***** parallel compressor rack systems

[COLD WANG]

in large cold storages, i heard ***** parallel compressor rack systems is used .how about this system?;)

[taz24]

in large cold storages, i heard ***** parallel compressor rack systems is used .how about this system?;)

large Rack (Packs, in the UK) are used in many aplications where there is a large fluctuation in the load or duty of the plant. They are very reliable and do the job they are designed to do well. They do have inherent problems such as oil distribution and liquid logging in the cond at low ambient temp conditions but on the whole have proven to be very reliable when used in the right aplication.

What exactly do you need to know:)

Cheers taz.

[Samarjit Sen]

***** Compressor racks are very much in use even in our country for large Cold Storages. Taz24 has very corerectly stated the advantages. To answer better to your question, we need to know what is your application and what do you exactly want to know.

[US Iceman]

The rack systems may be less expensive to install, but they may also be more expensive to operate than a central ammonia system.

Typically you will see about 30% less energy required to operate an ammonia system for the same capacity in a cold storage facility.

Can rack systems be used for this purpose? Yes, they can.

Is a rack system more efficient? No.;)

[COLD WANG]

The rack systems may be less expensive to install, but they may also be more expensive to operate than a central ammonia system.

Typically you will see about 30% less energy required to operate an ammonia system for the same capacity in a cold storage facility.

Can rack systems be used for this purpose? Yes, they can.

Is a rack system more efficient? No.;)

thanks iceman!
i really dont know that 30% less energy required to operate an ammonia system for the same capacity in a cold storage facility. in our contry many people regard the rack systems is more inexpensive to operate than a central ammonia system .according to you opinion this is inaccuracy?
could you explain this ? master .please teach me.

[US Iceman]

in our country many people regard the rack systems is more inexpensive to operate than a central ammonia system


Are these the same people who sell rack systems?;)

To understand the differences in energy use you first have to look at what uses the most energy... the compressors.

Now then, what affects the energy use to the greatest degree? Discharge pressure.

You have to find a method of lowering the discharge pressure to the absolute minimum required and what factors affect the discharge pressure.

There's nothing magic about this, but you have to spend the time to investigate how a system operates and what the controlling factors are.

A good teacher points the way for the student. Funny, that sounds like my judo instructor many years ago.:D

[Samarjit Sen]

In our country we have a large number of large cold storages which are being operated with ammonia system. Most of them are closing down due to high electricity bills. I personally had visited a number of Cold Storages and found that the power consumption are very high. What I could presume is that due to improper design and using obsolete models of reciprocating compressors and wrong systems, it is quite possible that the power consumption are due to these.

However as per my experience a rack system with ***** consumes much lower power. The units are all with water cooled condensers and due to water temperatures available here we can maintain a Condensing Temperature of 38 to 40 Deg. C.

I had personally not been involved with many ammonia systems, but after learning all about it from the RE and from people like you, I feel that if the systems are designed properly and efficient equipments are used, ammonia will definately be a superior refrigerant.

But then again we come back to the vicious circle. Most of the ammonia projects in our country are executed by technicians who may have worked in some plant as an operator and quotes a very low price which may not be even cover the cost. But then the customer is attracted by the price and ignoring all our hues and cry about energy efficient, we lose the job.

With ***** such problems are not faced.

[Peter_1]

US Iceman,

I've done once the test with the software of Bitzer and compared the same compressor under the same pressure conditions and the biggest savings are around -10°C (14°F) evaporating - 21% better COP , at -20°C (-4°F) reduced to +/- 11% and at -30°C (-22°F) reduced tot +/- 10%.

If you calculate this over the years, this can be of course a lot of money on the electricty bill.

But, you also have to take in account the pumps which you haven't in a DX system.
The power to drive these pumps is of course less then the power savings you make on compressor side.

But the initial cost of a NH3 system is much higher then a conventional system, so you have a higher payback time.
What we have in Belgium is that the techs working on an NH3 systems asks +/- 30% to 50% more/hour, so also this increases the payback time.

I personally think that you don't have to think on NH3 as long as the refrigerant capacity is less then 300 kW.
But, that's my opinion of it.

[US Iceman]

Most of the ammonia projects in our country are executed by technicians who may have worked in some plant as an operator...


Which typically means the systems are mirror images of those systems they used to operate. It's a circular pattern. The problems of the old systems are repeated in the new systems.

You also state the rack systems use less energy than the ammonia systems. That's certainly feasible. If the rack systems are water-cooled that is much better than air-cooled.

Energy use is a function of the system design. Any design can be manipulated to prove a certain point.

I'm not saying ammonia should be used for all applications. However, when the capacity requirements reach a certain level the use of an ammonia system can offer some significant ROI's.

Peter mentioned this also, and I fully agree. There may be some circumstances where a ***** system is cheaper than a large ammonia system. I would not argue that point either.



But, you also have to take in account the pumps which you haven't in a DX system.
The power to drive these pumps is of course less then the power savings you make on compressor side.


That's true. A good comparison would include all of the power input requirements. Refrigerant pumps do use power, but a small amount.

We should also compare the fan motor power differences between the air-cooled condenser and the water pumps and fan motors on the evaporative condensers.

The 30% value I used is based on the difference in energy use between an air-cooled system and a system using an evaporative condenser. Both at the same suction pressure.

Energy savings is a complex subject. First we have the basic system design issues. For example, is the system designed to use the lowest amount of energy at full load in the worst ambient conditions.

Secondly, we have the issue of how the system is operated. I've seen some systems that were operated and controlled better that acheived a 40-50% savings with no other changes to the system.

And third, how is the electricity used? In a cold storage facility the system can be basically shut off (or operate with greatly capacity). Then the system runs during the night when it's somewhat cooler and perhaps the electricity is cheapr and night.

There are a lot of issues to consider and I understand the problems you both mention.

What I'm trying to say is we should look at all options to determine the best system for cost and energy.

[Dan]

Typically you will see about 30% less energy required to operate an ammonia system for the same capacity in a cold storage facility.

Yes and no. I tried to compare equals to equals regarding refrigerant compressor combinations and I couldn't see this sort of advantage. But it was difficult to find such comparisons. Peter, perhaps has found equal comparisons of systems that operate with the same hardware and only change the refrigerant.

Let's make sure we distinguish the differences between which refrigerant is being used from the systems that we envision these refrigerants being used.

I do not see Ammonia as being 30% more efficient than R404 when you use the same machinery to accomplish refrigeration, so I will accept Peter's principles.

Uh, oh. The Peter Principle!:)

[COLD WANG]

Hi iceman

Today I ask york for calculating the cop of the same screw comp with r22 and NH3, they tell me in -45c/+40c ,the cop OF r22 is bigger than THAT OF NH3, what do you say ammonia is +/-30% more efficiency?

[COLD WANG]

HI PETER!YOU SAID:
"I've done once the test with the software of Bitzer and compared the same compressor under the same pressure conditions and the biggest savings are around -10°C (14°F) evaporating - 21% better COP , at -20°C (-4°F) reduced to +/- 11% and at -30°C (-22°F) reduced tot +/- 10%."

I DONT understand WHAT YOU SAID, your comparetion is ammonia and r404a? ammonia is higher cop? I REALLY DONT KNOW.

[Peter_1]

Download once the Bitzer software at http://www.bitzer.de/home/index.php?LNG=DE&CTR=DEU&CMP=DEU&NAV=1&&LNG=EN
and play a little with it.
You can choose and open machine and then make a selection with R404a and NH3 with the same compressor.
You get the COP's and the capacities. Try it once.

[COLD WANG]

yes r22 is more efficent than NH3.BUT r404a in not.

[Peter_1]

Indeed R22 and NH3, you started this thread with R22 and others added NH3 to it.

But look also once at the mass flow of NH3 compared to that of R22 at the same low pressure conditions: almost 8 times more mass flow wit R22!
So you can use much smaller tubes with NH3.

[US Iceman]

We are comparing apples and oranges. Energy efficiency and energy use are two different items as I see them. It might be playing word games, but bear with me.

For efficiency Peter is comparing the same compressor with the same operating conditions with two different refrigerants. That directly shows the link between the using different refrigerants. I did not check his numbers, but I agree in principle with what he said.

Of course, a system with a higher efficiency will use less energy. But the first thing I think we should consider is, what has the greatest impact on reducing energy (besides the refrigerant selected)?

The answer is discharge pressure. Lower discharge pressure requires lower energy input.

When I started this off, I said:



Typically you will see about 30% less energy required to operate an ammonia system for the same capacity in a cold storage facility.


This comment was based on the systems I have seen. Ammonia systems almost universally use evaporative condensers. Almost all ***** equipment will use an air-cooled condenser.

When you compare the energy use of an ammonia system (using an evaportive condenser) to a similar capacity with R-22 (using an air-cooled condenser), the energy use difference is close to 30%.

Part of this reduction is due to the lower discharge pressure when the evaporative condenser is used to condense at 95F (35C). Another part of this reduction is the use of ammonia.

A typical air-cooled system will condense at about 115F (46.1C). Of course the higher condensing temperature will have a higher energy input.

As Samarjit Sen stated,



However as per my experience a rack system with ***** consumes much lower power. The units are all with water cooled condensers and due to water temperatures available here we can maintain a Condensing Temperature of 38 to 40 Deg. C.


Again we see a different viewpoint. His comparison to an ammonia system is based on water-cooled condensers.

We don't know what the operating conditions are for ammonia systems he has seen, but if he says the ammonia systems use more energy than the water-cooled rack system I believe him.

We are comparing things based on preconceived ideas and methods.

I did also try to clarify my point in a later post by saying:



The 30% value I used is based on the difference in energy use between an air-cooled system and a system using an evaporative condenser. Both at the same suction pressure.


The point I'm trying to make (which I seem to be doing a poor job of) is that the systems should be designed to use the least amount of energy in the first place.

Then look at the benefits of using a different refrigerant. This is what Peter's examples show. At one set of operating conditions the ammonia systems are/or can be more efficient almost all cases.

When someone says one system is better than another, they can be right. But of course this is sometimes based on what they have seen.

There are many installed examples of bad refrigeration systems, although I tend to see more energy intensive applications using ***** (my apologies to DuPont for using their tradename as I'm using the word in a generic way to describe these refrigerants)

To compare refrigerants, you HAVE to compare them at the same operating condition.

To compare systems, you HAVE to compare the same system configuration.

[Samarjit Sen]

It is nice to study such threads as I personally am learning a lot about NH3 and to be very honest I feel it is more economical and power saving if the capacity of the plant is large. Specially for a Te below -5 Deg. C. Further I feel using Evaporative Condensers is more efficient than Shell & Tube Condensers. Please corrrect me if I am wrong as I am trying to learn more about NH3, and would like to to upgrade the plants which are closed due to wrong design and high electricity bills in our country.

[US Iceman]

I feel using Evaporative Condensers is more efficient than Shell & Tube Condensers.


In general, this is correct as long as you understand each condenser may be operating at different condensing temperatures.

Look at it this way... If you have two identical systems; one with a shell & tube condenser, and one system with an evaporative condenser and both are operating at 35C (95F) condensing. Which one will use more power?

Since the systems are identical and operating at the same pressures (suction and discharge), the system using the shell & tube condenser may have higher total power input. Why?

The water pumps for circulating the water from the cooling tower to the condenser will probably be higher than the pump power for the evaporative condenser.

Our notions of condensers have to be based on the available heat sink temperature.

For evaporative condenser, the normal condensing temperature is about 95F (35C) or lower. If you use an evaporative condenser with higher condensing temperatures you can run into problems with water quality.

If a shell & tube condenser is used, the normal condensing temperature is based on several factors such as the entering and leaving water temperatures and the approach temperature. You usually see the condensing temperatures in the range of 100F to 105F (37.7 to 40.5C).

This is a constraint due to the use of a cooling tower and ambient temperatures (wet bulb) as it affects the ultimate tower capacity.

However, if a cold water source is available such as well water the condensing temperatures could be much lower (if a shell & tube condenser were used).

This doesn't say the shell and tube condenser is more efficient than an evaporative condenser. It simply says the power input will be less if you can condense at a lower temperature.

If the cold water is available and does not pose any environmental hazards, the well water may be more cost effective and cheaper to operate.

On the other hand, if no water is available, then we would have to look at air-cooled condensers. The capacity of these are dependent on the entering dry bulb temperatures. Therefore, the condensing temperatures will be higher and use more total energy.

[Samarjit Sen]

Thank you for the input. We generally use shell & tube condensers for our plants with ***** and maintain a condensing temperature of 40 Deg C. The weather conditions in our country are variable and as such inspite of our attempts to lower the condensing temperature it is difficult to go below 40 deg. C. Theoritically engineers claim 36 or 38 Deg C, but it is not there in fact.
I remember one very renowned refrigeration firm wanted to lower the condensing temperature to 12 Deg C and asked me about it. I told them neither it is feasible nor the compressor operate at this condensing temperature. I have no idea as to what they finally did.

[Andy]

Thank you for the input. We generally use shell & tube condensers for our plants with ***** and maintain a condensing temperature of 40 Deg C. The weather conditions in our country are variable and as such inspite of our attempts to lower the condensing temperature it is difficult to go below 40 deg. C. Theoritically engineers claim 36 or 38 Deg C, but it is not there in fact.
I remember one very renowned refrigeration firm wanted to lower the condensing temperature to 12 Deg C and asked me about it. I told them neither it is feasible nor the compressor operate at this condensing temperature. I have no idea as to what they finally did.

Hi Samarjit Sen:)

it would be possible to condense at 18 or 20 deg c using bore hole water at 12 deg if enough bore hole water was available:)

Other than that I know no other way to lower the condensing temperatures in a warm country, unless you want to use a cascade system and use the the haet of the condenser for water heating.

Kind Regards Andy:)

[US Iceman]

In order for a 12C (53.6F) condensing temperature to be possible you would have to have a heat sink temperature available below 12C.

My question would be: How is the lower temperature being provided? I've seen this done on large chilled water systems? The condenser for the refrigeration equipment was supplied with 7.2C (45F) chilled water. It works just fine.

The issue then becomes; yes you are saving energy on the refrigeration system, but you are paying for it by using chilled water to do the condensing on the refrigeration systems.

The whole idea with selecting design condensing temperatures is to use the available medium (ambient air, ground water, etc.) at the best point of operation.

What are your design wet bulb and dry bulb temperatures in India?

Here in the US we deal with 35C (95F) condensing temperature all of the time with summer excursions up to 82F (~27.7C) entering wet bulb temperatures.

[Samarjit Sen]

In our country there is a great variation in temperature. If we go towards the Northern part, the weather is mostly extreme. In winter the temperatures are very low and in summer it goes above 108 Deg. F. On an average you may consider the temperatures are 110 Deg. F WB and about 95 Deg. F WB.

Now by lowering the condensing temperatures to say 53 Deg F, what is the gain. If you work on any of the Compressor Selection Software, you will not be able to locate the performance at such low condensing temperatures unless you are using a cascade system .

Generally for any application be it Air Conditioning or Refrigeration, we in India design the Condensing temperatures at 38 to 40 deg C with water cooled condensers and 50 to 54 Deg. C with Air Cooled Condensers. For extreme and severe conditions and applications like Steel Plants or Coal Mines, we design with air cooled condensers at 60 to 70 Deg. C Condensing Temperatures. But then these are special applications using refrigerants such as R 124.

The basic idea is how to reduce the electricity consumption so that specially the Cold Storages in our country can operate . I feel that by using Evaporative Condensers things may improve.

I would like your advice and suggestion on the same.

[US Iceman]

On an average you may consider the temperatures are 110 Deg. F WB and about 95 Deg. F WB.


:eek:

Which one is the correct value?

Generally, an evaporative condenser is sized for a maximum condensing temperature of 95F (35C). The highest wet bulb temperatures I've seen are somewhere near 80-82F (26.6 to 27.7C), unless there are some air re-circulation problems causing the moist discharge air to re-enter an adajcent condenser.

If the air has ANY re-circulation problems I can just about guarantee you the condensing temperature will be too high.

On the ammonia systems you have there, what type of condensers are used and what are some general operating conditions (discharge pressure, condensing temperatures, entering wet bulb temperatures, etc.)?

It's starting to look like this thread has been hi-jacked by an ammonia discussion, but I think it is relative to the ***** systems as well. My apologies to the original post.

[Samarjit Sen]

I am sorry. It is 110 Deg F DB and 95 Deg F WB

[Samarjit Sen]

As I had stated earlier in another post the present ammonia condensers are bare MS tube pipes over which water is trickling down. There is no air. It is the air which is blowing in the atmosphere. I know it is wrong, but then this is what is being installed in all the Ammonia Plants.

My intention is that although I have hardly done any ammonia projects, but some how my clients and the people I meet have some confidence in me. I am frequently asked to modify the system so that power consumption is lowered and the performance of the plant improves.

I have learnt a lot about ammonia from this forum and from you all who are frequently working on ammonia. You would be surprised that there are very few people in our trade who are aware of the proper ammonia system. Due to my age it may not be possible to do much, but I believe in giving it a try, so that some may pick it up and continue from there.

Yes the original thead of ***** Rack System has been hijacked, but then there are points which may be picked up from this and applied to *****.

I personally am very much involved with ***** system, but then I do gather and get a lot of information from the various threads which may or not be directly involved.

[DeB]

My experience in NH3 is fairly close to Zero but I think we can all agree per KG/LB circulated NH3 wins hands down. We can argue till the cows come home on energy costs, right down to the energy costs of producing the refrigerant itself. You could consider CO2 there are rack systems in Australia that are claiming 10 - 20% efficiency gains over HFCs as well as being a cheap refrigerant and no toxicity levels and no theoretical GWP. In Austalia and my turf Queensland, we work on higher average condensing temps. I almost drool when you can be so nonchalant about such water wastage as evap condensers bore (well) water etc.Here where we must water cars and gardens with buckets and then only on certain days of the week(Hosing is banned) aircooled condensers are the norm these days. I don't know if this contrbuted much to the debate but wanted to point out watercooled systems are not always an option. What about an Economised screw rack?

[Peter_1]

DeB, if we're speaking of a borehole, then in fact we're speaking of 2 boreholes. One for pumping up the water and one to inject it back into the ground, so no wastage of water at all.

In Belgium, it's forbidden to pump up water for cooling purposes, unless you pump it back in the ground.
On in- and outlet, you have to install 2 sealed mass-flow meters to prove that you just circulate the water.

[D.D.KORANNE]

RACK SYSTEMS SHOULD BE USED WITH VFDs (variable frequecy drives ) + semihermetic compressors with forced feed LUBRICATION systems.This can give a power saving to the tune of 25% in supermarket rack refrigeration systems .THE VFDs CONTRIBUTE TO AN IMPROVED COMPRESSOR LIFE. CONTACT ME WITH SPECIFIC DETAILS ON ddk@indiatimes.com withn a cpy to : contact@bockindia.com


D.D.KORANNE

d.d.koranne

[taz24]

[quote=semihermetic compressors with forced feed LUBRICATION systems.This can give a power saving to the tune of 25% in supermarket rack refrigeration systems .

D.D.KORANNE

d.d.koranne[/quote]

I thought the idea of a pack was to load the compressors up and down as and when the load was there.
I would imagine the saving overall would be marginal.
How do you arrive at a saving of 25%?
We have had companies promising us all sorts of saveings over the years but they rarely deliver the promised amount.

Cheers taz:)

[Andy]

I thought the idea of a pack was to load the compressors up and down as and when the load was there.
I would imagine the saving overall would be marginal.
How do you arrive at a saving of 25%?
We have had companies promising us all sorts of saveings over the years but they rarely deliver the promised amount.

Cheers taz:)

Hi TAZ:)
All our latest packs all have inverters:) The savings are relative to what you start with:D You could save 25% if the pack you replace was bog standard with not enough steps. An good pack will save you 10% plus with an inverter properly applied;) The suction and discharge is very constant, thus you liquid supply to the TEV's is improved. The suction pressure is steady and cuts out suction pressure hunting with compressors cutting in and out:)
A three compressor pack will be possible as the minimum turn down is improved with use of the inverter, four compressor is better with really good turn down to match winter and night time loads:)

Kind Regards Andy:)

[taz24]

A three compressor pack will be possible as the minimum turn down is improved with use of the inverter, four compressor is better with really good turn down to match winter and night time loads:)

Kind Regards Andy:)

Oh right:)
Are the comps individualy inverter controled and then brought online individualy as and when required or is the pak sized almost to the limit and then just one comp controls to fluctuation in pressure.

Cheers taz.