Understanding the variable speed refrigerant compressor’s performance

[Richard Kollmann]

The variable-speed Hermetically sealed compressor is the first real advancement to 12 volt pleasure boat refrigeration efficiency, no longer the one speed one size compressor fits multiple size box’s icebox conversion.

The purpose of having variable speed changes in refrigeration compressors is much the same as having Multiple speed transmissions shifting gears in your automobile. No longer will one size single speed inefficient compressor be used now that we have these third Generation variable speed small Danfoss BD compressors.
The true Energy Efficiency of updated variable speed compressors only occurs when they are operated in a small box where the fixed speed units at 2500 rpm can be reduced to 2000 RPM.
An example, in a three cubic foot box with a variable speed BD35 compressor at 2000 Rpm might reduce energy consumption by 50% of what the older fixed speed 2500 Rpm compressor consumed. There are a number of reasons why this variable speed is more energy efficient, the
biggest improvement is in Volumetric Efficiency. It may surprise you to know that new variable speeds above 2500 Rpm are only 12% more energy efficient than the fixed speed BD2.5 compressor. The variable speed Danfoss BD 35 and BD50 single piston and 3 cm3 cylinder is an upgrade from the BD2.5 cm3 displacements of older fixed speed compressors.

The noticeable change in the new compressor’s is their smaller size and less resistance to gas vapor flow. Volumetric Efficiency inside would be one key to their “improved” design. Volumetric efficiency is defined as a ratio of a gaseous substance entering a cylinder to the volume metric displacement of a piston. The little piston forces this gas into a small volume that creates pressure which is also energy if the gas enters the chamber and completely fills it compresse’s it is all transferred to the next part of the system having a higher Volumetric Efficiency. Compressor speed and Volumetric flow of refrigerant through a hermetically sealed compressor is not understood by people in the marine refrigeration industry. If it was understood there would be better compressor cooling. Compressor manufacturers are concerned with efficiency inside a compressor that depends on ambient pressure,temperature, suction pressure, piston clearance ratio to cylinder and its lubrication.

Ice box conversion refrigeration companies after selecting refrigeration components play a major role in designing an efficient system of refrigerant flow and the controlling of refrigerant TEMPERATURES through each system’s component. Most boat refrigeration people understand that a reasonable differantial pressure across the refrigerant flow control device is important but ignore the low pressure interting compressors importance in volumetric efficiency and compressor cooling. A compressor can only increase pressure of gaseous refrigerant by reducing its volume. Without reduction of gas particles there will not be high Volumetric efficiency from compressors. The low pressure side of the compressor pump is supplied by low density pressure returning refrigerant from the evaporator. As returning gas enters the open space in hemedicly sealed compressor where its refrigerant density is even lower due to internal compressor heat.

Hermetically sealed electrical compressors are cooled by subcooled returning refrigerant, some believe, returning gas to compressor inlet should be 60 degrees F because 70% of compressor’s heat is disposed of by gas flow. The remaining 30% of compressor heat normally is transferred to surrounding air movement around the compressor and last left over process heat is removed by the condenser. .A hot compressor results in low density refrigerant vapor and higher compressor temperature; this results in the ratio of the volume of gas actually delivered to the piston displacement from the evaporator causing compressor to be less efficient.

Design performance of the compressor remains with the Manufacturer, The job shop designing the Icebox conversion refrigeration system must not exceed the operating limits of the selected compressor. Once a system with a variable speed compressor is in operation there are simple ways to check its performance by measuring amperage, compressor heat and compressor cycle off and on running times. A simple test of these small 12 volt compressors is if you can not keep the palm of your hand on top of the compressor dome indicates it is overloaded and high amperage will confirm this.. Mobile boat refrigeration must be protected from overloading pressures caused by lack of cooling. The Danfoss BD system does not have a high pressure switch to stop the compressor before damage is done. Most Danfoss 12 volt systems use a back door method of stopping compressor by determining fan current draw across electronic module terminals F and small + (plus). Since none of the water cooled BD compressors use overload protection reliability of these systems are at risk, Module failures, compressor failures and catistrapic contamination that requires system replacement.

[Bill O]

Richard,
Thank you very much for this great and educational post.

[rslifkin]

Richard, correct me if I'm wrong on this one, but if I remember correctly from reading the datasheets, the BD35 is noticeably more efficient than the BD50, even if you have to turn up the RPM a bit (but not all the way) on the BD35 to get enough cooling? Do you know what is different between the 2 that leads to that? Or is it something to do with the test conditions and doesn't necessarily translate to the real world?

[Richard Kollmann]

Originally Posted by rslifkin

Richard, correct me if I'm wrong on this one, but if I remember correctly from reading the datasheets, the BD35 is noticeably more efficient than the BD50, even if you have to turn up the RPM a bit (but not all the way) on the BD35 to get enough cooling? Do you know what is different between the 2 that leads to that? Or is it something to do with the test conditions and doesn't necessarily translate to the real world?

At -10 degrees C and both at 2500 Rpm Yes, because COP difference the BD 35 has a COP of 1.74 while BD50 COP is 1,62. The higher the COP the more energy efficient. The design objective is work towards a system's electrical energy IN produces the same wattage of cooling energy OUT. I also see my displacement figures were incorrect the BD 35 is 2.00 cubic cm and BD50 2.50 cubic cm.

[Dirk01]

Originally Posted by Richard Kollmann

At -10 degrees C and both at 2500 Rpm Yes, because COP difference the BD 35 has a COP of 1.74 while BD50 COP is 1,62. The higher the COP the more energy efficient. The design objective is work towards a system's electrical energy IN produces the same wattage of cooling energy OUT. I also see my displacement figures were incorrect the BD 35 is 2.00 cubic cm and BD50 2.50 cubic cm.

Following - very very interesting

[Bowdrie]

I think that what Mr. Kollmann is explaining is in some ways similar to the operation of a naturally aspirated piston engine, (like a car going from sea-level to a mountain pass at 12,000ft., or an airplane gaining altitude).
The air becomes less dense, there's not enough WEIGHT of oxygen to make power no matter how much you push the gas pedal.
In a reefer compressor you can't compress something from nothing. As the incoming gas loses density there's less to compress, and at higher than optimum piston speed the cylinder(s) can't "fill up" with an optimum charge.
Heat is always the enemy in a reefer system.

[Richard Kollmann]

Originally Posted by Bowdrie

I think that what Mr. Kollmann is explaining is in some ways similar to the operation of a naturally aspirated piston engine, (like a car going from sea-level to a mountain pass at 12,000ft., or an airplane gaining altitude).
The air becomes less dense, there's not enough WEIGHT of oxygen to make power no matter how much you push the gas pedal.
In a reefer compressor you can't compress something from nothing. As the incoming gas loses density there's less to compress, and at higher than optimum piston speed the cylinder(s) can't "fill up" with an optimum charge.
Heat is always the enemy in a reefer system.

Bowbriet, Air mass change between sea level and 12,000 feet has a differential of 8.56 Psi and refrigerant is less dense and much colder. The closed refrigerant loop of this small system operates at a pressure differential of 110 psi creating high compressor temperatures. Pressure drop and compressor overheating of refrigerant is normally not controllable by a boater only system selection and avoiding compressor overheating problems are my reason for this posting..

[Bowdrie]

Mr. Kollmann, I understand, my analogy was somewhat "loose" in application.
My experience with the "new breed" of variable speed 12V units is very little.
Whilst I've done a lot of the older single speed units my "heart" is really in the more powerful 120AC and engine drive systems, (something with a TXV).
Yes, hermetic compressors are more critical of return gas temps to maintain acceptable cooling, (and oil return).
With the typical 1/4>1/2HP Tecumseh condensing units we could choose between High/Medium/Low temp applications.
They all ran at the same speed, but the valves are different sizes to maintain some volumetric efficiency at the different low-side return temps/pressures.
Engine drives also, I've seen lots of Sanden/Sankyo compressors "burned up", the heads and valve plates turned brown because they didn't get enough return gas to cool them, (they were never designed for "low temp" work anyway).
Perhaps the 12V units might be better with the large 140mm fans?
The little fans commonly used all seem borderline to me, and then people stuff the units into spaces with little airflow, or engine rooms.

[Richard Kollmann]

Originally Posted by Bowdrie

Mr. Kollmann, I understand, my analogy was somewhat "loose" in application.
My experience with the "new breed" of variable speed 12V units is very little.
Whilst I've done a lot of the older single speed units my "heart" is really in the more powerful 120AC and engine drive systems, (something with a TXV).
Yes, hermetic compressors are more critical of return gas temps to maintain acceptable cooling, (and oil return).
With the typical 1/4>1/2HP Tecumseh condensing units we could choose between High/Medium/Low temp applications.
They all ran at the same speed, but the valves are different sizes to maintain some volumetric efficiency at the different low-side return temps/pressures.
Engine drives also, I've seen lots of Sanden/Sankyo compressors "burned up", the heads and valve plates turned brown because they didn't get enough return gas to cool them, (they were never designed for "low temp" work anyway).
Perhaps the 12V units might be better with the large 140mm fans?
The little fans commonly used all seem borderline to me, and then people stuff the units into spaces with little airflow, or engine rooms.

You are jumping all over the place I will respond in the same non constructive way.
The 1/2 HP 120/220AC low back pressure compressor was my standard water cooled compressor. With a 1.5 ton water condenser.

Mobile refrigeration is best with TXV, refrigerant receiver and sight glass. also requires High and Low pressure switch.

The Sanden and Sankyo only fail in refrigeration systems when operated above 1600 Rpm or TXV was too large during pump down start up.

There are good and bad comments you could have said bout this thread instead of non related comments.

[Bowdrie]

Originally Posted by Richard Kollmann

There are good and bad comments you could have said bout this thread instead of non related comments.

Sorry, I'll try not to let it happen again.
Having built/repaired/serviced, and installed, well over 1,000 systems of all types sometimes I get carried away.

[OzeLouie]

The refrigeration industry, like most of the world’s industries and professions, has developed standard procedures, processes and practices that have become the proven foundation of system analysis, performance and comparison.
Compressors across the world are rated to indicate their efficiency as their Coefficient Of Performance (COP) This is a performance rating that tells us exactly how effective a system or compressor is at transferring heat versus the amount of electrical power it consumes, within the operating parameters used. Simple and accurate! See pic and COP ratings highlighted for an example.
Often the COP is quoted as W/W (or watts of heat removed for each watt of electrical energy consumed.)

To establish the theoretical power difference between a compressor at different speeds is simply a matter of checking the makers charts for COP ratings at each of the speeds in question. (all compressor makers produce them)

And don’t expect anything like the guesswork 50% figure!!. Consider also that the shift to a lower compressor speed corresponds with a lesser pumping rate and therefore higher suction pressure resulting in a lesser heat extraction rate, so it’s not all gain! Expect efficiency gains of a net 5 to 8% to be realistic.



Cheers and Merry Christmas to all...
OzeLouie

[Richard Kollmann]

Louie, You failed to mention you used CECOMAF metric industry standards and I used ASHRAE standards for COP comparison. As far as I know Technautics Cold Blue costumers
demonstrated that the BD 35 compressor with the right system design running at 2000 Rpm could cut daily energy consumption in half. I believe the success of this design was do to low compressor speed, oversize condenser, directional fan cooling and smaller gas cavity inside compressor all of these reduce compressor heat. You participated in a resent thread with exceptionally low energy performance from a BD35 Isotherm system although freezing section lacked low enough temperatures. The purpose of this thread was to point out there are ways to operate these units efficiently by pointing out some of the weeds in their designs.

[OzeLouie]

Originally Posted by Richard Kollmann

Louie, You failed to mention you used CECOMAF metric industry standards and I used ASHRAE standards for COP comparison. Absolutely, metric like most of the world. (The USA, Liberia and Myanmar the exceptions! ) But regardless the outcome is the same! So what is the point?

As far as I know Technautics Cold Blue costumers
demonstrated that the BD 35 compressor with the right system design running at 2000 Rpm could cut daily energy consumption in half. I believe the success of this design was do to low compressor speed, oversize condenser, directional fan cooling and smaller gas cavity inside compressor all of these reduce compressor heat. Richard, this is quite wrong.. The excellent product that Rich Boren and Charley manufacture is a EUTECTIC system which DO NOT benefit from reducing compressor speed but instead may benefit from increased speed when power supply is abundant. (See ECO2 at my web site for ref) Also they don't use 2.00cc BD35 compressors. The efficiency Rich quotes relates to their systems being eutectic and nothing to do with compressor speed!!

You participated in a resent thread with exceptionally low energy performance from a BD35 Isotherm system although freezing section lacked low enough temperatures. Yes I was involved and pleased that the member followed the suggestions and heat load calculations provided. The outcome for him was excellent and an example of how to do things properly! The purpose of this thread was to point out there are ways to operate these units efficiently by pointing out some of the weeds in their designs.

Best to stick to industry procedures if you want accurate outcomes.

[Richard Kollmann]

Originally Posted by OzeLouie

Best to stick to industry procedures if you want accurate outcomes.

I again will let readers decide as they have done before.

[malbert73]

So I assume my Frigoboat system with Merlin speed control and BD35 compressor achieves this by ramping down compressor speed once box is chilled down ?