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matt watson
07-06-2008, 05:36 AM
hi there just got a question for an assignment i am doing i have to draw a schematic diagram of a three screw comp system with a high stage comp low stage comp and a booster comp. there will be four rooms with flooded evaporators fed from the accumulator two rooms at -25 two at -15. With another room DX running at 0 degrees. My question is what should my three comps be sucking off.
Should i have my booster sucking off the economiser and discharging into the suction of the high stage and also have the high stage comp sucking off the accumulator and the low stage comp sucking off the evaps? or should the evap suctions be piped back into the accumulator and what should be sucking off the DX room at 0 degrees. Im sorry for the brief info i no it might be hard to understand but would appreciate any help or links to helpfull sights
Also is it possible to find a link to the official ISO symbols for refrigeration schematic drawing cant seem to find anything

US Iceman
07-06-2008, 04:28 PM
Hi Matt,

First some clarifications are in order. What you are proposing is not a three stage system. It is a refrigeration system with three temperature levels, i.e., -25C, -15C, and 0C. It's important to distinguish what we mean when we say stages. Usually this term implies stages of compression.

When this term is used it indicates the refrigerant is compressed from one pressure up to a higher pressure. This is called a booster or low-stage compressor, although the exact terminology will differ depending on locality. This higher pressure becomes the suction pressure for the high-stage compressor. The high-stage compressor compresses the refrigerant up to the condensing pressure.

Just because you have three evaporator temperature levels does not mean you have to use three stages of compression. The normal way of viewing this is; colder temperatures require more stages. Your problem is a very long way from needing three stages. In fact, two stages might be questionable. Your temperatures are simply not cold enough in my opinion, although you might use a booster on the -25C temperature up to the 0C level.

Then as an example you might use a single stage compressor for the -15C temperature level for compressing the refrigerant up to condensing pressure. Notice what the term I used here; single stage. This compressor is only pumping gas from -15C up to the condensing pressure.

Another high stage compressor could be used for the total loads at the 0 temperature level. This compressor(s) could compress the refrigerant up to the condensing pressure. Again, notice my terminology; when we use the term high stage compressor it implies this compressor has a low stage compressor discharging into it's suction (via an intercooler normally).

Selecting equipment is a complex task. There are thousands of ways a system can be designed; poorly or very good. They both might maintain temperatures however one design may do that more economically than the other.

System design also depends on the total cooling loads at each temperature. It's simply not enough to say I have "X" evaporator temperature so I need to do "Y". Hope this helps you understand the issue you are facing.;)

matt watson
08-06-2008, 01:25 AM
Thanks for your reply iceman

The reason i haved mentioned the three compressors is because it is a question out of one of my assignments and that is what info i have to use and in corporate to finally draw up a final pipe schematic of the whole system. This is where i am confused as to how i should have my pipes configurated. Mainly where my suction pipes should be. If you have any good links which could point me in the right direction that would be great.
Thaks for your time

US Iceman
08-06-2008, 03:25 AM
OK then Matt.

What I would suggest is to use then is three separate suction levels. One each for each temperature.

Draw three boxes, one on top of the other with some space between them. Consider these the three compressors. To the right and above draw another box. This one is the condenser. Now connect all three compressors (from the right side of the compressor boxes) to the condenser. This makes it a common discharge for all three compressors.

From the condenser box go straight down to another box. This is your receiver.

Now, from the receiver you have to supply high pressure liquid to the DX evaporators in the 0C rooms. The suction from these evaporators goes to an single accumulator, which is then connected to the suction of the 0C compressor suction.

The high pressure liquid is also fed to the liquid make-up valve train on the surge drums for the flooded evaporators in the -15 & -25C rooms.

From each room (the -15 & -25C rooms) you will have one suction. One suction line goes to the compressor suction for -15 and one suction line goes to the compressor suction for -25C compressor.

With this arrangement you have three single-stage compressors, each operating on a separate suction pressure, but with a common discharge line and liquid feed to the evaporators.

That is about as simple as it gets.;)

taz24
08-06-2008, 11:41 AM
OK then Matt.

With this arrangement you have three single-stage compressors, each operating on a separate suction pressure, but with a common discharge line and liquid feed to the evaporators.

That is about as simple as it gets.;)


I agree with this layout as a single stage but the question does mention a booster.

It may not be the best solution but he does have to answer the question as it is worded.

Cheers taz.

US Iceman
08-06-2008, 07:37 PM
Well taz, there are probably a hundred different ways this could get put together. And if a person wanted to use a booster compressor then the obvious use is from -25 up to the 0C temperature and then let the high-stage compressor handle the total load of the 0C combined loads.

Frankly, I'm a little surprised this is being asked as part of a training program unless it is intended for system designers instead of service or operations people.

taz24
09-06-2008, 02:40 AM
Well taz, there are probably a hundred different ways this could get put together. And if a person wanted to use a booster compressor then the obvious use is from -25 up to the 0C temperature and then let the high-stage compressor handle the total load of the 0C combined loads.

Frankly, I'm a little surprised this is being asked as part of a training program unless it is intended for system designers instead of service or operations people.


I agree with you 100% and I am not questioning the answer you gave :).

But I know that trainers like to pose improbable questions to get the trainees thinking. I think somtimes they ask the trainee to design unacceptable systems just to make them think and for it not to be a straight forward answer.

I'm doing My HNC at the moment here in the UK and the lecturer always sets questions with weird design specifications and I have argued with him in the past that he sets questions that do not follow what I know from industry experience.

He wants to see the workings out more than a correct solutions and he also wants the trainee to understand the limits of system design.

Cheers taz.

brian_chapin
09-06-2008, 12:08 PM
I'm doing My HNC at the moment here in the UK and the lecturer always sets questions with weird design specifications and I have argued with him in the past that he sets questions that do not follow what I know from industry experience.

Keep in mind that those lecturers are giving that same class for the thousandth time and they tend to do really stupid things to their students so they can keep themselves interested in their own lectures.

You will find more often than not the old adage is true:

"Those that can, do. Those that can't, teach."

US Iceman
09-06-2008, 03:56 PM
You will find more often than not the old adage is true:

"Those that can, do. Those that can't, teach."


Ouch! I'm glad you said this so I did not have to.:D

matt watson
10-06-2008, 07:26 AM
Yea that is the hard thing about it i know there are plenty of different options but thanks for the comments. It wont be the last time i seek for advice.

shooter
10-06-2008, 11:12 AM
make the system for -15 (=-22) with the low stage
and a intercooler. running on -7 celsius
add the 0 room to it and feed to the high stage.
the booster for -25 (-32) can be added to the low stage, however better to put on high stage. calculate if possible to do as discharge temp is critical.
normal high stage is half capacity here it will be different.