Re: Economizers and screw compressors
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....
Re: Economizers and screw compressors
Quote:
Originally Posted by
RefJGM
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 .
Re: Economizers and screw compressors
Quote:
Originally Posted by sterl
...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
Re: Economizers and screw compressors
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 .
Re: Economizers and screw compressors
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.
Re: Economizers and screw compressors
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 ?
Re: Economizers and screw compressors
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...
Re: Economizers and screw compressors
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!!!!
Re: Economizers and screw compressors
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.
Re: Economizers and screw compressors
Quote:
Originally Posted by
sterl
...
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.
Re: Economizers and screw compressors
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.
Re: Economizers and screw compressors
Quote:
Originally Posted by
afza
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.
Re: Economizers and screw compressors
does anyone have any information on how gas densities at different pressures/temperatures play into economizer design and efficiency?
Re: Economizers and screw compressors
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 !
Re: Economizers and screw compressors
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.
Re: Economizers and screw compressors
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
Re: Economizers and screw compressors
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.