what is useful of Economizer?

[Ahmed.ragab]

hi dear sirs,
i want to know wt is useful of economizer and wt is the type of it and use for wt?
thanks

[NoNickName]

Economiser is what is called a plate heat exchanger, vapour to liquid, used to increase subcooling at the expense of
1) superheating
2) some liquid evaporated in the other channel of the PHE

[US Iceman]

The term economizer is used to describe a process of increasing the compressor capacity. The increase in capacity comes from one of two methods:

1. The liquid refrigerant is cooled below the condensing temperature using a heat exchanger. This heat exchanger is an evaporator used for cooling the liquid refrigerant. In one side of the heat exchanger liquid refrigerant flows through it, while on the other side of the heat exchanger refrigerant is boiling (just like a normal evaporator). This type of economizer produces high pressure subcooled refrigerant.
2. The second example is called a flash type economizer. In this type the refrigerant flows through an expansion device into a vessel. The liquid flashes off into vapor (which goes into the economizer connection on the compressor). The cold liquid falls to the bottom of the vessel. This liquid is saturated at the economizer pressure.

In either example, the cold liquid refrigerant has a lower enthalpy which increases the amount of heat each pound or kilogram of liquid refrigerant can absorb (Net Refrigeration Effect or also called NRE).

Because of this increase in the NRE the compressor capacity is increased, while the power required by the compressor motor only increases slightly.

[SteinarN]

The term economizer is used to describe a process of increasing the compressor capacity. The increase in capacity comes from one of two methods:

1. The liquid refrigerant is cooled below the condensing temperature using a heat exchanger. This heat exchanger is an evaporator used for cooling the liquid refrigerant. In one side of the heat exchanger liquid refrigerant flows through it, while on the other side of the heat exchanger refrigerant is boiling (just like a normal evaporator). This type of economizer produces high pressure subcooled refrigerant.
2. The second example is called a flash type economizer. In this type the refrigerant flows through an expansion device into a vessel. The liquid flashes off into vapor (which goes into the economizer connection on the compressor). The cold liquid falls to the bottom of the vessel. This liquid is saturated at the economizer pressure.

In either example, the cold liquid refrigerant has a lower enthalpy which increases the amount of heat each pound or kilogram of liquid refrigerant can absorb (Net Refrigeration Effect or also called NRE).

Because of this increase in the NRE the compressor capacity is increased, while the power required by the compressor motor only increases slightly.

The sentence in bold is the essence of the whole economizer idea. It isn't really the decreased liquid temperature which is responsible for the increase in COP or cooling capasity, it is the compressor operating with a higher pressure suction port in addition to the main suction port. For a single compressor this is possible only on a screw or scroll compressor. It could be possible in a reciprocating compressor with more than lets say 5 or 6 cylinders if the suction of one cylinder was separated from the main suction and utilized as the economizer port. If for one or another reason the evaoprating pressure in the economizer should happen to be at the same pressure as the main suction, then there would not be any gain in COP or capasity even though the liquid temperature after the economizer clearly would be very low.

[US Iceman]

It isn't really the decreased liquid temperature which is responsible for the increase in COP or cooling capacity,...

I have to disagree.

The increased compressor capacity is a result of the increased net refrigeration effect created with the lower liquid enthalpy being fed to the evaporator(s). Since the compressor is essentially a constant volume device (at full load) the mass flow is increased due to the lower liquid enthalpy thus the compressor capacity is increased.

The side port connection (economizer connection) is simply a port that is available at a pressure greater than suction pressure. Without any pressure regulator the side port pressure determines the evaporating pressure for the economizer (heat exchanger or flash vessel).

By comparison to a reciprocating compressor... when the piston starts to rise and the pressure in the cylinder starts to increase; if you could install a pipe connection in the cylinder at a suitable location (just as the piston starts to rise) this would allow a reasonable pressure you could use as a side port connection for economizing also.

The side port pressure is just the pressure from which the flash gas is compressed back up to discharge pressure. The small amount of flash gas that must be compressed from this intermediate pressure (between suction & discharge) back to discharge pressure is the reason the power goes up in small increments for the increase in compressor capacity.

This is an identical process for either type of economizer.

[Billy Ray]

US Iceman / SteinarN

I understand the concept in increasing the plant/compressor capacity via the usage of the plate heat exchanger to lower the liquid enthalpy, this therfore providing a greater enthalpy change across the evaporator & therfore increased cooling effect.

What i do not unsderstand is the relavance of having a higher pressure on the suction side of the plate heat exchnager & inputing this higher suction gas into the screw compressor ecomiser port.

Why the need for an economiser port? Why can this not be at system suction pressure?

Billy

[US Iceman]

Why the need for an economiser port? Why can this not be at system suction pressure?

We make this more difficult than it needs to be.

Think about it like this:

The compressor pumps the gas from suction pressure to discharge pressure. It provides a certain amount of what we call cooling capacity. For this capacity it takes specific amount of power to compress the refrigerant vapor from a low pressure to a higher pressure.

We know subcooling is good and increases the compressor capacity for any given operating condition. The tricky part is understanding the cost of providing the subcooling to increase the compressor capacity.

We could cool the liquid with a separate refrigeration system (mechanical subcooling). This increases the larger system capacity that is being subcooled however we have to pay for the cost of operating the little refrigeration system doing the subcooling.

Or we can back up liquid in the condenser (which ultimately has the potential of raising the discharge pressure if not done properly). This offers very little subcooling unless the air temperature is cold (assuming air cooled condensers here).

Or, we can use an economizer. Think of an economizer as the little refrigeration system mentioned above for mechanical subcooling. We are only cooling the main liquid supply (for the larger system) down to some intermediate temperature. Because of this the evaporating temperature for the little system can be much higher, which saves some operating cost (higher COP).

Also, since the subcooling of the liquid only requires a small amount of actual cooling capacity the cost for cooling the liquid is relatively low to what you can gain.

When an economizer is used on screw compressors, the side port connection is at a higher pressure (greater than suction and lower than discharge). Therefore you can consider this intermediate pressure to be the evaporating pressure for the mechanical subcooler.

Now, instead of using a separate compressor to pump the flash gas generated by subcooling the main liquid supply...you pipe the flash gas into the side port. Since the port pressure is higher than the suction pressure you are only compressing the gas at lower compression ratio.

If you pump the flash gas from suction pressure to discharge it costs you more power. And... it also costs you some compressor capacity to do the subcooling since this would be a separate evaporator operating at the main suction pressure.

Understanding economizers and side ports can be difficult because of all of the processes that occur. Each type of economizer has some specific requirements that need to be provided in order to function properly.

[SteinarN]

I have to disagree.

The increased compressor capacity is a result of the increased net refrigeration effect created with the lower liquid enthalpy being fed to the evaporator(s).

Yes, as long as you see it from the evaporator point of view. But not from the compressor point of view. From the compressor point of view, the reason for the increase in capasity and COP is the economizer port having a higher pressure than the main suction. This leads to the compressor being able to compress a larger mass flow, than would otherwise be possible with it's fixed displacement, if all suction gas was at main suction pressure. Imagine a 6 syl. compressor operating at -30°C. Then imagine the same compressor with one of its sylinders cut off from main suction and in stead operating that sylinder at an economizer pressure of 0°C. The mass flow through the evaporator would decrease. The cooling capasity would increase due to the decreased liquid temperature more than offset the reduction in mass flow through the evaporator. The mass flow at compressor outlet and through the condenser would increase due to the economizer suction operating at a higher pressure than the main suction.
The compressor power consumption would increase, but not as much as the cooling capasity, hence better COP. But this requires the economizer to be at a higher pressure than the main suction.

Imagine a 3 syl. compressor with one of it's sylinders operating an economizer. If this large economizer capasity causes the economizer pressure to be as low as the main suction, then no increase in capasity or COP would occure despite the very low liquid temperature at economizer outlet. You could rather operate the economizer port at main suction also without any difference in capasity. Hence my standpoint it is the higher economizer pressure which is the essence of the economizer idea.

Since the compressor is essentially a constant volume device (at full load) the mass flow is increased due to the lower liquid enthalpy thus the compressor capacity is increased.

On this I have to partly disagree.
The mass flow rate is not increased in the evaporator and compressor main suction port. It is however increased at compressor discharge and in the condencer.

The side port connection (economizer connection) is simply a port that is available at a pressure greater than suction pressure.

This is exactly what I said and ment in my previous post with the essence of the economizer idea is to have a suction port at a higher pressure than the main suction port

Without any pressure regulator the side port pressure determines the evaporating pressure for the economizer (heat exchanger or flash vessel).

By comparison to a reciprocating compressor... when the piston starts to rise and the pressure in the cylinder starts to increase; if you could install a pipe connection in the cylinder at a suitable location (just as the piston starts to rise) this would allow a reasonable pressure you could use as a side port connection for economizing also.

In this I agree.

US Iceman / SteinarN

I understand the concept in increasing the plant/compressor capacity via the usage of the plate heat exchanger to lower the liquid enthalpy, this therfore providing a greater enthalpy change across the evaporator & therfore increased cooling effect.

What i do not unsderstand is the relavance of having a higher pressure on the suction side of the plate heat exchnager & inputing this higher suction gas into the screw compressor ecomiser port.

Why the need for an economiser port? Why can this not be at system suction pressure?Billy

See above. If the economizer port is at the same pressure as the main suction, then the reduction in capasity due to decreased mass flow through the evaporator is exactly equal to the increase in capasity due to the lower temperature liquid entering the evaporator/TEV. Better leave the full suction capasity operating the evaporator alone without any economizer.

[SteinarN]

US Iceman, a moment I saw the possibity for a lively discussion here.
I think however we are saying almost exactly the same thing here, we are mostly saying it in a slightly different way.

[daddyfixit]

Very interesting. What are the common applications for the economizing compressor systems? I've never come accross any...yet.

[SteinarN]

Since the port pressure is higher than the suction pressure you are only compressing the gas at lower compression ratio.

If you pump the flash gas from suction pressure to discharge it costs you more power. And... it also costs you some compressor capacity to do the subcooling since this would be a separate evaporator operating at the main suction pressure.

Here we are at the core of the economizer idea.

[Thermatech]

Allthough this is posted in the industrial section I hope you dont mind if I provide an example from air conditioning.

Mitsubishi Electric have used a subcooler circuit in the City Multi VRF systems for nearly 20 years now.
The subcooler is used to cool the liquid as it leaves the condenser on the outdoor unit.
Because VRF systems use multiple indoor units which have an LEV electronic expansion valve, it is important to eliminate all bubbles from the liquid so that only pure liquid arrives at each indoor unit. This helps the indoor unit to accurately control the opening % of the LEV valve & therefore helps the overall system efficency.

It works like this in cooling mode.
As the warm liquid leaves the condenser it first goes through a tube in tube heat exchanger. After subcooling any bubbles are gone. Then as subcooled liquid it flows along the liquid line to all the indoor units.
Directly after the subcooler a very small volume of subcooled liquid is diverted through an LEV valve to flash off in the subcooler tube in tube heat exchanger thus cooling the primary liquid passing through the larger tube. The vapor then is dumped back into main suction line just before the suction accumulator.
The outdoor unit control board monitors a number of pressure & temperature sensors. Some of the data is used for Subcooler LEV valve % opening control.
1/ TH5 liquid leaving condenser temp.
2/ TH7 subcooled liquid leaving subcooler temp
3/ TH8 refrigerant vapor after leaving the subcooler.
4/ TH2 main suction line before accumulator temp.

So the outdoor unit constantly monitors all this data & very precisely adjusts the % opening of the LEV valve to achieve the target subcooling required.

This is some typical data example taken from an old Mitsubishi service manual for system running flat out at standard japan summer temps of 35 deg c db outdoor ambient & 27 deg C db indoor on coil.

TH5 liquid leaving condenser 40 deg C
TH7 subcooled liquid after subcooler 27 deg C
TH8 vapor leaving subcooler 8 deg C
TH2 main suction line 6 deg C

The manual quotes a compressor suction temperature of 7 deg C at this condition.

Only a very small volume of liquid is used to achieve the subcooling of the liquid & the impact on comp suction temp very small.
But the lower liquid temp helps cooling performance & makes sure there are no bubbles in the liquid line.

Just manually shut the subcooler LEV valve & see the impact on the indoor unit performance. You will suddenly see a dramatic reduction in cooling perforance at the indoor units & you then will realise how important it is to have pure liquid with no bubbles in the liquid line.

The Mitsubishi Electric design enginners had this all worked out over 20 years ago & its been a standard part of the VRF systems ever since.

Sorry for long post but hope it shows how benifical the subcooling can be.

[US Iceman]

Hi SteinarN,

You are correct about the mass flow argument I posted. I did not say it correctly.

On an economized compressor the suction mass flow is constant at full load. The discharge mass flow will be comprised of: suction mass flow + flash gas mass flow = discharge mass flow. You were correct.

What I was trying to say is that given a specific cooling load it may be possible to use a smaller compressor displacement if the smaller compressor is economized.

The increase in refrigerating effect would of course require a smaller mass flow.

Hopefully, that will clear up my error in trying to post a fast explanation. My bad.

[US Iceman]

Only a very small volume of liquid is used to achieve the subcooling of the liquid & the impact on comp suction temp very small.

That in a strict sense is only mechanical subcooling, not economizing. If the gas created by subcooling is vented to the suction line the subcooling load becomes part of the main evaporator load.

To fit into the scheme of economizing the gas created by subcooling would have to vented/piped back into the compression stroke of the piston (or degrees of rotation for a screw) at a pressure slightly greater than the main suction pressure.

[US Iceman]

US Iceman, a moment I saw the possibity for a lively discussion here.

Don't give up yet. It may still happen.

[SteinarN]

Don't give up yet. It may still happen.

All right, I'll give it one moore try.

I clearly see the main suction displacement on a screw remains the same when utilizing the economizer port, but in principle the effective available displacement is increased as long as the economizer pressure is higher than main suction. The cycle drawn on a log p-h chart is exactly the same in this instance as in an instance when utilizing the suction of one of the cylinders in a multi cylinder compressor as the economizer port. The difference is that this cylinder alternatively could operate at main suction also, which is not the case in a screw.

The reason to the increase in COP is the higher pressure in the economizer suction port and the reason for the increase in capasity is the increase in the effective displacement of the compressor and/or that some of the displacement is utilized at a higher suction pressure. When utilizing an economizer port on a screw, the effective displacement increases, when adding an economizer cylinder to a resip, the effective displacement increases and when "stealing" one of the cylinders of a resip and utilize it as an economizer cylinder, then some displacement is utilized at a higher suction pressure. All three cases give an increase in capasity, albeit the last example give less increase in capasity but also less increase in power consumption, in addition to the increase in COP.

Edit:
I'm not sure what I want to say.....
We dont disagree enough

[US Iceman]

...but in principle the effective available displacement is increased as long as the economizer pressure is higher than main suction.

That is one way of looking at it. The view I hold is the mass flowing into the side port increases the thread pressure (where the side port is located). If you push too much mass into the side port the pressure will start to rise and could become higher than the evaporator connected to the side port.

In effect, the thread becomes supercharged with the boost in pressure which also increases the power input.

This tends to happen more on applications where the side port is utilized for many evaporators or other purposes instead of only subcooling liquid.

I think we are in agreement now... (unless you decide to correct me again)

[Billy Ray]

US Iceman / SteinarN

This is getting very good!!

Shall we proceed further!!!

Questions....

To what extent do you consider the effectivness of the sub-cooled liquid to the plant.

i.e to what limit would you consider sub-cooling the liquid 5 / 10 / 15 / 20 degK ?

where does the effective limit end?

is there a negative effect (assuming too much sub-cooling) on the typical DX evaporator?

a consideration must be given to the evaporator distributor/leads?

Billy Ray (pupil)

[SteinarN]

I'm heading for bed now. I leave Billy Ray to you US Iceman.

[US Iceman]

To what extent do you consider the effectiveness of the sub-cooled liquid to the plant.

i.e to what limit would you consider sub-cooling the liquid 5 / 10 / 15 / 20 degK ?

where does the effective limit end?

You cannot cool any lower than the saturation temperature of the side port at full load. If the slide valve unloads the port pressure begins to decrease to the main suction pressure of the compressor.

If you use a heat exchanger (instead of a flash economizer) the lower limit is the side port saturation temperature plus the approach temperature allowed by the economizer design.

There are some minor corrections you have make for pressure loss in valves, piping, etc but the general concept is as listed above.

Sometimes depending on how the system is design someone may try to subcool the refrigerant too low.
If you cool the liquid below the evaporating temperature it will not flash off when it flows into the evaporator (sensible heat transfer instead of boiling).

Just like with an expansion device you have to correct the valve capacity for liquid temperature and pressure differential. No different in that respect.

[TXiceman]

One way I have explained it to non-engineer type is that you take part of the load and do it at a higher temperature where it is more energy efficient than doing it all at the lower temperature.

It is basically the same as an intercooler on a centrifugal compressor.

You can use a flash type economizer or subcooler type.

To really get to the meat of the discussion you need to get into the thermodynamics of the Rankin cycle.

Ken

[SteinarN]

One way I have explained it to non-engineer type is that you take part of the load and do it at a higher temperature where it is more energy efficient than doing it all at the lower temperature.

Ken

Exactly! This is the core of the economizer idea.

[Billy Ray]

Thats know understood.

I assume this is the same principle on a economiser system on a two-stage recip. Injecting into the intermediate stage.

Would i be correct in saying that it also provides cooling to the intermediate stage? or is there a requirement for liquid injection aswell?

Billy Ray

[RANGER1]

The only one in this post who i agree with would be US Iceman in regard to explanation of economizer.
Its not the interstage of a 2 stage recip
Its not as suggested converting 1 cylinder on a 6 cyl machine to pump gas on an economizer subcooling liquid line ie at a higher pressure
To say in a similar way as US Iceman
eg 6 Cylinder comp (***** not ammonia)
suction -40 deg C SST Discharge +40 deg C
Piston at top dead centre and as piston travels to bottom of its stroke sucking gas from main suction line, at bottom of stroke a port in cylinder is open to economizer (open or flash type,doesnt matter in this case) gas flows in as long as port is open as piston travels up discharging gas .
The compressor is using all 6 cyls on main suction line and on economizer suction making it as efficient as possible under those conditions.The subcooled liquid from this system has increased refrigerating effect from the same size equipment, but electric motor current draw would increase slightly.
Rule of thumb 1 deg C subcooling = 1% increase in capacity
Screw comp is the same, as female rotor thread opens to suction and gas fills vane .As male rotor turns interlobing into the female rotor this vane is isolated from suction and discharge line at a slightly higher than suction pressure.At this point the economizer port is open to this vane.The economizer (suction)gas flows in to this lower pressure than economizer pressure so cooling of economizer liquid takes place(subcooling).We know comp is most efficient at 100% load etc.
This system is usually applied to med to low temp applications as it is these systems with lower suction pressures that the economizer really starts to work signifficantly.More power is required to run compressor as with recip.
Mycom 160VLD at conditions previosly stated R22
no economizer economizer

capacity 95.6KW 123.7KW

Absorbed 92.8KW 104.3KW
power

COP 1.03 1.19

30.4 KW INCREASE with economizer

This is my understanding anyway.

[US Iceman]

I assume this is the same principle on a economiser system on a two-stage recip. Injecting into the intermediate stage.

Would i be correct in saying that it also provides cooling to the intermediate stage? or is there a requirement for liquid injection aswell?

A two stage or compound compressor is different than an economized compressor. On a compound compressor you will usually see an intercooler or an intercooler and a subcooler. Either the subcooler or intercooler operate at the interstage pressure, which seems similar to an economized compressor.

However, the gas load generated by the intercooler or subcooler are part of the high stage suction. This is a little complicated.

Anytime you add gas to a compressor suction you decrease the useful compressor capacity for something else (like the main evaporators).

However, by using an intercooler and subcooler on a compound compressor the additional gas from these is offset by a capacity increase due to the intercooling and subcooling.

One way of thinking about this is; If the gas load from any cooling process is dumped directly into the compressor suction this is not economizing. This would simply be part of the suction load for the compressor.

However, if the gas load from any cooling process is injected into the compressor at a pressure slightly greater than the compressor suction pressure this would be called economizing.

It all depends on where you compress the gas from and the impact to the overall efficiency.

[Sergei]

I don't see big difference between two stage compression and compressor with economiser. For example, first stage compress gas from 0 psig to 25 psig and discharge this gas into suction of high stage. Reciprocating compressors require desuperheating at this stage, but it isn't issue for screw compressors. At this stage additional mass flow will be added to the suction of high stage. This mass flow will be created by high temperature load and by flash gas due to subcooling main liquid flow. No difference between two stage compression and one stage with economiser. Capacity of this stage in compressor with economiser are more limited than for high stage of two stage compression. Efficiency of compressor with economiser is lower that two stage compression due to higher pressure difference between high pressure load and pressure in injection port of this compressor.

[TXiceman]

If you are looking at a flash economizer, this is essentially the same as an intercooled centrifugal. You are flashing the gas to a lower level to lower the liquid enthalpy. The gas that is flashed off at the intermediate level is taken into the side port of the compressor (or on a centrifugal between stages. The lower temperature liquid is then fed down to the evaporator level and you have a greater NRE (net refrigerating effect) or a larger delta enthalpy at the evaporator. This means that for the same mass flow as on a noneconomized system you are now doing more cooling. Remember capacity = mass flow x delta enthalpy.

Now a subcooler economizer will subcool the remaining liquid in a shell and tube of shell and coil vessel (economizer). This lowers the liquid temperature which increases the delta enthalpy in the evaporator. BUT, you loose a few percent in evaporator capacity since the liquid is typically cooled to within 10 dF of the side port pressure.

I do agree that on an ammonia system you will get a 1% capacity increase for each degree you cool the condense liquid at the condenser. When you go to a subcooler economizer design and subcool the liquid 50 dF of more, you will not see a 50% increase in capacity. So you do need to be careful about such a broad statement when we are discussing a subcooler economizer system.

Each manufacturer locates the economizer side port in a slight different location relative to suction and will have a different side port pressure, thus will have some difference in the comparison of economized and non-economized ratings.

The side port is at a fixed location and when unload the machine, the side port pressure will drop and change the economizer effect. Once most compressors are below 80% slide valve travel, the side port is at suction pressure. I feel that you need to control the pressure to the economizer with a economizer pressure regulator, otherwise the subcooler temp will drop like a rock. If the system is a flash economizer, you need to hold up the pressure so that you have enough pressure to feed to the evaporators.

The flash type is used on close -coupled systems where you do not need high pressures to push the liquid to remote evaporators.

Now lets all crack open a cold one.

Ken

[US Iceman]

Thanks Ken. That was a good addition!

Have one for me.

[Billy Ray]

Thanks all,

Billy Ray

[US Iceman]

I wonder what happened to the original poster who asked the question and never came back?

[nike123]

I wonder what happened to the original poster who asked the question and never came back?

Maybe he has got more answers then he could absorb and he then exploded from that! And now, he is not able to respond in that state of mater.

[SteinarN]

I wonder what happened to the original poster who asked the question and never came back?

Well, at least it has been an interesting topic for other people in here

[US Iceman]

Well, at least it has been an interesting topic for other people in here

This seems to happen a lot and I don't know why.

Or perhaps nike123 is correct. Does that make us responsible for those people?

[melisaltinoluk]

"Economizers" or "Economisers" are mechanical devices intended to reduce energy consumption, or to perform another useful function like preheating a fluid.
The term economizer is used for other purposes as well. In simple terms, an economizer is a heat exchanger.

M.A.
Industrial Engineer, Energy Consultant