PDA

View Full Version : Economizers and screw compressors







US Iceman
05-06-2007, 03:01 AM
I was recently asked via a PM from a RE member to discuss this topic. As there is a lot of information that could be covered in this discussion I'll start with some basics and we can proceed from there.

There are two basic types of economizers for screw compressors:

Flash type
Subcooling typeThe flash type is relatively simple. High pressure liquid from the receiver is fed into a vessel through an expansion device. The liquid flashes off from receiver pressure down to the pressure in the vessel. The flash gas flows out of the vessel and back to the screw compressor side port. The cold liquid is at an intermediate pressure (between suction and discharge pressure). And, the liquid is saturated at this intermediate pressure. It is not subcooled, but rather a cold saturated liquid.

The subcooling type uses a heat exchanger of some type. On one side of the heat exchanger flows high pressure liquid from the receiver. The other side of the heat exchanger is simply an evaporator. This type of economizer only subcools liquid from one temperature to another lower temperature at approximately the same pressure.

Imagine a DX chiller. Instead of cooling water or a glycol mix we are simply cooling liquid refrigerant from a warmer temperature to a cooler temperature. This occurs at a relatively constant pressure, so this is subcooling. The gas that is created by cooling the liquid (in the cooling section of the heat exchanger) is diverted back to the side port of the compressor.

With the flash economizer, the liquid is at saturation temperature so the liquid can still flash off due to heat gain or pressure drop in the liquid line.

With the subcooling type of economizer, the liquid is subcooled and will remain so until the liquid has warmed back up to the saturation temperature of the receiver pressure, or if there is a lot of pressure loss in the piping.

Let's see where this discussion goes...

absrbrtek
07-06-2007, 10:10 PM
Nice post US Ice. The only thing I see you left out is why there is an economizer. The economizer is for energy savings and to help prevent flash gas from happening in the evaporator.

If there was no economizer in most cases you would need a larger compresser along with larger piping due to the flash gas creating added vapor volume in the evaporator. The closer the liquid temperature entering the evaporator is to the evaporator temperature, the less flash gas you have.

By using the economizer you only need to compress the gas part way through the compression cycle in order to drop the liquids temperature. Systems without an economizer compress the flash gasses created in the evaporator through the entire compression cycle using more power consumption to due so. Flash gas is wasted energy in an evaporator. JMHO

US Iceman
08-06-2007, 12:29 AM
Funny, the person who asked me about this has not said anything yet??? :(

His request was for some more in depth information, but I thought we should start with some basics before it got too complicated. Maybe later.:confused:

nh3wizard
08-06-2007, 01:49 AM
I always love reading post by the USIceman, he explains so well, now I look forward to post by adsrtck

US Iceman
08-06-2007, 02:22 AM
The only thing I see you left out is why there is an economizer.


:) I did that on purpose. There are a lot more details to this that I was hoping to cover as the discussion progressed.

However, you did an excellent job of adding some more information.

Here's a question to consider: What happens to the flash tank pressure when the compressor unloads?

How does this affect the liquid supply from the flash tank?

Samarjit Sen
08-06-2007, 05:44 AM
I shall look forward to this thread. Iam sure that as usual US Iceman has more to offer on this subject, which will in turn educate all of us.

church2k
09-06-2007, 11:22 AM
Lo all!, specially iceman :), thank you for your exlpanation i,m really sorry about reading this so late, but i was very busy making an start-up of an equipment very far away my home. Ok, taking the bussiness again im really interested in this question. What i was talking with iceman,u RE members, was that i had a discussion about the effect of the economizer to the oil.Here it goes: when the economizer is working, it chills the oil in the compressor?, im talking about a flash economizer but i think that the effect is the same with flash or subcooling type(i think that the eco does not chill the oil), because hte most of the refrigeration is the cold vapour proceeding from the evaporator(sorry im talking about a liquid feeded nh3 system with the intermediate stage), and the flashing temperature is never going to be colder than the return of the ev. so i think that the eco does not chill the oil, instead of this i think that it heats more the oil, because you are compressing a higher quantity of gas, am i wrong??


About the other question im not pretty sure what are you talking about the unloading in the compressor, it depends i fyou are talking about a flash system used always for mantain, for example, a fresh store, or a subcooling type that u are using only when u need it(usually joined to the 100% stage of capacity of the compressor and depending of the discharge pressure).

US Iceman
09-06-2007, 03:15 PM
Hi everyone,

First, the easy question...

If you use a flash economizer you also need to provide an inlet pressure regulator on the flash tank suction line. Remember, the flash tank is at the intermediate pressure of the side port. The liquid in the flash tank is also at this same intermediate pressure (saturated).

When the compressor unloads below +/- 75% of the full capacity slide valve position, the side port pressure is reduced to suction pressure. The reason is the slide valve is now in the position where the gas flows back to the suction, instead of a intermediate pressure during compression. In effect, the useful length of the rotor has been shortened.

Therefore if you have a flash tank on a screw compressor and you are using this as a flash economizer, the liquid pressure decreases down to suction pressure when the compressor unloads. You still have compressor capacity, but no pressure to push the liquid to the evaporator.

The discharge pressure has very little effect on this since the side port pressure is a function of % capacity and the suction pressure. If the discharge pressure is reduced, the side port pressure will decrease but not very much. However, if you change the suction pressure (compressor @ 100% capacity) the side port pressure changes quickly.

Second part:

The cool gas from the economizer (either type) flowing into the compressor at full load increases the oil cooling load, not the oil temperature.

An economizer is similar to having a separate compressor operating at a higher suction pressure. Whatever mass flow that comes into the side port is added to the suction mass flow. The higher total mass flow increases the total power input from the compressor motor. You have a higher total power input, but the COP has been improved.

With the increase in total power input the oil cooling load increases. If the oil cooler is selected to maintain the oil injection temperature at a constant temperature, the discharge temperature remains constant also.

Realistically, the cold gas entering the side port undergoes does try to cool down some gas, but the resulting energy balance looks something like this: Suction mass flow is greater than side port mass flow, so the side port mass flow has little effect on oil temperature.

The oil cooler does the oil cooling not the economizer mass flow.

At the same time the oil cooling load increases when an economizer is used because of the higher total mass flow the compressor is compressing (suction + side port). The higher total mass flow increases the oil cooling load.

church2k
10-06-2007, 12:31 AM
Uderstood, so i was correct in certain way. My god, iceman, where you working at?

US Iceman
10-06-2007, 02:12 AM
I'm self-employed as a consultant. I used to work for one the major industrial refrigeration manufacturers here in the US. Due to this I was exposed to a lot of different problems and systems. No matter where you work, you work for a paycheck, but learning lasts forever. Well, almost!:D

TXiceman
10-06-2007, 03:03 AM
Next thing you need to discuss is proper economized piping with two machines in parallel service.

Also, using the flash economizer as a combination flash economizer and receiver by using a sump on the condenser and a high side float to dump the liquid to the economizer.

Ken

US Iceman
10-06-2007, 04:04 AM
Also, using the flash economizer as a combination flash economizer and Receiver (http://www.refrigeration-engineer.com/forums/glossary.php?do=viewglossary&term=88) by using a sump on the Condenser (http://www.refrigeration-engineer.com/forums/glossary.php?do=viewglossary&term=61) and a high side float to dump the liquid to the economizer.


Yep, that's a good one. I won a few orders with that little trick.;)

Another one is using an economizer on two boosters (in parallel) in a two-stage system. Howden had to back me up on this when a competitor said it was not possible to get the capacity out of the boosters like this.

I know there are a couple of schools of thought on the parallel screws on a common economizer Ken. Some I have seen have more control valves than an air unit with hot gas defrost.

My opinion was to use separate valve trains on each screw side port with check valves downstream of the regulators on each screw.

How do you take care of this? You have probably worked on more of these than I have.

church2k
11-06-2007, 07:56 PM
Please tx iceman and us iceman, can u explain better what u talking about, i,m really interested in this but i cannot understand very much.

US Iceman
11-06-2007, 08:08 PM
I will try to explain the one Txiceman mentioned on using the receiver as a flash economizer.

Imagine a standard refrigeration system laid out in this order.

Compressor
Condenser
Receiver
High pressure liquid is fed to the expansion device feeding the...
Evaporator

This version causes the liquid to flash off in the evaporator.

What Txiceman mentioned is this:

Compressor
Condenser
High-side float valve feeding the receiver (now a flash economizer or flash tank)
Now, the intermediate pressure feeds liquid to the...
Evaporator

This version causes the liquid to flash from high pressure to intermediate pressure in the flash tank. Then we feed the liquid to the evaporator as you normally would when a flash economizer is used. In this case, you simply size the receiver/flash tank large enough to hold the system refrigerant charge.

church2k
11-06-2007, 10:24 PM
I understand that you are feeding the receiver with the liquid apiled up in the condenser, and when you have a determinated liquid level in this one, the float valve opens and feed the receiver isnt it?, but in this case what happens with the condenser type?,it has to be larger than an usual one. I think thats more inportant to grow up your condenser that the receiver that is really controlled by the float valve.

SteveDixey
12-06-2007, 12:21 AM
Next thing you need to discuss is proper economized piping with two machines in parallel service. Ken

I had a problem with a parallel machine set up. Two SAB 163 screws on an open flash economiser, level fed by liquid off the HP receiver via a solenoid valve and hand regulator, and a sub-cooling coil again coming off the HP receiver and then on to the open flash intercooler.

I could never get these machines to take flow evenly; it went to one machine or the other. I got the feeling that there was never enough flash gas generated to build up enough mass flow to positively flow through the flow \ pressure control valves fitted to the side ports.

One of the issues might have been the rather tortuous route the pipework took to the sub-cooling coil, (along with the thermosyphon oil cooling pipes that were another issue). The idea was that high mass flows would generate more flash gas for the side ports but even at full load, the machines never shared the economiser output.

Steve

US Iceman
12-06-2007, 01:24 AM
...what happens with the Condenser (http://www.refrigeration-engineer.com/forums/glossary.php?do=viewglossary&term=61) type? It has to be larger than an usual one.


Not really. The system is what we call critically charged. You only charge enough refrigerant to make the system work. No more, No less.:cool:

Basically, you have to size the flash receiver as a separator and as a receiver for pump down purposes.

TXiceman
12-06-2007, 11:11 PM
The beauty of using the flash tank as a receiver and eliminating a HP receiver is you manage to do away with one veessel. When skid packaging a system, this will save you some space on the skid. Also, a siple high side float sytem is in expensive as well and does provide modulating control.

When running two screws in parallel with economizing, I have always installed check vlaves on each compressor. I use two regulators to prevent the unloaded machine from taking too much flash load.

I have a system now that we are revamping that is two stage screws, with a flash type interstage level and a subcooler type economizer on the booster or low stage compressor. The subcooled liquid from the booster subcooler feeds a thermosiphon plate type chiller mounted some distance from the compression skid. What is interesting about this type system is NONE of the compressor manufactures have a program that will do the compressor calculations. You have to run the standard ratings and then hand calculate the intermediate conditions as the low stage compressor does not have an oil separator, but goes directly to the high stage suction. The high stage does not have an injection oil feed.

This is an example of where you have to understand the basics of the thermodynamic process and have to do mass/energy balances around the system. Once you get flow rates and intermediate temperatures, you can make the high stage compressor runs and check them based on flow and temperature, not tons.

Ken

ABDUL
18-09-2007, 09:40 AM
Good Morning

I will appreciate if someone can send me relevant technical guidelines for performance evaluation of Ammonia Storage Refrigetaion facility. We are operating three tanks with Refirigeration facility. We have screw compressors but not giving good performance. Please note that Purger is out of service.

samiam
18-09-2007, 03:04 PM
Thank you for the explanation.
I think it was Newton who said "I sood on teh shoulder of Giants..."

garyb
23-09-2007, 11:45 AM
I had a problem with a parallel machine set up. Two SAB 163 screws on an open flash economiser, level fed by liquid off the HP Receiver via a Solenoid Valve and hand regulator, and a sub-cooling coil again coming off the HP Receiver and then on to the open flash intercooler.

I could never get these machines to take flow evenly; it went to one machine or the other. I got the feeling that there was never enough Flash Gas generated to build up enough Mass flow to positively flow through the flow \ pressure Control valves fitted to the side ports.

From my experience I suggest two possibilties for Steves symptoms. We have a number of systems with multiple compressors drawing from a common open flash economiser vessel.

Firstly are there back presure control valves in the eco suction lines to each compressor? If there are, they will never balance the flow consistently to each machine. One of our systems had this set-up and caused a current imbalance of aound 80amps to the machine taking the eco load and no matter how finely we tried to set the two EPR pilots it would not remain the same to each machine. Eventually we converted the PM valves to ordinary solenoid valves and cut a new PM valve into the common eco suction and the current imbalance disappeared. System was two SAB-87Es on a tunnel freezer

Secondly are the eco suction check valves the same brand? On another system a third compressor set was added some years after the first two and the spring rate on the new set check valve was different to the original two machines and we had to modifiy the new valve spring to compensate. System was three SAB-85Es on a tunnel freezer.

SteveDixey
23-09-2007, 05:01 PM
The set up is exactly the same for the two screws.

Eventually, the users just ran with it as I was starting to come around to a similar idea that was going to cost money and they choked on their breakfast at the quotes.

I did my job and it is now up to them. Somehow the concept of spending money to save it doesn't seem to register with this lot.....

Steve

TXiceman
24-09-2007, 03:01 PM
Garyb, you are correct, however, you do need check vlaes at each compressor to prevent back flowing in one economizer line. The side port pressure on the compressor will vary according the the slide valve position. The side port pressure will be open to suction at around 80% slide valve.

Ken

gwapa
26-09-2007, 03:11 PM
Hi US Iceman
It is really a good post. I would like to invite every body to see the presetation of Joseph W Pillis from York(Frick) or Johnson controls . It could be heldfull .

www.irc.wisc.edu/ASHRAE-TC101/programs/Annual%202001%20Pillis.pdf -

After we undertand the role of the economizer we should ask ourself when could we use it?

I think that depend of the Refrigeration system we are dealing with . We could use economaizer or not.

Normally an economizer system is used where we have a low pressure evaporator (-45C) a mid range evaporator (-5 to 5C) .Of course we have to check that the mid range refrigeration load can be handle by the side port of the compressor(s).
Also the refrigeration system must be in some way stable and the pick load should be in some range .

Some time I prefer use a two stage system with flash intercooler.It is a little spendcier but "fool proof"

regards

US Iceman
26-09-2007, 03:55 PM
Hi gwapa,

I have seen the presentation you posted. It highlights something a lot of people do not know about; you can overload the side port by trying to push to much gas into it. As a result, the motor size must be increased due to the additional mass flow through the side port.

However, you should also be aware of the fact that when the compressor unloads the side port load opens to suction pressure. If this cause the suction pressure to rise, then the compressor loads back up.

Not the best arrangement in my mind.

A lot of people use the side port as a cheap way to get capacity for a cooling load at an intermediate pressure, but you have to be VERY careful in applying this technique.

gwapa
26-09-2007, 05:26 PM
Hi USiceman

What do you think if we work a combination of economizer with a compresor which motor is moved by a VLT (variable frecuency converter) .

US Iceman
26-09-2007, 07:38 PM
What do you think if we work a combination of economizer with a compressor which motor is moved by a VLT (variable frequency converter) .


Not a bad idea. This would help to keep the capacity of the compressor variable, but still keep the slide valve fully loaded. It depends on how much the frequency drive costs. Some larger sizes are quite expensive I'm told.

Another good approach would be to use a small recip. compressor to run the side load and use a frequency drive on the recip also.:cool:

gwapa
26-09-2007, 10:56 PM
Nowadays some customer are using softstar and also VLT. They keep the starting Amp down. Biside you can get from 10 o 15% more capacity at full load (more that 60hz).

Back to the economizer which will be in your opinion the best aplication for it?

US Iceman
27-09-2007, 03:11 AM
I don't think I would recommend over-speeding a screw compressor unless the entire package was designed for the increase in speed.

The only time I would recommend using the side port is strictly for economizing to get cold liquid. I know people use the side port for other things, but I'm not too fond of doing so.

gwapa
28-09-2007, 03:42 AM
I am not sure with the following statament,Just to discous.
If we use a VLT in a compressor to control its capacity I think we have to leave the slide valve full open and without controling capacity.
If it is true, the side port will keep open when the speed of the compressor slow down in low capacity load .The side port keep open but handling a lower flow mass.
The pressure in the side port will be set according the compression relationship .What do you think?

HallsEngineer
28-09-2007, 08:57 AM
We lock our loading at 100% with a cuff when using inverters so the economiser is always working. The load with the inverter doesn't mater because it is like you are changing the size of the compressor every time you load or unload to match the required load so the required gas flow ratios stay around the same.
Usually we dont use the economiser on the boosters because the pressure differential is normally minimal and you can normally just add the load to the intermediate stage suction or add it to the low stage load and use a back pressure valve. Economisers only become very worth while on large DP because the compressor is working harder to compress the gass we inject at the right point and hey presto compressor gets a helping hand and you get some free cooling.
Another point is that you must ensure the back pressure valve is correctly set up when you can unload the compressor with the slide valve as the flash econ can come to a much lower pressure and the DP over you expansion device doesn't work correctly due to this. Next thing you know youv'e got liquid entering your compressor mid-compression (D'oh). We use danfoss valves with a solenoid shut off so when the compressor comes below 75% we can shut off the economiser.
You must also be careful if you have a multiple compressor circuit. There must be a suction non-return present or you can end up with a big ice ball for a compressor. There must be controls in place to ensure the economiser only comes in when all machines are running or you watch the suction non returns very carefully. Oh and your compressor doesn't whinge as much as you think when you've got liquid going in your economiser ports, your oil just starts to dissapear quickly.
Oh and electronic valves are perfect for this in subcooling mode!

US Iceman
28-09-2007, 02:57 PM
Hi gwapa,

If you use a frequency drive on a screw compressor the best efficiency is seen when you leave the slide valve fully loaded and vary the compressor speed. This allows the port pressure to remain fairly constant.

The side port pressure should remain relatively constant until the rotor speed slows down very low. At this point, you begin to loose the sealing effect on the rotor tips and more gas begins to bypass from discharge back to suction.

Sergei
28-09-2007, 05:36 PM
We lock our loading at 100% with a cuff when using inverters so the economiser is always working. The load with the inverter doesn't mater because it is like you are changing the size of the compressor every time you load or unload to match the required load so the required gas flow ratios stay around the same.
Usually we dont use the economiser on the boosters because the pressure differential is normally minimal and you can normally just add the load to the intermediate stage suction or add it to the low stage load and use a back pressure valve. Economisers only become very worth while on large DP because the compressor is working harder to compress the gass we inject at the right point and hey presto compressor gets a helping hand and you get some free cooling.
Another point is that you must ensure the back pressure valve is correctly set up when you can unload the compressor with the slide valve as the flash econ can come to a much lower pressure and the DP over you expansion device doesn't work correctly due to this. Next thing you know youv'e got liquid entering your compressor mid-compression (D'oh). We use danfoss valves with a solenoid shut off so when the compressor comes below 75% we can shut off the economiser.
You must also be careful if you have a multiple compressor circuit. There must be a suction non-return present or you can end up with a big ice ball for a compressor. There must be controls in place to ensure the economiser only comes in when all machines are running or you watch the suction non returns very carefully. Oh and your compressor doesn't whinge as much as you think when you've got liquid going in your economiser ports, your oil just starts to dissapear quickly.
Oh and electronic valves are perfect for this in subcooling mode!
I didn't get about free cooling.
Usually, we have to steps capacity reduction.
1. Inverter reduces the speed of compressor from 100% to minimum allowable speed(very often 50% of normal speed). Slide valve at 100% capacity.
2. Further reduction of capacity should be done by slide valve.
Liquid will enter port side only due to poor design of intermediate vessel and piping. Capacity of port side is very limited and, usually, lowering pressure(due to compressor unloading) in this port side won't pull the liquid from intermediate receiver. Solenoid will shut off port side at 75-80% capacity, because at this point port side will connect to the suction and benefits of economiser will be lost.

HallsEngineer
28-09-2007, 08:43 PM
WE slow ours down to around 20% speed and slightly overspeed to increase capability if required. and if you dont monitor the differential pressures when running with a flash type intercooler you can cause big problems. If you have to unload the compressor as well as slowing the speed down you have got a very poor plant design.

Sergei
29-09-2007, 05:16 PM
Minimum allowable speed of compressors is different from one compressor manufacturer to another. It is nothing to do with plant design.

HallsEngineer
30-09-2007, 10:36 AM
Its everything to do with plant design if you need a greater turndown ratio you should use two smaller compressors to cover the duty

Sergei
05-11-2007, 07:10 PM
I found questionable statement about economizer.
"As screw compressor unloads, the economizer port will drop in pressure level, eventually being fully open to suction. Because of this, an output from the microprocessor is generally used to turn off the supply of flashing liquid on DX economizer when capacity falls below 45-60%. This is done because the compressor will be more efficient operating at a higher slide valve position with economizer turned off, than it will be at lower slide valve position with the economizer turned on..."
What is your opinion?

RefJGM
02-12-2009, 06:56 PM
We have a system using a subcooler (brazed plate HX) with a TXV with the gas then being returned to the economizert port of the compressor.

The problem we are having is that the liquid feeding the TXV is flashing upstream of the valve. The TXV seems to be passing a mixture of gas and vapour and is hunting, producing SH of 25-50F. The pressure is also higher in the economizer line (about 15-20 psi)

Comming out of the condenser there is about 8F subcooling. We have redone the piping to make sure it is not too restrictive. Were we branch off to the sub cooler TXV we have a solenoid and then a sight-glass. It is there we see the flashing.

We have tried several things toate:
- Changed the piping to make it as short as possible. - Change the refrigeratio charge (+/-30%) in 0.5 lb increments
- changed the condensing and evaporating tempeatures
- Adjuted the SH on the TXV (not resposive)
- Checked the lication of the equalizing line and the location of the bulb. for the TXV.

But no noticable change in reducing the upstream flashing or the economizer performance.

The main line of refrigerant going to the evaporator that we are trying to sub cool is getting some effect and the sight glass is clear, but not the full effect.

Is there anyone who has come accross something like this before or has options that we can try.

Any input would be greatly appreciated.

US Iceman
02-12-2009, 09:09 PM
The TXV seems to be passing a mixture of gas and vapour and is hunting, producing SH of 25-50F


Seems like you have a liquid supply issue to the TXV. Little to no liquid and high superheats.

Is the liquid supply to the economizer solenoid valve from a high pressure receiver? If so, you probably have a flash gas problem in the main liquid feed line or the receiver liquid seal on the dip tube is periodically lost.

Segei
03-12-2009, 05:25 AM
Is this ammonia plant? Where receiver is located compare to HX(above or below)?

sterl
09-12-2009, 07:48 PM
Vapor will flow into the side port of a screw only when the passing void of the rotor is at a lower pressure than the 1) evaporating pressure of the subcooler or (2) the holdback pressure of the regulator on a flash vessel. With slide valve unloading, the pressure of the side port itself decreases to approach suction so the regulator is now passing vapor from a fixed inlet pressure to a lower outlet and that same gas is compressed through a higher pressure ratio...At some point of balance in all this, the advantage of cooling the liquid before it goes to the evaporator is lost because the "flash gas" is compressed across the full pressure ratio either way.

So a Big Ratio (High Vi) machine with long rotors is the better candidate for either economizing or side port loading than a short, large diameter rotored machine at lower Vi.

2-Stage arrangements often end up with both machines at Low Vi; cause the pressure ratio is split between the 2. Should the intermediate temperature load otherwise be a high proporition of the total high stage load, and the low stage be thereby operating at a high pressure ratio: economizing the boosters may make sense, feeding intermediate pressure liquid through the economizer; but this does benefit from motor driven valves....Even though the pressure ratio may be high, the total pressure difference available will be low so valve and pipe line sizing gets quite critical.

Boosters with oil pumps operating at high Vi's will also perform very nicely with VFD's : The big rotors and consistent oil injection all works out at speeds to 25% and lower; and the motors do not need to be outsized to start up under a high pressure difference.

As a retrofit: if you have an excess of condenser capacity and you are a little short on high stage compressor, adding economizer to the boosters might be a lot more cost effective than finding a home for a bigger machine on the high side....

RANGER1
11-12-2009, 05:23 AM
We have a system using a subcooler (brazed plate HX) with a TXV with the gas then being returned to the economizert port of the compressor.

The problem we are having is that the liquid feeding the TXV is flashing upstream of the valve. The TXV seems to be passing a mixture of gas and vapour and is hunting, producing SH of 25-50F. The pressure is also higher in the economizer line (about 15-20 psi)

Comming out of the condenser there is about 8F subcooling. We have redone the piping to make sure it is not too restrictive. Were we branch off to the sub cooler TXV we have a solenoid and then a sight-glass. It is there we see the flashing.

We have tried several things toate:
- Changed the piping to make it as short as possible. - Change the refrigeratio charge (+/-30%) in 0.5 lb increments
- changed the condensing and evaporating tempeatures
- Adjuted the SH on the TXV (not resposive)
- Checked the lication of the equalizing line and the location of the bulb. for the TXV.

But no noticable change in reducing the upstream flashing or the economizer performance.

The main line of refrigerant going to the evaporator that we are trying to sub cool is getting some effect and the sight glass is clear, but not the full effect.

Is there anyone who has come accross something like this before or has options that we can try.

Any input would be greatly appreciated.


Is TX valve big enough ?
Take bulb off pipe , hold it in your hand and see what happens .

US Iceman
12-12-2009, 04:42 PM
...adding economizer to the boosters...


Hah, I did that on a system in a chem plant about 15 years ago.

Another manufacturer complained to Howden that the compressor ratings were inflated and how could they stand behind those fictional ratings!

It worked just fine. Funny thing about compressor ratings... You can do a lot of interesting things if follow thermodynamics...:D

RANGER1
13-12-2009, 06:02 AM
As a retrofit: if you have an excess of condenser capacity and you are a little short on high stage compressor, adding economizer to the boosters might be a lot more cost effective than finding a home for a bigger machine on the high side....[/quote]



Sterl how does this work ?
You have increased mass flow rate on boosters , so how does this help high side compressor .
Would'nt interstage pressure just increase to new balance point?
Just economize at intersage pressure as its near enough same thing without complicated controls .

US Iceman
13-12-2009, 05:37 PM
Economizing does not increase suction mass flow at full load. The increased capacity comes form the lower liquid enthalpy of the colder liquid supplied by the economizer.

The only increase in mass flow is the discharge mass flow because: suction mass flow + side port mass flow = discharge mass flow.

The increase in booster discharge mass flow has to be compressed by the high stage, so that would cost you high stage capacity.

RANGER1
13-12-2009, 10:58 PM
Thanks but that was why I was questioning Sterl's comment on high stage compressor a bit low on capacity , but with plenty of condensing .
It would make things worse unless you economized high stage compressor , maybe thats what he meant ?

technog
17-04-2010, 10:02 AM
Please, could you tell me which is the best way to size the economizer suction line? I mean, the gas line that connect the eco to the screw compressor port. I have noticed that the gas velocity, at the eco port, is always very high...

low0tech
29-05-2010, 07:19 PM
Hallo!the thread i read was very enlightning for me after a seminar i got a little bit confused and after many hours of reading and searching my main question remained wasn't answered....on a air to water chiller with no economiser, what kind of subcooling can we measure?Does this thing even exists? Until now i thought that an insignificant subcooling was created only by a slight oversized condenser....Help!!!!

sterl
29-05-2010, 08:25 PM
Ranger et al:

Considering "Normal" for a 2-stage plant to mean taking the liquid makeup for LT vessels from intercooler...

The flash gas from this second step of liquid expansion is suction CFM to the boosters without effecting any refrigeration.

Take the same liquid flow from intercooler to flash economizer connected to the side port of a booster and that portion off the flash gas goes through a lesser extent of compression, thus has less HP applied; and the flow through the LT vessel is correspondingly reduced....So for the same Mass Flow out the booster discharge. less horsepower was applied at the booster; therefroe less heat at the intercooler for the HS machines to remove....

BUT

That acutally makes your boosters more capable: So if the plant will tolerate, raise the intermediate pressure a few PSI and the High Stage machines will make more capacity; but you can't do that and make sense of it overall without a ittle excess in condenser, because higher head tends to reduce what you gained by economizing....


ALSO

presuming the intermediate pressure can't be raised because of the intermediate suction loads:

If you direct HP-Cond'g temp liquid through a shell and tube Econ on the HS machines and thence through a shell and tube Econ on the Boosters: both machines will be dealing with the FG load at relatively reduced pressure ratio and the mass flow of the plant overall will increase; the high stage machines have been unloaded because some of the flash load shifted to the boosters and liquid makeup to all the vessels is at low temp but high pressure: Nothing magical in terms of valves on the makeup stations; the existing stuff is seeing the pressures it always did...And you have net effect diluted the HS load but marginally increased the LS load.

US Iceman
30-05-2010, 02:08 AM
...

Take the same liquid flow from intercooler to flash economizer connected to the side port of a booster and that portion off the flash gas goes through a lesser extent of compression, thus has less HP applied; ...



yes, but... the liquid feed to the low temperature evaporators/recirc. vessel now has much lower liquid pressure available. Therefore you will need BPR's on flash tank and possible larger liquid feed control valves. If you don't use the BPR's the flash tank pressure will go to booster suction when the compressors unloads to about 75% of full side valve.

I did something like this about 15-16 years ago and it works great.

afza
03-11-2010, 01:31 AM
i was informed by our refrigerant package designer that by utilizing economizer (in this case flash), they can reduced the motor BHP about 20% with the same cooling capacity.

But i read above, US iceman did mentioned that by adding the additional flash load from econ. to the compressor side port, you actually increase the compression flowrate which in relatively will increase the compressor BHP as well and this is make sense.

Segei
03-11-2010, 12:46 PM
i was informed by our refrigerant package designer that by utilizing economizer (in this case flash), they can reduced the motor BHP about 20% with the same cooling capacity.

But i read above, US iceman did mentioned that by adding the additional flash load from econ. to the compressor side port, you actually increase the compression flowrate which in relatively will increase the compressor BHP as well and this is make sense.
Economizer will improve efficiency of the compressor. Efficiency is energy use per unit of refrigeration(BHP/TR). I would say that economizer will increase power by one step but it will increase capacity by 2 steps. To achieve the same capacity, you need larger compressor without economizer and it will use more energy(20%) then economized compressor.

GCAP CoolCast
09-11-2010, 02:49 PM
does anyone have any information on how gas densities at different pressures/temperatures play into economizer design and efficiency?

chillerman2006
01-09-2011, 01:15 AM
Big Thanks to Iceman

for such a clear & informative thread

R's chillerman

Ps: am going to have a butchers at your other threads now....well worth a read everyone !

Homer1
04-10-2011, 05:22 AM
Thanks so much for this information...Fantastic thread!

I have another question that I can't get an answer on anywhere. I have noticed that 06N compressors have a model number that designate an E for economizer and an N for no economizer as the 11th letter.

My question is - can you use one compressor for either application?
I can't see any physical difference in the compressors as such as both E and N series have 2 different unloader plates - a large square one or an oblong one (which I originally thought was the difference between the economized and non economized models) so I can't see what the difference is between these two designations and does it really matter?

Any help would be appreciated.

TXiceman
05-10-2011, 01:34 AM
96n...you are talking about a Carrier/Carlyle screw? On larger compressors, the side port is simply plugged on the non-economized machine and pull the plate off and use it as an economized machine with the appropriate economizer.

Does Carrier open the port on both machines? It has been a few years since I worked with the carrier screw.

Ken

Homer1
05-10-2011, 03:07 AM
Thanks for the reply. These are the smaller Carrier/Carlyle non serviceable screw compressors...model number example 06NW2300S5NA.

They seem to have 2 different side plates where the unloaders are. I first thought these were the difference in economiser models and non economiser models but they both seem to have them so I am wondering what to look for on the compressor so i can swap one to the other if needed.