Can we use a VFD for reciprocating comp.

[ukram]

we have double stage reciprocating compressor for minus 40c. we are planing to operate the motor through VFD for power saving. Is it posible. Is there anybody who already done this thing.

[US Iceman]

Yes, but on single stage compressors only. The principle would be the same for two-stage compressors.

How do figure there will be energy savings?

Recip. compressors already have linear unloading performance. The percentage of load is very nearly equal to the percentage of power required.

The VFD basically provides unlimited adjustment of capacity control.

Just be careful that you do not slow the compressor down too much as you can loose oil pressure at slow speed operation.

[Andy]

Yes, but on single stage compressors only. The principle would be the same for two-stage compressors.

How do figure there will be energy savings?

Recip. compressors already have linear unloading performance. The percentage of load is very nearly equal to the percentage of power required.

The VFD basically provides unlimited adjustment of capacity control.

Just be careful that you do not slow the compressor down too much as you can loose oil pressure at slow speed operation.

As Iceman states it is usually on single stage compressors. The energy savings are from the quick reaction to varing load.
On low temp systems loads do not react as quick. I can't see any saving being realised.
Try it and let us know but the pay back on capital investment might be non-existant.
Probably better to spend your money on smarter controls

Kind Regards Andy

[Josip]

Hi,

we have double stage reciprocating compressor for minus 40c. we are planing to operate the motor through VFD for power saving. Is it posible. Is there anybody who already done this thing.

yes you can find a lot of VFD, but on new plants or new compressors without separate capacity control, on screws without slide capacity valve and usually only one (last compressor) on the same system. You must made a very good plant design regarding capacity covered by multiple compressors.

The VFD basically provides unlimited adjustment of capacity control.

This is true, i.e. with one compressor you can cover capacity of two , but what if that one is broken

On low temp systems loads do not react as quick. I can't see any saving being realised.
Try it and let us know but the pay back on capital investment might be non-existant.
Probably better to spend your money on smarter controls

This is also true, VFD is quite expensive and...

But if you like it...do it and as Andy said let us know..

Best regards,

Josip

[US Iceman]

This is true, i.e. with one compressor you can cover capacity of two , but what if that one is broken

Very true Josip. The "what if" part of this applies to any decision affecting the system design and operation. These are the things most engineers do not watch carefully. Most will just install several big compressors to meet the full load capacity without looking at part load efficiency and operation.

You will get very good capacity control using a VFD, but I don't think you will see very much energy savings. Unless, a good control system is used to control the kW (demand) and kWh (energy) during average capacity requirements.

[Josip]

Hi,

The question was:

Re: Can we use a VFD for reciprocating comp.

Just be careful that you do not slow the compressor down too much as you can loose oil pressure at slow speed operation.

This can be one side of the problem but here we forget is it allowed to speed up that reciprocating comp for 50% or 100%.

You can try to speed it up, but my suggestion is, stay far away during test

Maybe I am wrong, but I believe compressor must be designed for that kind of operation.

Best regards,

Josip

[US Iceman]

Hi Josip,

I'm not sure what you mean with this statement:

...but here we forget is it allowed to speed up that reciprocating comp for 50% or 100%.

Are you talking about the compressor unloaders or the speed range for the compressor?

Best Regards,
US Iceman

[Andy]

Hi Josip,

I'm not sure what you mean with this statement:



Are you talking about the compressor unloaders or the speed range for the compressor?

Best Regards,
US Iceman

Iceman in my experience he is talking % speed

Grasso make a E range compressor that will run to 1750 rpm (PEAK valve material instead of plates, plastic looking thing), any Sabroe SMC will do this also. Best not go there 1500 rpm is about fine down to 750 rpm.


Kind Regards Andy

[Josip]

Hi, US Iceman

Hi Josip,

I'm not sure what you mean with this statement:
Are you talking about the compressor unloaders or the speed range for the compressor?
Best Regards,
US Iceman

Iceman in my experience he is talking % speed

Grasso make a E range compressor that will run to 1750 rpm (PEAK valve material instead of plates, plastic looking thing), any Sabroe SMC will do this also. Best not go there 1500 rpm is about fine down to 750 rpm.

I am talking about speed. At high speed valves are practically open, there is not enough time for springs to push and close valve plate. The problem is linear movement of masses (pistons) with very fast acceleration and deceleration (very big moment of inertia). That can be very dangerous if you cross the limits you can find your piston stick to ceiling or to wall

For that reason I suggest to stay far away during test

Best regards,

Josip

[US Iceman]

Hi Josip,

I am talking about speed. At high speed valves are practically open, there is not enough time for springs to push and close valve plate. The problem is linear movement of masses (pistons) with very fast acceleration and deceleration (very big moment of inertia). That can be very dangerous if you cross the limits you can find your piston stick to ceiling or to wall

I agree with you. Since VFD's have the ability to change the motor speed the person should be careful to not over-speed the motor. That's why manufacturers have speed limits, although some are not very good at pointing this out.

The range I usually work with is between the manufacturers maximum rated speed and about 50%. This should be safe.

With recip's the unloaders can also be used during speed regulation. If this is plotted out it approaches almost unlimited capacity adjustment ... just like a screw.

[star882]

When I worked with a local group to design the cooling system for some "supercomputers" (basically 4 or 8 Britney CPUs cooled by refrigerant), the VFD was one subsystem that drawn a lot of attention.
I have studied the design, and it's impossible to overspeed the motor - the PLL is limited to 64Hz.

Their latest design (for an 8 way Britney and a 2HP scroll compressor) can run the compressor as low as 20Hz since it has a separate oil pump motor. Along with the EEVs and 200MHz Jessica Simpson CPU, it can regulate temperature very well (-30C+-3, IIRC), even during load swings.

[US Iceman]

...can run the compressor as low as 20Hz since it has a separate oil pump motor.

This is one of the areas to be careful of. Sure you can run a compressor at lower speeds, but, you also have to ensure sufficient oil pressure for lubrication and/or unloader operation. The external oil pump is one method of doing this.

The over-speed is based also on the compressor design. Some small compressors I have heard are capable of operation at 80 Hz. Danfoss I believe.

The important message to remember is to check the manufacturers maximum and minimum application limits.

What works for one compressor may not work for another.

[Renato RR]

I have seen one new Dorin with VFD.It was instold in new supermarket.The compressor was new design special for VFD sistem.VFD was Danfoss.It can keep evaporation on -20 in all situation.System like this cost almost double then normal instalation kit.

Renato

[refteach]

Low speed operation is also limited by the suction and discharge valving on the compressor.

A compressor with ring type valves you start running into a situation where the velocity of the gas is not great enough to keep the discharge or suction valves fully open during the discharge or suction periods of the stroke. If the velocity is too low the valve will have a tendency to wobble and roll around the seat, usually wearing the outside seat on the head or cylinder. Sort of like dropping a coin and having it roll around. The valves need to be fully lifted to prevent this type of wear.

Even a compressor with reed or diaphragm valves may have a problem, if the speed is low enough, the reduced velocity of the gas can even cause these valves to have a problem with fluttering. There is always a oil film on the valves and the oil film will create sticking between the valve and the seat delaying the release of the valve so instead of a simple over pressure, they valve waits to open so once open the pressure drops and the valve recloses and it goes on and on during the portion of the stroke.

[Jack Marcotte]

I am not sure I understand why everybody thinks there will be little if any energy savings. Has anyone heard of the affinity laws? Under part load conditions for a recip, at 50% speed the theoretical energy consumption would be about 1/8 of the power at full speed and you would still have 1/2 the output.

[NH3LVR]

Welcome to the Forum.
Usually people introduce themselves on their first post and tread a bit lightly at for awhile.
Yes, I have heard of the Affinity Laws and so have many others.
I do not have a comparison chart for different speeds immediately available. However THEORETICAL and real world can be quite different. You would need to have a motor one half the size. But you still have to have the torque to start it.
Other factors come into play in Compressor Energy Usage as well.
Are you saying that if I install two compressors instead of one, running them at half speed, I will drop my power bill by 75%?
We will need some real world figures from a Manufacturer confirming this.

[Andy]

I am not sure I understand why everybody thinks there will be little if any energy savings. Has anyone heard of the affinity laws? Under part load conditions for a recip, at 50% speed the theoretical energy consumption would be about 1/8 of the power at full speed and you would still have 1/2 the output.

Hi Jack
that law applies for fans and pumps but not compressors.
50% speed equates to roughly 50% energy input

The savings are in the refrigeration system, due to the close control of the suction and discharge pressures, the motor that is inverter controlled is actually less effecient than the standard motor

If you read my last post you would think I was anti inverter, far from it all our supermarket packs have it and any of the industrial plants that I can make it pay back on are fitted with inverter

Kind Regards Andy

[US Iceman]

Under part load conditions for a recip, at 50% speed the theoretical energy consumption would be about 1/8 of the power at full speed and you would still have 1/2 the output.

The Affinity Laws do not apply to positive displacement machines, only kinetic energy devices such as centrifugal pumps, fans, and compressors.

Andy is spot on with his reply.

With recip's and screws you do get linear part load performance. This is where the energy savings comes from, expecially on screw compressors.

[Peter_1]

Jack has invented the warm water again.
If it was that easy, then the solution of NH3LVR A install two compressors instead of one, running them at half speed, I will drop my power bill by 75% should have been widely used.
With other words, this statement isn't true.

[D.D.KORANNE]

Yes, VFD is a good idea for linear power saving. Only a word of caution : Do not allow the speed of copressor to fall below minimum as per compressor manufacturer`s recommendations . 2) oIL return to compressor to be ensured as speed of compressor reduces in accordance with load demand.

D.D.KORANNE

[gopalakrish]

Hi,
VFD drives, I saw, mainly used in screw, scroll and rotary compressors - compressors with high volumetric efficiency. If it is used used in a reciprocating compressor, there is chance for further reduction in volumetric efficiency at low speeds. Also pls consider the folowing points also:
1. Oil separator, if not used at present, may be required for low speed operation as the refrigerant velocity will be too low to return oil back to compr.
2. If the compressor is hermetic or semi-hermetic type, motor cooling at low speed may be at risk as the refrigerant mass flow reduces.
3. Lubrication system will be in problem, as the flow rate and pressure will decrease at low speeds.

[Peter_1]

If it is used used in a reciprocating compressor, there is chance for further reduction in volumetric efficiency at low speeds.

I should just say the opposite, at high speed, the discharge valves are more prone to flutter.

You have a point with the oil separators.

Reduced mass flow to cool the compressor: the absorbed power decreases +/- in the same way as the cooling capacity decreases, so you need also less mass flow to cool the compressor. I have somewhere a bulletin - think it's from KIMO - which states that you even need less mass flow in comparison with a compressor at full speed.

Lubrication system: it will not be a problem but it can be a problem. We have Bitzers running, mostly at 25 Hz and the oil pressure varies +/- 0.25 bar from the pressure at 50 Hz. I never investigated an oil pump but they're certainly designed with a certain spare flow to counteract thick oil, clogged oil filter, restricted suction port, some wear of the gears in the pump, the inbuild availability for speed reduction,...

[gopalakrish]

Volumetric efficiency of Recip compressors are less compared to Scroll/Screew compressors because:
1. Wire drawing effect - valve pressure drop
2. Re expansion and
3. eakage past piston rings during compression.

When the speed reduces, duration of compression process increases and hence the leakage past piston rings. This is not applicable in scroll or screw. This will surely reduce volumetric efficiency at low speeds in recips.

[US Iceman]

When the speed reduces, duration of compression process increases and hence the leakage past piston rings. This is not applicable in scroll or screw.

I disagree. When the rotor tip speed is reduced (by use of inverter), the tips loose their sealing ability against the compressor housing. The effect is quite dramatic around 1/2 of the nominal full speed.

The valve pressure losses should actually decrease as the recip. compressor slows down as you are pumping less mass flow due to the decrease in swept volume of the compressor.

Re-expansion is due to the pressure ratio seen at the cylinder conditions, so I can't see the speed affecting this.

I'm having a llittle trouble trying to accept the notion of piston rings leaking more at slower piston speeds.

You mentioned the "high" volumetric efficiency of other compressors being superior to recip.'s. There might be a small difference in this at low pressure ratios, but it is not quite as bad as you make it out. At least not on anything I have checked.

Where the screws provide an advantage is when you have a higher pressure ratio application. The re-expansion does limit the recip. compressor volumetric efficiency at higher pressure ratios. This is independent of rotating speed.

The arguments presented seem like a "sales presentation" from a screw compressor manufacturer.

[Andy]

Hi Peter and Iceman

It is hard for engineers to accept that the old reciprocating compressor can perform in the modern age of screws and scrolls

But perform they do

Now I'm talking industrial, recips give better part load and seasonal EER's on chillers.

Compound Recips have better COP,s than even booster screw set up,s for low temperature

What is lacking is not the performance of the recip, but mainly the skills required to carryout overhauls annually. Most customers do little to their screws other than a slide kit or a shaftseal.

Now I not saying that screws don't have their place, but the only reason they are being used so much is due to sales hype and lack of basic compressor rebuild skills

I am working on a little chiller project (about 500kw on ammonia) on ammonia, with a recip and an inverter, with no secondary refrigerant pumps other than the chiller recirculation pump. No packaged screw chiller will beat the seansonal EER on this one, or produce chilled water cheaper.

Grasso have developed a new valve plate for their recips made of "peak" material, looks a bit like kevlar plastic, this removes a lot of the valve bounch and quite a lot of the noise from the compressor

Me I will fit the odd recip where appropriate, other can fit screws as they feel fit

Kind Regards Andy

[US Iceman]

...but the only reason they are being used so much is due to sales hype...

I think that's about 90% of the problem. I can't remember how many times I have heard the same arguments about recip.'s.

As you say Andy, there are places where screws can or should be used and some good reasons for doing so. I am in agreement with that.

But to have a lot of people say screws are best for a bunch of technical sounding reasons is bogus.

Screws are good compressors too. But they do have their areas of application which make sense, and others that just don't.

[stan1488]

hi everyone does anyone have available any case studies or articles written on this topic specifically return on investment for both recip and screw, and different applications specifically heavy-industrial refrig? would be great, i have worked with this applic w vilters on ice rinks ,great control of loading, but my area of interest is roi and projective savings for field retrofit regards , merry xmas!! keep it cold stan

[NH3LVR]

1. Oil separator, if not used at present, may be required for low speed operation as the refrigerant velocity will be too low to return oil back to compr.

Since we know that the oil separator is less than 100% efficient, and that oil will leave the compressor, can we also presume that if the refrigerant velocity is less than required to return oil, that we will eventually have trouble with oil logging in the evaporator?
Unless of course the system only operates at lower speeds for a relatively short period of time.
I occasionally work on plants in cold climates where the loads are very low in the winter for an extended period of time.

[US Iceman]

Since we know that the oil separator is less than 100% efficient, and that oil will leave the compressor, can we also presume that if the refrigerant velocity is less than required to return oil, that we will eventually have trouble with oil logging in the evaporator?

You are on the right track. But we also have to recognize if the system uses ammonia or one of those other refrigerants .

An ammonia system already has a tendency to oil log evaporators if the piping and system are not designed properly, or if the oil pots are not drained. That's just a fact of life.

On the "*****" systems this is much more critical. In one aspect you need velocity to return oil. Suction line/evaporator velocity is developed by compressor volume thru a fixed cross section area (the pipe).

So if the compressor volume is reduced due to a speed where the suction line velocity is lower than the minimum oil return velocity, the oil just lays there.

Now having said that, what I have seen on a VFD recip. compressor on ammonia (all high stages) the total oil consumption is drastically reduced. In fact, on the one system I have been watching the oil consumption has been almost zero for over two years.

Yep, you heard that right. No oil has been put into the compressors in that time. The oil levels just don't change.

I attribute this to the fact of longer operation with much fewer start-ups. The suction pressures are more stable and do not fluctuate like a normal constant speed compressor will see. The oil separators on this system are the old style with demister pads in them, so they are not even close to the performance of a coalescing element.

With "*****" systems you could also program the controls to periodically ramp up the speed for a specified time to increase the oil return velocity.

We need to be careful to address the systems based on certain criteria. Ammonia systems have some different operational details that differ from "*****" systems.

[Mauro]

expensive colleague I do not know which it is the type of installation that voce has, but is very relative. The installation is type production, or conservation of cooled or congealed products already? if it will be production line does not have because to control the compressor, since the same it needs to win the production time, it will be the maintenance of the temperature of the product ai yes has as to reduce the consumption of the compressor.

[Peter_1]

I occasionally work on plants in cold climates where the loads are very low in the winter for an extended period of time.

Just throw this here in this forum: is the load anyhow lower on the evaporator side (while running) in cold winter period?

Let me try to explain: let's take the summer condition.
We have to maintain a temperature in the coldroom of 1°C (33.8°F) and a DT of 2K before cooling again.
So when it becomes 3°C (37.4°C) the compressor starts.
So, what load is the evaporator seeing? An airflow of x m³/h at a RH of x% which must be cooled from 3°C to 1°C.

This load will be the same in the winter. The only thing which will change then very slightly is that the room will reach faster the set temperature because heat infiltration through the walls will be smaller (very minor influence if PU panels are used)
If the compressor is not maintained on a certain pre-set HP, compressor capacity will increase with a lower evaporating pressure (or a bigger DT over the coil) as a result.
Due to the colder outside temperature, liquid temperature will drop, so an increased enthalpy.
But the load seen by the evaporator doesn't change, so the mass flow of the refrigerant flowing will also stay +/- the same.
We service a poultry slaughtery (+/- 6000 chickens/hour) and the compressor were maintained the whole year at a pre-set Hp with pressostats.
They had +/- the same running hours during winter and summer. The only explanation for the longer running hours during winter were because we couldn't hold the HP low enough to the pre-set HP (fans were running continuously)

So the compressor

[NH3LVR]

Just throw this here in this forum: is the load anyhow lower on the evaporator side (while running) in cold winter period?

Peter_1;
Thank you for pointing out my error.I understand the point you are making. The load that a compressor sees is not dependent on the outside temperature and that the coil/room TD is the major factor.
In the Plant I cited however the circumstances are different. I used this Plant as an example, it's design is not one that has an oil return problem.
The Plant is Pump Recirculated, two stage. It has one large evap in the -10F (-24C) area. (This Evap is fed continuously, except during defrost.There are twelve evaporators in the Medium Temp areas.
The normal running mode is with a Screw Booster running at about 90% during the Summer, 30% in the
Winter. The High Stage operates at about 90% Summer/50% Winter. The Compressors cannot be cycled On/Off.
The Winter Average Temp is down 42F (23C). The indication of the reduced load is the reduced load on the Compressors.
But this is not caused by the outside load directly. The Evaporators cycle by the opening\closing of a Liquid Solenoid. It is not uncommon to see only one or two Medium Temp Evaps working in the plant at any one time during the Winter.
So you are correct that the Mass Flow through the Evaps does not change during the time the coil is operating. However the flow through the common suction does.
Cycling the Compressors is the difference in this case.
But lets us look at the case of a system using TX Valves. (Not my best area of expertise, I might say, so feel free to correct me)
During a cooling cycle the Liquid Line Solenoid will come on and admit Liquid to the TX Valve. At first it will feed quite heavily. As the load drops off with the room temperature, the TX Valve lowers the feed rate to maintain the Superheat Setting. The Mass Flow drops off. It can only fall to a certain point as the Liquid Line Solenoid closes when the temperature is satisfied at the Thermostat. In most cases this will still maintain sufficient Mass Flow to return the Oil.
I have seen Blast Freezing Systems (poorly designed/installed) that would not return oil at the end of the freezing cycle. These systems are left to run at low temperatures without any provisions for cycling.
In the example I used of the Summer/Winter loads in this Plant there are many factors involved in the reduced Winter load, besides the outside Temperature.
Infiltration, forklift usage, occupants and other factors all come into play here.

[Peter_1]

NH3LVR, it was not my intention of pointing on an error.
It was more sometimes I allways think on when someone says that load decreases when it becomes cold. This seems not so acceptable for me but it's sometimes difficult to explain your doubts.

[Peter_1]

But this is not caused by the outside load directly. The Evaporators cycle by the opening\closing of a Liquid Solenoid. It is not uncommon to see only one or two Medium Temp Evaps working in the plant at any one time during the Winter.
So you are correct that the Mass Flow through the Evaps does not change during the time the coil is operating. However the flow through the common suction does.

You're completely right that the speed in the main suction header will decrease a lot at reduced load because this header is calculated for max capacity.

How we solve this: we open on a regulary base (4 to 6 times /day) all the SV's of the whole plant for some time to ensure oil return.
Main header is also calulated at minumum 12 m/s (sometimes 15 m/s) at max load conditions.

We once had a freezing tunnel with a pack with big troubles with the problems you mentioned: oil not coming back. The service tech filleld the oil receiver back up again and that was his solution. After some weeks, fill it up once gain and eventually once again.
Till one compressor tripped on overload and we opened the compressor: pistons were broken and we even found pistons turned 90° in their housing.
This freezing tunnel started at 0° (32 °F) and went to -32°C (-25.6°F), so a very big decrease of load.

Another thing that botters me and I'm not sure my point of view is followed by other members: load on the evaporator in a cooling room or a freezing room varies almost never: the heat content of air at 3°C (37.4°C), 2°C (35.6°F) and 1°C (33.8°F) is so small that this can be seen as each time the same load.
So I don't see the need for the so praised EEV's which can adapt fast and precise to various load conditions. But the load doesn't change, only the running time.

[stan1488]

good description of plant peter lol, now who has info on energy reduction w vfd control? soft starting less cycling steady load control etc , has anyone done analysys before and after?

[NH3LVR]

Peter_1
We are digressing from the original point of this thread. I wonder if it would be better if we continued this on another a new thread, related to the percentage of total load caused by the box load. Others could contribute much easier than I. I have not worked these calculations in a long time, and with the software many members use, they could run examples for us very quickly.

[US Iceman]

Well, it looks like this thread got a good workout by Peter & NH3LVR while I was on vacation. Good discussion gentlemen.

It seems this thread was started then abandoned by the original poster.

...now who has info on energy reduction w vfd control?

This is a very in depth topic. The main difference in energy use (for a screw) is due to the part-load efficiency of a screw at full speed versus the part-load efficiency with variable speed.

Part-load efficiency at full speed is dependent on the slide valve position, while the part-load efficiency at variable speed is dependent on the variable tip speed of the rotors with a VFD and various slide valve positions.

In general the part-load performance for a variable speed screw is almost linear from about half speed to full speed. What that means is at 50% speed, the power & capacity will both be approximately 50%.

On a screw operating at full speed with the slide valve in a 50% capacity position, the percentage of power input will be around 75% (this is a number off of the top of my head, just to illustrate the point. It could be a little lower).

So... When the two examples are compared, it looks like you are saving about 25%. That's how it works.

If you do not have the software from the compressor manufacturer you will not be able to do this comparison. Call your manufacturer and they should be able to work up some examples for you.

Does that help explain the process?

[Peter_1]

USIceman, this is the new thread http://www.refrigeration-engineer.co...ead.php?t=6628

[laf100]

My 2 pence for what its worth is; a lot of expense and hassle to fit inverter drives, with probably only marginal energy savings, however if during experimentation/set up you take out a compressor.....or two!!! Thats your savings blown out of the water for the next few years, surley?

[Peter_1]

...however if during experimentation/set up you take out a compressor.....or two!!! Thats your savings blown out of the water for the next few years, surley?

Why you have to take out a compressor? Because you used a VFD? A compressor doesn't fail due to the fact its controleld by a VFD. It's just the opposite.
Or did I misunderstand you?

[stan1488]

thanks US iceman for input and sorry no response , been busy, so deffinitely savings there at variable speed, i should think the soft starting should be more than ample lra savings as well, although im not inclined to mess with screws in the field for a number of reasons ,auxillary oil pumps and overall risks which i can leave to design engineers in factories,, the market i`m interested in infiltrating is medium size plant w open drive compressor vilter, sabro, grasso, etc which there are alot of where i am ,applying vfd , this seems win/win application as well as on condensor fans, steady, consistent plant, eliminating hard starts , short cycling , and bouncing around typical unloaders give us , I have always since had interest in this appl. sine the ice rink plant i have mentioned before, keep it cool boys stan

[NH3LVR]

The Plant is Pump Recirculated, two stage. It has one large evap in the -10F (-24C) area. (This Evap is fed continuously, except during defrost.There are twelve evaporators in the Medium Temp areas.
The normal running mode is with a Screw Booster running at about 90% during the Summer, 30% in the
Winter. The High Stage operates at about 90% Summer/50% Winter. The Compressors cannot be cycled On/Off.
The Winter Average Temp is down 42F (23C). The indication of the reduced load is the reduced load on the Compressors.
In the example I used of the Summer/Winter loads in this Plant there are many factors involved in the reduced Winter load, besides the outside Temperature.
Infiltration, forklift usage, occupants and other factors all come into play here.

I had hoped someone would jump in with a real world example of how loads change in colder weather.
I used this example of a plant that had no process loads.
However, I must apologize the the group as I find I have provide inaccurate information.
During a discussion of this plant a few days ago I was reminded that as part of a Corporate Policy on energy savings the Operator had changed the Suction Setpoint from the 2lbs we had been running to 6lbs (.01BAR to .04BAR).
Therefore the Slide Valve Percentages I gave were of no use to the discussion and misleading.

[US Iceman]

...the Operator had changed the Suction Setpoint from the 2lbs we had been running to 6lbs (.01BAR to .04BAR.

So, with this change are they still running a low slide valve position? Raising the suction pressure is a good thing, as long as temperatures are maintained.

However, if due to this change the slide is forced to operate more unloaded, the raise in suction pressure could cost more than the savings produced.

[NH3LVR]

So, with this change are they still running a low slide valve position? Raising the suction pressure is a good thing, as long as temperatures are maintained.

However, if due to this change the slide is forced to operate more unloaded, the raise in suction pressure could cost more than the savings produced.

Good Morning Iceman
I do not think I explained myself properly.
The Corporate decision was to change Storage Temps from -10F to +2F (-17C to -23C).
The operator adjusted the set point on the Booster to 6lbs from 2 lbs. This unloaded the Booster and the High Stage screw, producing some energy savings.
The total amount of savings is unclear. I would have to compare power use before and after.
As we know, especially with Screws, energy savings are not Linear with loading.
Were you thinking about the difference in Power Usage at the lower slide valve position, but higher Suction?
I wish I had a Software Program like Frick uses. They can give me figures at any conditions or VI instantly.

[US Iceman]

Good afternoon,

Were you thinking about the difference in Power Usage at the lower slide valve position, but higher Suction?

That's it. If the screw were operating at a higher slide valve position at the lower suction pressure, then the compressor capacity would increase at the higher suction pressure.

This would result in the slide valve unloading the screw, and possibly increased power use even at a higher suction pressure.