screw verses reciprocating compressor

[ukram]

we are planning to installed a new refrigeration system for spiral freezing. Capacity of the refrigeration plant is 355kw at minus 40. we prefer to used reciprocating compressor because we are already using reciprocating compressor for different application. we are familiar with the recip compressor.But our consultant praposed screw compressor for new refrigeration system. I just want know the advantage & disadvantage of the both. Can sombody explain me.

[RANGER1]

Advantage for screw would mainly be price .
If no or little intermediate load , a single stage economized single screw comp would handle job .
Someone can correct me but you would need 4 to 5 reasonable size recip compressors to do same job ie 3 booster compressors and 2 high side .
Recip compressors would all have to serviced every say 2 years as opposed to screw full service 30-40 000 hrs .
Screw option could be more expensive if you want redundancy with a spare machine . I think if system design is good screw would be very reliable and if worried about redundancy for breakdown buy a few spares like elec motors , shaft seals , pumps etc .
There are a lot of screws running plants with no redundancy and all ok .
( a spare is always nice though )

[sterl]

Most reciprocating compressors respond to varying operating pressures immediately due to the nature of their valves: so pressure in the cylinder need only rise a fraction over that of the discharge pipe before gas is expelled.

Nominally reciprocating compressors for R-717 compress "dry" that is without any substantial volume of oil in the compression space. Because most recip compressor pass the refrigerant through the crankcase on the way to the compression chamber, oil is picked up; some have oil delivery to the space between rings to reduce piston and ring wear;, and of course those machines tend to pass a little oil. Always there is some wear of the rings against the cylinder because when the piston reverses direction, there is no realtive motion and thus shear on stagnant oil film; so the contact between ring and cylinder wall is imminent.

Almost all reciprocating compressors for R-717 have head cooling and often jacket cooling, desiigned to recirculate water or glycol.

A screw compressor compresses from the point where the rotors first pass the suction inlet port to the point where they first make that same cavity "see" the discharge port: Which means the machine can overcompress internally, or undercompress. The moving parts that compensate for this condition and adjust capacity are linked logically and shifted by servo motors or hydraulic cyliinders pressurized with oil. Either arrangement move pretty slowly, in search of the right extent of capacity and compression ratio...While the capacity adjustment is itself a continuum, as opposed to the steps of a recip, an extent of hunt is inherent to the machine, and the volume ratio adjustment is not happening every compression stroke as it does on a recip.

The rotors of a screw compressor do not contact the housing or end plates except at the bearings. The potential for contact is between the male and female rotor of a twin-screw; only one is driven and the pressure seals between rotors and between rotor and housing is effected by bathing the internals with a lot of oil...The shear of the oil between moving parts both isolates the parts and acts like the piston ring on a recip. The oil travels through the machine from low pressure end to high and absorbs some of the heat of compression out of the refrigerant vapor; this does make for a more efficient compression process. It also serves to keep the vapor cooler on the discharge side compared to a recip running at high pressure ratios...

The normal "package" that accompanies a screw compressor has a sophisticated oil separator, oil cooling, and oil circulation circuit and so on. With the cooler gas the oil temperatures tend to stay quite low and are easily separated out on the discharge side without oil breakdown; and the machine body is not cooled . The difference in quality between one screw compressor package and another has as much to do with the effectiveness of oil separation, oil cooling and circulation and the effectiveness of the control panel and operator interface as it does with the compressor itself.

Economically, the sophistication of controls and oil recovery on a screw, large or small, is much the same; so a low capacity screw is an expensive piece per unit capacity; a high capacity one is relatively inexpensive, especially once you go past where a single machine, "big" recip will take you.

Vibration on a screw compressor is different as is the sound. The travelling parts in a reciprocating compressor reverse twice each rotation, so there are 16-mechanical jerks on the crank of an 8-cyl compressor, each full rotation. Pressure pulses in a screw are generated at motor speed times the number of lobes on the driven rotor, often 3550 times 6, which is about 22,000. An 8-cylinder recip at 1000 rpm makes 8000. The stresses on a screw compressor's rotors are associated with pressure differences and transmitting torque, while those on a cranshaft are related to both of those plus the reversal of the moving parts.

Service is different; mcuh of the regular service addresses the oil circuit. Twin rotor screws have operated 75,000 hours and more without opening the machine (I'm sure that will stimulate some anecdotes, from advocats from both sides of the number). Recip teardowns tend to happen a little more frequently, but do not require the sophisticated measurements and handling equipment that a big-rotored screw will.

With the crankcase upstream, a little debris getting past the suction housing of a recip is not a DOA. With a screw: anything hard metal larger than a fingernail clipping is probably going to make the machine a write-off. A screw will tolerate a minor porportion of liquid carryover steady state; and a pretty good drink in standby providing the machines outlet is to an accomodating space because the refrigerant will literally fall to the botttom of a stationary machine.

A Spiral Freezer at Minus 40 represents a challenge in terms of oil transport, removal and so on....A machne that puts less oil in the circuit would be a big advantage. A "normal" recip won't make the pressure difference in one stage of compression. The application leans "screw" as opposed to some others....But with very good oil separation and good machine cooling, a 2-stage recip plant can do it reliably. An econmized, thermosyphon screw can do it with less total pieces and fewer total controls, and less input maintenance wise. Course when the time for Big Maintenance comes along, it may be of greater duration unless a second machine is on hand....

[US Iceman]

Recips.'s you can rebuild yourself. Screws should be rebuilt in a shop capable of doing this. Screws at high compression ratios will have increased wear on the thrust bearings, while recip.'s on 2-stage system will last a long time if you keep liquid out of them.

[RANGER1]

ukram,
What type of recips are you familiar with ?
Also what type and brand is consultant proposing , if any

[chilldis]

just a personal preference here, screw compressors with thermosiphon oil cooling over a recip. with water/glycol cooled heads for for a couple of reasons. longer service life between planned maintenance,i.e. valve plate replacement. more precise capacity control,most "packages" have all controls included,and my favorite, a quieter compressor room.

[Andy]

Recips.'s you can rebuild yourself. Screws should be rebuilt in a shop capable of doing this. Screws at high compression ratios will have increased wear on the thrust bearings, while recip.'s on 2-stage system will last a long time if you keep liquid out of them.

Hello

Also a properly designed recip system will us less electricity. But which is the cheapest when you consider whole life cycle costing

Screw or Recip.

Screw less maintenance but more electric.

Recip less electric but higher maintenance.

Kind Regards Andy

[ukram]

ukram,
What type of recips are you familiar with ?
Also what type and brand is consultant proposing , if any

First of all thanks to showing keen interst in our case.
We are familiar with Indian make Kirloskar Make compressors. Our consultant praposed to us a Grasso make screw compressor.

[ukram]

Hello

Also a properly designed recip system will us less electricity. But which is the cheapest when you consider whole life cycle costing

Screw or Recip.

Screw less maintenance but more electric.

Recip less electric but higher maintenance.

Kind Regards Andy

Dear Andy,

Thanks for reply
Another wild question:-Do you have any operating cost comparision on screw verses recip. it will give me a rough idea.

best regards,

Ukram

[ukram]

I sincierly thanks to you for elabarative reply on this case. If you have any operating cost comparision of both compressor then it is good.

best regards,
Ukram

[US Iceman]

I sincierly thanks to you for elabarative reply on this case. If you have any operating cost comparision of both compressor then it is good.

best regards,
Ukram

The operating costs vary depending on how much time the two different compressors operate at part-load. Screw compressors are terrible for part load operating performance and hence have much higher operating costs. However, if you use an inverter for driving the motor on a screw compressor the part-load performance can improve greatly.

[Andy]

I sincierly thanks to you for elabarative reply on this case. If you have any operating cost comparision of both compressor then it is good.

best regards,
Ukram

Im afraid to say the whole life cycle costing must be carried out on a plant by palnt basis.

Costs to be considered.

1/ Captial costs
2/ Running costs
3/ Miantenance costs

If you ask the suppliers tendering to supply this information and tie this into the contract, with agreed cooling capacity against the running and maintenance costs you will be able to determine hich plant is the best for you.

I would assume that your home built recips will be more cost effective than an imported screw pack.

Kind Regards Andy

[D.D.KORANNE]

NH3 ( R-717) :
The new generation of ammonia recip involving finer design of valve reeds deliver commendable performance . For two reasons , a) reeds are finer b) compressors operate at high speed 1450 rpm . At times direct coupling can add to the performance further by minimizing losses v/s belt transmission.Conventional recip compressors are too heavy & operate at slow speed ; 600-900 rpm.
As pointed out earlier in the disc above , compressor dry run ( free of liquid ) is essential.
Contact ddk0123@gmail.com

[bpard001]

So in general, screw compressors require less maintenance but consume more electricity compared to a recip. compressor?

[Magoo]

I generally have a preference for screw compressors at -40'C, due to the reduced number of moving parts and tollerance for saturation.
But given the capital costs of local recips versus imported compressors, local knowledge of similar applications and high tech PLC's and VSD's etc., etc... The plant specifier and client should agree on plant design and long term servicablilty.
magoo