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kengineering
03-01-2006, 10:55 PM
A system with a 1/3 HP compresor med temp with a 1/4 ton TXV and a fan coil. Is it possible to get more capacity from a 1/2 HP compressor keeping all other components the same? I would think the valve would fill the coil with the same quantity of refrigerant even though the compressor is capable of pumping more liquid. When there is not enough refrigeration efffect the design engineers where I work insist a bigger compressor is allways the answer. seems this causes higher head pressure and does nothing to get the fixture the few degrees we need.

star882
03-01-2006, 10:57 PM
even though the compressor is capable of pumping more liquid.
In a properly designed system, there should be no liquid refrigerant in the compressor.

US Iceman
03-01-2006, 11:09 PM
Is it possible to get more capacity from a 1/2 HP compressor keeping all other components the same?

Yes and maybe.

If the TXV is adjusted for a slightly lower evaporator superheat, a small increase can be gained by a lower suction gas temperature. Lower superheat in the suction gas decreases the specific volume of the refrigerant and increases the mass flow. So, you get a small capacity increase.

Liquid subcooling can pick up the compressor capacity. The problem with this is, how do you get the subcooling. It has to be accounted for somewhere.

Another trick is to use more evaporator surface area to increase the evaporating pressure. Higher evaporating pressure means higher suction pressure and more capacity.

A similar thing happens with the condenser. If you can add more condenser heat transfer surface, the condensing temperature can decrease. Lower condensing temperature equals lower liquid temperature and a slight increase in capacity. It sounds like they are adding a larger compressor without increasing the condenser capacity. A loosing proposition in my book.


...even though the compressor is capable of pumping more liquid The compressor only pumps gas. That's it. It is what you do with the gas pumped that matters.

Just putting on bigger compressor is not the answer. The whole system should be engineered correctly.

Tons by itself is meaningless. If the compressor has a 1/4 Ton capacity at some operating point, the TXV should be matched at the same point.

kengineering
03-01-2006, 11:15 PM
I should have stated it differently. Will the larger compressor put more refrigerant in the coil using the same size valve hith the same superheat? Thanks Ken

phil68
04-01-2006, 12:16 AM
This is what the boss of the test chamber company I used to work for would do when test chambers came in for refurbishment to increase pull-down speeds. In place of a 15 hp compressor would go a 20, with no other mods. It worked, but only a marginal increase was affected. As the other guys have said ideally the rest of the system should be sized accordingly.

US Iceman
04-01-2006, 01:08 AM
Hi Ken,


Will the larger compressor put more refrigerant in the coil using the same size valve with the same superheat?

The larger compressor will pump more gas, yes. At the same time the larger volume of gas flow can increase the heat rejection requirement to the condenser. Now, if the same size condenser is used the condensing temperature will increase. Assuming the air flow over the condenser remains constant.

The suction pressure will decrease slightly, unless more heat is applied to the evaporator (not likely). At the same time, the pressure difference across the TXV will increase slightly, so the valve capacity could increase and try to control the evaporator superheat.

All of this is like trying to hit a moving target. When one point of operation changes, the whole system tries to re-balance at some new condition.

When the suction pressure decreases with the larger compressor, the TD of the evaporator does increase. At the same time the lower suction pressures causes a decrease in the compressor capacity. In fact, the balance point will be slightly lower than the original operating condition.

The net effect is, you may get a slightly lower air temperature off of the evaporator temporarily, but could be loosing capacity.

wambat
04-01-2006, 01:30 AM
A higher HP compressor does not equate to more capacity per say with respect to compressor displacement, various horsepower capacities are available for compressors with the same displacement. What this means is that the potential for horsepower is available. The actual horsepower delivered is that which is required, not desired. A 5-hp load is a 5-hp load. A 10-hp compressor will deliver 5-hp and no more until the load increases. The difference is that the 10-hp compressor will not overheat and the motor windings will not burn out on a 10-hp load. The 5-hp compressor might meet the 10-hp load, but not without harmful overheating. :confused:

Lc_shi
04-01-2006, 02:44 AM
The system matching is a essential requirement. if the compressor has more capacity than matching point,two results will come out 1)the head pressure will increase,the consuming power will be more;2)if the capacity too much more,the system can't work or start/stop frequently due to the high pressure switch

rgds
LC-:)

Erik Detroit
04-01-2006, 04:19 AM
This is a very normal situation in automotive refrigeration. All other conditions and components remain the same, and the compressor capacity (speed) increases due to the driver's right foot exerting more pressure. For the sake of argument I'm going to assume the superheat and subcool changes are negligible.

The suction pressure decreases since the compressor has more capacity. Since there is more capacity, the condensing pressure must increase in order to reject more power in the condenser. This increase in compressor discharge pressure decreases slightly the volumetric efficiency of the compressor (tends to spoil compressor capacity). Since the suction pressure has decreased, the suction gas density has decreased, and this again tends to spoil compressor capacity. The result is that the system reaches a new operating condition with:
Higher condensing pressure
Lower evaporating pressure
Lower COP
Higher Cooling Capacity

LC is of course correct that you can go too far and cause problems, I am talking here about a generic increase in compressor capacity.

US Iceman
04-01-2006, 04:39 PM
The lower suction pressure means higher specific volumes and so per revolution the compressor will be displacing less refrigerant mass while each kg of refrigerant will be affecting a reduced net refrigeration effect

No argument there...


Coupled with the head pressure rising and the fact that the mass flow will reduce with pressure (density decline) we will have a reduced COP. The COP is reduced further on account of the compressors volumetric efficiency declining in proportion to the isentropic efficiency per thermal mass flow (comp cooing) decline on account of higher compression ratios.


The way I look at it is the reduced COP is a function of the combined factors of lower volumetric and isentropic efficiency. We are saying the same thing, but in different ways.


The net effect is, you may get a slightly lower air temperature off of the evaporator temporarily, but could be loosing capacity.

Let me take another stab at this. If the evaporator surface area is fixed, any increase in compressor capacity (more volume flow) will reduce the evaporating/suction pressure. This would increase the evaporator capacity due to an increased TD. Since the evaporator capacity is increased, the enthalpy difference in the air stream would also increase. So, I think we agree on this point.

My previous comment was based on a constant compressor volume, so I confused the issue.

kengineering
04-01-2006, 11:33 PM
I love this site. You have all given me so much help and information to consider. Thanks, Ken

Lc_shi
05-01-2006, 02:45 AM
"any increase in compressor capacity (more volume flow) will reduce the evaporating/suction pressure. This would increase the evaporator capacity capacity due to an increased TD. Since the evaporator capacity is increased, the enthalpy difference in the air stream would also increase. "
More volume flow of compressor should increase the volume flow of evaporator and the evaporator pressure should increase /and capacity increase;the TD increase is not meaning the capacity increase necessarily. I doubt this conclusion.
Hope more discussion and make it clear.

thanks
LC-:)

US Iceman
05-01-2006, 03:20 AM
..More volume flow of compressor should increase the volume flow of evaporator and the evaporator pressure...

How will the larger compressor increase the evaporator pressure???

If a larger compressor is installed on an evaporator that was fitted to a smaller compressor, the suction pressure should decrease. Since the evaporator pressure is now lower, the TD on the evaporator is slightly greater. I think you will see a marginal increase in system capacity.

Plot out the compressor capacities at various evaporating temperatures. Then cross plot the evaporator capacity at different TD's. You can see the balance point move as you change the compressor capacity.

Dan
05-01-2006, 04:14 AM
Excellent and broad ranging discusion! My two cents worth. Original question:

A system with a 1/3 HP compresor med temp with a 1/4 ton TXV and a fan coil. Is it possible to get more capacity from a 1/2 HP compressor keeping all other components the same?

Absolutely! Pardon me, but I am a refrigeration person and I need to up the horsepowers a bit for convenient example. I will work with 1/3 of a ton of refrigeration versus half a ton.

Let's say you had 1 hp low temperature compressor (Copeland model KAJB-0100) providing you 3,492 Btu/hr at -25 Deg F Evaporating Temp and 105 Deg F Condensing Temp.

And you change it out to a 2 hp low temperature compressor (Copeland model EAVB-210). You will now have a compressor providing 5,733 Btu/hr at -25 degrees Evaporating Temperature and 120 deg F condensing temperature. A 64% improvement in capacity when looked at this way.

A better way to look at it is how much colder you need the refrigerated space to be, because this is probably what is motivating you to increase the horsepower size.

Let's say the 1 hp compressor was only achieving a space temperature of -5 deg F with a -25 deg F evaporating temperture (20 deg F TD between space and evaporating temperature). But you need a -20 deg F space temperature.

The 2hp compressor will provide 4,000 Btu/hr (1/3 ton) at -40 deg F evaporating temperature and actually even more, because you will now be condensing at 115 deg F because of the lower suction required to meet the desired space temperature.

So you will have effectively decreased the space temperature from -5 deg F to -20 deg F, or more. Quite significant if you are trying to keep ice cream brick hard.


More volume flow of compressor should increase the volume flow of evaporator and the evaporator pressure should increase /and capacity increase;

No, Lc. Perhaps you are thinking of it backwards. What you say is true if we look at it this way:

If the load on the evaporator increases, not so much the capacity of the evaporator, then you will have increased volume flow. The compressor has a fixed displacement, thus it can only absorb this higher load by operating at a higher suction pressure as it pumps more molecules during each stroke or revolution. Think of refrigeration being the victim of the load instead of master of the load.:)


the TD increase is not meaning the capacity increase necessarily. I doubt this conclusion.

The TD increase is what the evaporator requires to attain additional capacity. For example, an evaporator that has a capacity of 6,000 Btu/hr at a 10 deg TD, will have a capacity of 12,000 Btu/hr at a 20 deg TD, broadly speaking. If the original question speculated what would happen with an evaporator change... or a load change, then that would be another variable to introduce.

Quite a robust discussion!

Renato RR
05-01-2006, 08:43 AM
However, it will make use of it's extra 15% capacity permitted or accommodated now on account of the increased TD.

So if I have machine with two Bitzer 8GC60.2Y (60HP)and replace them with two 8FC70.2Y(70HP) i would get acording to you 345 kW instead of 300 kW.Well this is good way to save some money.

I found machine from 1986 (chiller 200kW) never repaired until 2005.(Smoller compressors) and didnt found yet with biger compressors that old but stil I am very young.

Try to be polite Marc You are beter tehnician but that is all nathing more.

US Iceman
05-01-2006, 04:20 PM
I have to agree with Dan's comments on the discussion. This topic can be a lot like playing chess on three levels. There are a lot of dynamics that have to be included.

The important part to remember is; If you move one point of operation, something else has to occur.

Lc_shi
06-01-2006, 04:22 AM
I like this discussion:)

if the load keep unchanged ,the suction pressure should be lower. It's correct.

rgds
LC

Dan
06-01-2006, 04:32 AM
So if I have machine with two Bitzer 8GC60.2Y (60HP)and replace them with two 8FC70.2Y(70HP) i would get acording to you 345 kW instead of 300 kW.Well this is good way to save some money.

Renato, I just used an example that was convenient to me and looked at the performance curves published by the compressor manufacturer. If you have Bitzer performance curves you might find it an interesting exercise to replicate what I did with the Copeland data, which is available as a download from the Copeland/Emerson website. But it is quite possible that you can attain a capacity increase in the proportion you suggest that I suggested.:)


The important part to remember is; If you move one point of operation, something else has to occur.
As Iceman notes, once you change one thing, such as evaporating pressure and you follow the single line of that change, you will have a result that shows that this one change affected condensing pressure, and you have to recalculate the capacity again with the newly discovered condensing pressure. It's fun stuff for the mathematcally challenged person such as me.

US Iceman
07-01-2006, 04:46 AM
No matter which way all of the contributing factors are calculated or assumed, the argument is nevertheless based on the difference between theoretical and real world values reported by the manufacturer.

As both volumetric and isentropic efficiency are concerned it is possible to contrive many factors which combine to create the specific efficiencies.

The manufacturers have included these as a result of their testing process. The important part we are interested in is the illustrated in the following.

At a specific speed, a compressor of fixed displacement has specific value for volume flow, i.e., cubic meters per minute. The refrigerant mass flow is established by the vapor density. In turn, this mass flow produces the refrigeration effect by the respective difference in enthalpies.

Whatever mass flow that is developed is in vapor form the actual cubic meters per minute of refrigerant flow for this capacity.

The compressor displacement divided by the actual volume flow is the real volumetric efficiency. This combines all of the many factors that cause a loss of net pumping capacity.

A similar effect is seen when the theoretical power input required is calculated versus the manufacturers table value. The calculated values are always lower than real world operation.

As a result, you find the compressor does not compress according to a constant entropy, but deviates from it.

In effect by using a fixed specific heat ratio as shown in your script, you have introduced an assumption that the specific heat ratio is constant. While I have no problem with the methodology presented, it should not be assumed that this is an exact science.

Most specific heat ratios are values based on a fixed condition. This again is not true during a compression process as you will only find a discrepancy in the isentropic compression process and call it part of the efficiency.


As far as prognosticating new system balance points go one would need to overlay TEV, Evap, Compressor and condenser capacity curves taking into account changes in pipe friction and increased liquid temperature and subcool.

I agree. This is called system modeling. In lieu of creating a complex model, basic assumptions can be generated in the form of a spreadsheet curve which serve to provide the rudimentary facts to establish an approximate balance point.