booster compressor

[matt watson]

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]

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., -25°C, -15°C, and 0°C. 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 -25°C temperature up to the 0°C level.

Then as an example you might use a single stage compressor for the -15°C 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 -15°C 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]

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]

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 0°C rooms. The suction from these evaporators goes to an single accumulator, which is then connected to the suction of the 0°C 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 & -25°C rooms.

From each room (the -15 & -25°C 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 -25°C 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]

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]

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 0°C temperature and then let the high-stage compressor handle the total load of the 0°C 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.