reciprocating compressors vs screw, for NH3

[lraja]

Hi

For ammonia recirculation systems, how do the reciprocating compressors compare with those of screw compressors, particularly in the aspects of
a) Performance,
b) Efficiency,
c) Oil/lubricant related issues and
d) Maintenance?

Raja

[Josip]

Hi, Raja

Hi
For ammonia recirculation systems, how do the reciprocating compressors compare with those of screw compressors, particularly in the aspects of
a) Performance,
b) Efficiency,
c) Oil/lubricant related issues and
d) Maintenance?
Raja

There is no simple answer to the constantly recurring question of whether it is best to use screw compressors or reciprocating compressors. Both types are viable alternatives in virtually all applications, and both types are normally capable of doing the job.

Please visit this web site to find more about:


http://www.sabroe.com/Information/Sc...%3F/index.html

Hope you'll get your answers

Best regards, Josip

[lraja]

Hi Josip

Thank you for the reply. Well, I believe what you mentioned is correct in general. However, I thought there may be certain points our forum members couldpointout baesd on their experience on the specific application I inquired, i.e. ammonia recirculation systems....

raja

[US Iceman]

raja,

Your question is too broad to answer. It seems as if you are looking for a simple answer where there is none.

It all depends on what you want to do in the system.

If you ask a screw compressor salesman he will of course say screw's are best. I think if you ask a salesman who sells both screw's and recip's he will say... it depends on what you are trying to do.

I believe this why Josip provided the answer he did.

[Josip]

Hi, Iraja

Hi Josip

Thank you for the reply. Well, I believe what you mentioned is correct in general. However, I thought there may be certain points our forum members couldpointout baesd on their experience on the specific application I inquired, i.e. ammonia recirculation systems....

raja

Based on our expirience we can tell you so much that finally you will be (believe me ) much more confused but:

Looking for some help you must give us as much as possible information, not expect us to guess what you want. There is no other way

It all depends on what you want to do in the system.

Best regards, Josip

[lraja]

Hi Josip

Thank you for the comments; I guess I shall rephrase my question based on what you and US Iceman said.

Let us say for an industrial cooling application with evaporation at about –8 oC, and condensing at about 50 oC, an ammonia system is considered, say a liquid recirculation system with pumps. The capacity is can be about 100 ton R.

Now for this application, the salesman want to give me a screw compressor, whereas from what I leant, the screws are not very efficient at smaller capacities (say less than 1200 kW), and at part loads. This plant is likely to run on part load about 40% of the running time. My selection is a reciprocating (perhaps a Sabroe), but the client thinks a screw compressor is better when it comes to regular maintenance saying it requires relatively lower attention than a reciprocating one.

Do you think this is the case ?

Kind regards

Raja

[US Iceman]

the salesman want to give me a screw compressor,

That's what I was afraid of. If this system is going to be operating at 40% of full load most of the time, you want a recip. compressor, not a screw.

-8C is a good suction temperature for a recip., although I question why you are using 50C for an ammonia condensing temperature. Is this an air-cooled condenser?

I would suggest you size the compressor for the nominal maximum speed and use a VFD on the compressor motor.

Use the speed range for first step of capacity control, say 25% to 100% of the maximum rated speed. When the load reduces to a lower point, then unload one step of capacity on the compressor, and then use the speed range again.

You get excellent part load performance and very good capacity control.

...the client thinks a screw compressor is better when it comes to regular maintenance saying it requires relatively lower attention than a reciprocating one.

I have seen some recip. compressor run for many years with no maintenance. Particularly when they run at low speeds.

[TXiceman]

As noted above, you need to look at the part load performance of the equipment. A screw running at part load is not an efficient choice. I have sold and installed both screws and recips and I have to look at each application for duty and part load cycles.

It is not a cut and dry answer. Another thing to complicate the issue is the possibility of multiple machines for redundnecy or back up. What to look for and make the decision easier comes from many years of experience in both design and service.

A salesman that only has one compresssor type to sell will try to make any project fit what he has to sell. Hopefully you will have the time to look at the various options and decide which is better. All too often a customer will fall for a slick sales job from a sales person and not get what he really needs. This is where I strongly encourage the customer to hire an independent refrigeration engineering consultant to evaluate the project and develope a specification to have the contractors bid. Then have the engineer look at the bids and see who has the best solution that best addresses your requirements at a realistic cost.

Ken

[Josip]

Hi, Iraja

Hi Josip



Let us say for an industrial cooling application with evaporation at about –8 oC, and condensing at about 50 oC, an ammonia system is considered, say a liquid recirculation system with pumps. The capacity is can be about 100 ton R.


My selection is a reciprocating (perhaps a Sabroe), but the client thinks a screw compressor is better when it comes to regular maintenance saying it requires relatively lower attention than a reciprocating one. [/FONT]

Do you think this is the case ?

Kind regards

Raja

Your selection is OK, but having that high condensing temp, you should have a lot of problems with any compressor

Agree with US Iceman.

That's what I was afraid of. If this system is going to be operating at 40% of full load most of the time, you want a recip. compressor, not a screw.

-8C is a good suction temperature for a recip., although I question why you are using 50C for an ammonia condensing temperature. Is this an air-cooled condenser?

I would suggest you size the compressor for the nominal maximum speed and use a VFD on the compressor motor

.

A salesman that only has one compresssor type to sell will try to make any project fit what he has to sell. Hopefully you will have the time to look at the various options and decide which is better. All too often a customer will fall for a slick sales job from a sales person and not get what he really needs. This is where I strongly encourage the customer to hire an independent refrigeration engineering consultant to evaluate the project and develope a specification to have the contractors bid. Then have the engineer look at the bids and see who has the best solution that best addresses your requirements at a realistic cost.

Agree with TXiceman, also.

With good design of complete plant each compressor (with regular maintenance) will run nice.

Come back, if you have some other doubts

Best regards, Josip

[TXiceman]

A properly designed recip system will provide many years of god service. Design to keep the piston speed under 600 feet per minute. On a system like this with an industrial recip, you can easily get 2 years between top end maintenance jobs and 5 years on total rebuilds.

For your design I would use two 50% design compressors and alternate the lead/lag on them to equalize the hours. How much of the year are you going to operate at 40% or lower. A really slick design would be 3 machines at 40%. This would allow some back up and also the potential to grow. If future growth is expected, put in two 50% machines with tie in's for a third machine and additional condensing.

As for a good machine, Grasso, Mycom, Hasegawa or Vilter. I would not use any of the long stroke machines as they tend to run higher piston speeds and higher discharge temps and you do not need to push the machine anymore than you have to with the 50 dC CT.

I would really question the 50 dC condensing temp, unless this is an air cooled job. If this is the case, I'd go for more condensing surface and drop the CT as much as is practical.

Ken

[SimonBrown]

Ken[/quote]

For your design I would use two 50% design compressors and alternate the lead/lag on them to equalize the hours.

------------------------

hi Ken

How do you think ' the alternate the lead/lag ' should be arranged, I mean the selection/arrangement of implementation of hardware and contros to achieve above

simon

[TXiceman]

If you use a PLC for the controls, it is simple enough to swap based on run time of each compressor. If one compressor gets XXX hours ahead of the other one, the high hour machine becomes the lag machine and the low hour machine is the lead machine.

Ken

[Mike W]

[quote=lraja;51716]Hi Josip

whereas from what I leant, the screws are not very efficient at smaller capacities (say less than 1200 kW), and at part loads.

Hi Iraja,
Your comment that screws are not efficient at capacites less than 1200kW seems odd. There are plenty of small screw compressors that can go right down to 100 - 200 kW range.

Regards

Mike W

[Samarjit Sen]

I totally agree with Ken. It would be be a much better proposition to go in with his selection and design. Using two of 50% load recip would be much better. However it would be better if you use water cooled condensers and bring down the condensing temperature as the same will not only reduce the power consumption but will also increase the performance. regarding the rpm, I have always considered that to get a better performance and longer life run the recips at 75% of the maximum rated rpm. Vilters is a good compressor and so is Mycom.

[lraja]

Hi Guys

Well, I revisited this thread and thought of asking this

Lets say the evaporation and condensing temps of this system are at – 8 and + 35 C, with the possibility of part load running at 40% full capacity, how would a variable speed screw (selected to match the full capacity) compressor do in the aspects discussed, I mean efficiency and reliability etc

[US Iceman]

A variable speed screw compares quite well with the part load efficiency of a constant speed reciprocating compressor. Based on this comparison, you woul dhave better part load control with the VFD screw compressor as the variable speed offerd unlimited capacity control steps. A constant speed reciprocating compressor is limited to it's actual steps of capacity reduciton available.

On th eother hand, if you use a VFD on a recip compressor you again have unlimited capacity steps. The big benefit of doing this on a recip. compressor is that you can unload the compressor at various speeds (above the minimum speed of course) and still retain the capacity control benefits.

If you unload a screw compressor with variable speed you begin to loose the part load benefits.