inverter vs liquid amplification

[fridge51]

anybody got any thoughts on which is a better option

liquid amplification or inverter driven compressors with or without using electronic exp valves?

[US Iceman]

As a starting point, I like the use of inverters for a wide range of capacity modulation and stable control. This also helps the TXV's in my opinion, since the pressure changes within the system are much smaller.

When designing a system we are faced with several desired requirements. Low energy use during all operating conditions (full load & part load, Winter & Summer), and stable control.

I believe the inverters help to provide the stable control for condensers and compressors. Inverters used on the evaporators also provide a two-fold purpose, i.e., lower power use for the fans, and reduced refrigeration load due to the lower power input to the fans.

If we can obtain the lower stable control and capacity modulation with the inverters, the next area to address is the compressor operating conditions.

Since the power use decreases rapidly (with a smaller associated increase in capacity) as the discharge pressure decreases, this will provide the largest reduction in power input.

If the electronic TXV's (or balanced port TXV's) can accommodate the much lower condensing temperatures (& the resulting lower pressure differential), the HySave system only provides marginal gains.

In other words, if the TXV's can control the evaporator superheat and capacity requirements down to 15C to 10C you are already getting the biggest gains without a refrigerant pump.

Note: you need to check with the compressor manufacturer to see if their compressors can operate at 10C. Some may have lower limits on condensing temperatures.

My thoughts for a beginning to this discussion...

[fridge51]

hi us iceman

nice to see an informed reply

a couple of points i would like to add, there are already compressor manufacturers that produce semi hermettic recips with inverter control, that work to low evap temps, and work fantasticly well with electronic txv`s, in fact a certain chain of supermarkets in this country are already convinced with them all new plant is to be along these lines, trials have shown 30% + energy savings.

i fully expect amajor increase in compressor manufacturers following this line of attack,

regards

fridge51

[Peter_1]

Liquid amplification and VFD controlled compressors are something completely different.

They both interfere in the system in different fields, different places in Marc's log/p.

Both can be used in the same system and both will give you savings, different savings, but they will save energy not the same way, not from the same source. (difficult to explain in English for me)

[US Iceman]

Hi fridge51,

...trials have shown 30% + energy savings.

I do not doubt it. I have done some supermarket work to know this is completely reasonable. In fact, I think a 50% savings could be generated by getting more aggressive with system design, inverters, and very good controls.

One of the problems with compressor manufacturers is that they provide the compressor ratings only down to about 24C. Copeland for one. The Discus compressors have a lower condensing limit at this temperature.

They do have a relatively new Discus compressor model that is supposed to be rated down to about 10C.

I used some of the first generation of Discus compressors on R-12 and R-502, where we could condense at about 4.5C in the winter.

For this project I used the balanced port valves, and they worked great for this application.

This worked very well. The compressors just purred like a kitten. Oh, by the way... I also used an evaporative condenser on this project.

The highest energy input requirement is during the summer. The evaporative condenser alone makes a big difference since you can drop the condensing temperature to 35C as a maximum versus, say 46C or greater with an air-cooled condenser.

One of the most important areas to concentrate on is the liquid quality being feed to the TXV'x (or EXV's). If you can provide 100% quality liquid to the TXV's (and maybe some subcooling for good measure), this should take care of most of the problems.

The second area is the compressor ratings... Most use a constant return gas temperature for the rating tables. If the evaporators are set up to provide a small amount of superheat, this will minimize the return gas temperature and increase the compressor capacity.

Since most supermarkets have long piping runs, the potential for liquid flood-back is minimal.

I suppose there are a lot of things we can do and make these systems work very well. Some manufacturers depend on too many gadgets, instead of well designed systems.

[US Iceman]

Always being mounted high on the roof they manage 100% liquid delivery even when the head has floated so low the liquid line frosts.

Static head is the cheapest form of subcooling, but it is the most dependent on the installation.

...at around 10% speed, by inverter speed drive regulation...

What is the minimum speed to maintain the oil pressure in a semi-hermetic compressor?

I am assuming you are saying this is 10% of design (a 90% reduction), not a 10% reduction in speed. If so, I would expect the oil pressure to be near the minimum oil differential pressure, if not below.

[Argus]

When all is said and done and the relative technical advantages are compared, you may also consider that, in the UK, both technologies qualify for Enhanced Capital Allowances (ECAs).

Your client?s accountant will be interested in the respective claim values, which differ and are claimable in the first year.

.
________
BMW 3 Series (E46)

[Peter_1]

What is the minimum speed to maintain the oil pressure in a semi-hermetic compressor?
I am assuming you are saying this is 10% of design (a 90% reduction), not a 10% reduction in speed. If so, I would expect the oil pressure to be near the minimum oil differential pressure, if not below.

I wrote some time ago a mail to Bitzer because we had a compressor (+/- 7,5 HP) which we tested some days at 15 Hz (instead of 50 Hz)
We monitored oil-pressure every second and the oil pressure remained allways at a DP of +/- 3 to 3,5 bar which is more then sufficient.
At 10 Hz we still had 2,8 to 3,0 bar.
So I was thinking, als long if there is enough oilpressure to lubricate everything inside the compressor, why not let it run at 10 hz?

This is the mail I received form Bitzer Germany.

The information isn't a big surpries for us. that an operation of the 4PC in conjunction with R404A and to/tc=-5/+37°C can work down to 25Hz.
We think also that we won't see a big influence to the bearing lifespan if the operation range between 25...30Hz will appear only for a small percentage of the total operation time and the specified conditions.
Nevertheless is the warranty limited to the 30Hz value.

Frequencies below 25Hz we don't recommend even with the specified conditions.
Please consider that it is a certain shaft speed necessary to create the necessary oil film in the slide bearings.
In the case that we will have to low speeds the friction can change over to the area of static friction.
Then we will have a much higher stress for the bearings and a reduced lifespan.

[US Iceman]

Frequencies below 25Hz we don't recommend even with the specified conditions.
Please consider that it is a certain shaft speed necessary to create the necessary oil film in the slide bearings.

In the case that we will have to low speeds the friction can change over to the area of static friction.
Then we will have a much higher stress for the bearings and a reduced lifespan.

Thank you Peter. This is what I expected. The variable speed recip compressors I have worked on (4 to 8 cylinder, industrial) begin to have fluctuating oil pressures at 20 Hz (motor speed, not compressor speed). At this speed it appears the oil pump pressure regulator begins to modulate, and the oil supply pressure experiences rapid pressure changes.

Below this speed, the oil pressure begins to decrease. For this reason, I use 25-30 Hz as the minimum speed.

Of course, if the compressor is belt driven the motor is running at 30Hz, but the compressor speed can be much lower depending on the speed reduction ratio of the compressor pulley diameter and the motor sheave diameter.

So I was thinking, as long if there is enough oil pressure to lubricate everything inside the compressor, why not let it run at 10 hz?

That was my thought also. However when we ran the compressor at the lower motor speeds, I noticed the oil pressure was fluctuating rapidly. This caused me to increase the motor speed, thereby increasing the minimum compressor speed.

If the oil pressure was changing due to reduced speed, I did not want to damage a compressor or the bearings/shaft/shaft seal due to the changing oil pressure.

From the Bitzer letter, it appears the bearing loads are increasing at lower RPM and can contribute to a shorter life.

There has been several applications where we have used a separate oil pump driven by a constant speed motor. This was mounted next to the compressor and supplied the minimum oil pressure for the compressor, regardless of the compressor speed.

The smaller, electrically driven oil pump was piped into the compressor oil passages and provided oil pressure for the bearings and shaft seal at normal pressure.

This worked well...

Thanks for sharing the information with me.

[milo]

Have to say, it sounds like old wives tales. Assumptions that fluctuating oil pressures are a problem, assumptions that bearing loads increase at lower speeds.

It's one of those topics that will give you a different story depending on who from the manufacturer you talk to.

My notes expaining the results of experimentation and installations permitting 10% recip speeds were surprising to me for their lack of mentioning auxillary oil pump configurations.

I suspect Bitzer don't want to risk speeds lower than 25Hz and so find explainations to support their concern - it's called "ad hoc science" the fallacy of seeing it if you believe it".

I presume you are familiar with boundary lubrication and the stribeck curve? - granted these are difficult to quantify in this particular instance

[milo]

In a well designed compressor it is certainly wouldnt be a problem- however not everybody builds their equipment to those standards! As I'm sure you can appreciate it is difficult to accurately measure these things.I would much rather follow the manufacturers reccomendations than risk the customers equipment long term.

[Peter_1]

I presume you are familiar with boundary lubrication and the stribeck curve? - granted these are difficult to quantify in this particular instance

Euhhh.... no... tell me more. Is it the capillary action between 2 narrow parts which takes care for lubrication?

Marc, you say 10%...so it may run at 5 Hz.

[milo]

Boundary lubrication is the breakdown of the oil film between moving parts - usually in high load/ low speed environment

[US Iceman]

They can operate at 3600rpm 60Hz and 3000rpm 50Hz.

Who makes recip compressors that run this fast? Or, is the speed reduction that great through the belt drive system?

[US Iceman]

Can you provide the source document for this? I have read some of his papers and they are quite good.
Thanks

[dallan]

The previous discussions seem to revolve around recip compressors, how well could it be applied to scroll compressors in a refrigeration application.

[Peter_1]

The notes don't say, it just says they are small high speed compressors. This is on the page before the graph that shows recip capacity versus % Power with a low limit of 10% capacity. I

Where can I find these notes Marc?