1 reply to this topic

[Kathari]

Greetings,

I am an undergraduate performing a project on improving the efficiency of a reciprocating compressor. The details of the compressor is attached in this post.

 

I have proposed an idea to improve the efficiency by replacing the existing capacity control system with a variable capacity control system. The company suggested a manufacture Hoerbiger which manufactures HydroCOM which is basically a electro hydraulic variable capacity control system. I have attached a presentation that the engineer gave me to study about it.

 

The compressor is a double acting 2 stage horizontally opposed (Duplex) compressor. Capacity control in existing system is a combination of Step control, Clearance pocket and recycle control.

There are 4 unloaders for each suction side and a clearance pocket.

 

for 100%,  All unloaders are loaded

for 75%, the clearance pocket placed at the head of each cylinder is unloaded

for 50%, the head side of cyl 1 and the crank side of cyl 2 is unloaded

for 0%, all the valves are unloaded.

 

Currently the required capacity is 31% and hence 19% is recycled back.

 

The idea of my project is that, by the proposed system, recycle control is eliminated and hence the efficiency can be improved.

 

I am having some trouble grasping the basic concepts of stepless control.

 

1. Why can't we use a flow control valve at the end of discharge to control the capacity of the compressor like we do in household pumps?

 

2. I am not sure how the existing system works. Say the compressor is running at 75%, when the gas passes the first stage does the capacity reduce from 100% to 87.5% then after passing the second stage to 75%?

 

3. Can we use a VFD to achieve capacity control <50%? The engineer who I consult said that it is not efficient but did not explain the reason.

 

My project is basically using Electro Hydraulic unloaders to precisely time the closing and opening of suction valve such that the discharge of the compressor. The engineer gave me a flash animation which explains how it works. I can understand how it works for a Single acting cylinder as in animation. But I am having trouble visualising the concept for a double acting 2 stage compressor.

 

Thanks in advance

 

Attached Files

•  Spreadsheet.xlsx   13.88KB   19 downloads
•  02_HydroCOM_General presentation.pdf   1.99MB   42 downloads

[xavio]

Hello Kathari,

 

1. Your compressor is reciprocating type, not a centrifugal one. It is basically a constant volume machine, so putting throttle valve at discharge will not enable you to adjust the capacity. In fact, it is not recommended because you might inadvertently build up too much backpressure which could damage the compressor.

 

2. When your compressor is under 75% operating mode, clearance pockets are activated. It means that the compressor can only handle 75% of its rated capacity. It has nothing to do with how many stages you have there, each stage would handle 75% of rated capacity.

 

3. Each VFD motor has limit of minimum speed it can operate in stable manner, for sure it cannot run at 0 rpm. Also, your compressor's main lube oil pump is crankshaft driven, meaning you cannot run slower than certain speed, otherwise your main lube oil system will not work and you must switch on the backup pump. Finally, your compressor itself needs delicate load balancing between stages, too low a speed may cause mechanical stability problem. The manufacturer should have considered all the factors and established a minimum allowable speed for your compressor. Whether or not you can use VFD up to below 50%, it depends on your compressor rated speed (max), it could be possible for higher speed compressor. All in all, you should consult the vendor to make sure.

 

Lastly, to visualize how double-acting compressor works, try this:

https://www.google.c...ting compressor

 

Good luck!

 

xavio