between 12-14 for the cooler months and during the warmer months maybe 10. If it gets really hot go as low as 8. During ambient temps of above 100 you may want to just put a high limit on the discharge as wetbulb can get quite high. You would also want a low limit on the floating setpoint during the winter, I like 90-100.

You may have to run a couple 20-30 hp condensers fans/pumps a little harder to achieve the lower head but your 200-300+ hp compressor(s) having an easier go of things should more than make up for the smaller motors running harder. You are correct, however, we need to watch and make sure we are not costing ourselves energy but this can be easily checked with our min/max setpoints. All systems, as you know, are different to varying degrees and the operator needs to fine tune to his conditions. The product is the bottom line.

I also like to see drives on the condenser fans as well as the evaporators. This can help smooth out the load (Plus saving tons more energy). A drive on a compressor used as the trim machine is an added bonus.

Optimum wet bulb approach depends of 2 factors.
1. Condenser power. One condenser can use twice more energy than another one for the same heat rejection.
2. Level of wet bulb temperature. From my experience optimum wet bulb can very from 8F to 30F.
I prefer to have winter head pressure as low as possible down to 60-70psig.
I don't think that operator can do fine tuning of the plant.

[US Iceman]

Funny, the OP never returned to carry on with the discussion.

[US Iceman]

I prefer to have winter head pressure as low as possible down to 60-70psig.
I don't think that operator can do fine tuning of the plant.

Agree with both points.

Optimum wet bulb approach depends of 2 factors.
1. Condenser power. One condenser can use twice more energy than another one for the same heat rejection.
2. Level of wet bulb temperature. From my experience optimum wet bulb can very from 8F to 30F.

Just want to add one point to the above...

The limit on the approach you can get with an evaporative condenser is function of the condenser capacity also. If the condenser is over-sized the approach temperature will be lower.

If the condenser was sized for the required heat rejection at 95°F (35°C) condensing the approach will be higher than what it would be it the condenser were over-sized. So, I think the 8° (F) minimum approach number you quoted is a relatively good value. Anything much smaller and the condenser cost would be very high and have a low ROI (return-on-investment).

[Sridhar1312]

If it is air cooled you can put adiabatic cooling pad (celldeck) and spray water to lower the entering air to condenser.
This will bring down the condensing temperature and save energy.

[MeanMack]

Optimum wet bulb approach depends of 2 factors.
1. Condenser power. One condenser can use twice more energy than another one for the same heat rejection.
2. Level of wet bulb temperature. From my experience optimum wet bulb can very from 8F to 30F.
I prefer to have winter head pressure as low as possible down to 60-70psig.
I don't think that operator can do fine tuning of the plant.

Good points. Some Fans are terribly inefficient and should be turned off as soon as possible to allow more efficient ones to carry the load. Also the operator should be able to fine tune a few parameters as seasons change such as minimum and maximum allowable floating head pressure, the approach, in what orders fans come on/off and at what rates we do this. 60/70 psi of head in the winter months, that's great, I love it.

[MeanMack]

One thing I have been thinking about is having a floating Approach to go along with the floating head (and floating suction). Anytime I float anything I would put in a min and max so the ideal world of math won't screw up the reality of the world we live in. But as the ambient temperature rises you could lower the approach by a certain calculated value or even at a certain rate, and as the ambient lowers you could increase that number. In my humble opinion I would not want to ever see it higher that say 18 or lower than 6. I would be interested in others thoughts about this.

[Gary]

When the ambient temp drops to or below the refrigerated space temp, the operating costs should be close to zero.

What remains is to find ways to make it happen.

[Segei]

Good points. Some Fans are terribly inefficient and should be turned off as soon as possible to allow more efficient ones to carry the load. Also the operator should be able to fine tune a few parameters as seasons change such as minimum and maximum allowable floating head pressure, the approach, in what orders fans come on/off and at what rates we do this. 60/70 psi of head in the winter months, that's great, I love it.

Minimum and maximum allowable head pressures should not be changed. For example.
Minimum is 120 psig, because of hot gas defrosting or liquid injection oil cooling or oil carry-over....
Maximum is 180-200 psig. This is safety issue.
I don't see reason to change these settings. However, to save energy minimum should be reduced as low as possible.

[Segei]

One thing I have been thinking about is having a floating Approach to go along with the floating head (and floating suction). Anytime I float anything I would put in a min and max so the ideal world of math won't screw up the reality of the world we live in. But as the ambient temperature rises you could lower the approach by a certain calculated value or even at a certain rate, and as the ambient lowers you could increase that number. In my humble opinion I would not want to ever see it higher that say 18 or lower than 6. I would be interested in others thoughts about this.

Two major factors influence on optimum wet bulb approach. Level of wet bulb temperature and condenser efficiency(power use per unit of heat rejection). I prefer the range from 8 to 30degF. Any settings have certain dead band. To keep 8, wet bulb will change from 7 to 9degF. 7 is very close to border where condenser power use can jump. So I prefer 8.

[US Iceman]

Part of the lower limit on approach temperature is how much condenser capacity you have and and how much the total heat rejection load is. Larger condenser will allow a lower approach just as a lower heat rejection from the system will. Therefore, I think the actual optimum approach has t take into account what is happening in the system and not an arbitrary value.

Although, for a specific system the value could be determined for that system and then used as a target.

[Segei]

I agree. One of the conditions for floating head pressure is that condenser should not be undersized. Idea of this floating that condenser capacity should be reduced when it is oversized compare to current refrigeration load. If initially designed condenser was undersized, most likely it will never be oversized even when refrigeration load is reduced.

[US Iceman]

I wish the OP would come back and be involved in the discussion....

[Segei]

I hope so.

[MeanMack]

Minimum and maximum allowable head pressures should not be changed. For example.
Minimum is 120 psig, because of hot gas defrosting or liquid injection oil cooling or oil carry-over....
Maximum is 180-200 psig. This is safety issue.
I don't see reason to change these settings. However, to save energy minimum should be reduced as low as possible.

There would always be limits so the operator couldn't put in too high of a discharge setpoint. The low end is typically set to 90 or 100 and left. Even if the minumum pressure for a defrost is 120 you can keep the pressure down as low as 90 (if that is Wet-Bulb) and only bump it up during defrost then back down during normal refrigeration. The only reason for this is energy savings.

[MeanMack]

Two major factors influence on optimum wet bulb approach. Level of wet bulb temperature and condenser efficiency(power use per unit of heat rejection). I prefer the range from 8 to 30degF. Any settings have certain dead band. To keep 8, wet bulb will change from 7 to 9degF. 7 is very close to border where condenser power use can jump. So I prefer 8.

The lowest I've seen the approach is 6 during the summer months. Usually when ambient starts rising above 100 degF floating head will either need to be clipped by a upper limit or use a manual setpoint. During the winter the pressure is usually running way below the minimum setpoint even at an approach of 30. I like 8 during the summer and just let it hold at 14 during the winter.

[MeanMack]

I agree. One of the conditions for floating head pressure is that condenser should not be undersized. Idea of this floating that condenser capacity should be reduced when it is oversized compare to current refrigeration load. If initially designed condenser was undersized, most likely it will never be oversized even when refrigeration load is reduced.

We run into undersized condensers all the time on older facilities. They typically have capacity issues during the summers months, temps over 100 degF. Hopefully any newer facilities will address this in thier design and add larger fan motors.

[Segei]

The lowest I've seen the approach is 6 during the summer months. Usually when ambient starts rising above 100 degF floating head will either need to be clipped by a upper limit or use a manual setpoint. During the winter the pressure is usually running way below the minimum setpoint even at an approach of 30. I like 8 during the summer and just let it hold at 14 during the winter.

Probably, you mentioned about 100 degF of dry temperature. For evaporative condensers wet bulb temperature is important. In Florida design wet bulb is one of highest in USA(around 80 degF). Assume that sometimes it goes up to 82 degF. 82+8=90 degF condensing temperature or 166 psig condensing pressure. It is not bad to have maximum 166 psig of head pressure. Upper limit should be 190-200 psig. So wet bulb approach shouldn't be issue. No significant difference for energy savings between 6 and 8 degF of wet bulb approach. Very rare you can oversize condenser when wet bulb 80 degF or higher. Undersized condensers is real issue for many plants.

[Segei]

We run into undersized condensers all the time on older facilities. They typically have capacity issues during the summers months, temps over 100 degF. Hopefully any newer facilities will address this in thier design and add larger fan motors.

I prefer larger heat transfer surface and smaller fan motors. Increased size of the condensers will give us opportunity to float head pressure and maximum head pressure will be lowered. During initial design size of compressor el. motors can be reduced as well.

[US Iceman]

I prefer larger heat transfer surface and smaller fan motors. Increased size of the condensers will give us opportunity to float head pressure and maximum head pressure will be lowered. During initial design size of compressor el. motors can be reduced as well.

This is my recommendation also. When you evaluate condensers your are looking for the ratios of fan/pump power to heat rejection. When the motor sizes are smaller the condenser coil size quickly increases.

Ideally, there is a lower limit of using more fan/pump power than what you save on the compressors motors if the intent is to make an academic exercise out of it.