Reciprocating Compressor 3

[FAIZANYOUSUF]

Dear Experts
I am confused about the subject compressor operation. The compressor does not make discharge pressure by itself but the discharge pressure of the compressor is the characteristic result of a system i.e. system offers resistance due to which discharge pressure is generated. Thats clear but we have learnt that by compressing the gas its pressure increases so as same as of recip compressor? Does not it mean thar compressor produces discharge pressure?
Also, the compressor must has to produce discharge pressure to get through spring loaded discharge valves(plate type valves) so in this case it is also evident that compressor produces discharge pressure. Please clear my confusions
Thanks

 

[BrianPetersen]

Oh, my. Where do I start.

Draw a box around the compressor, draw a box around the system that it's connected to. Look at what's crossing in and out of each box.

The box around the compressor will have mechanical power going in, and uncompressed air at a certain temperature, pressure, etc going in. It will have compressed air at some other temperature, pressure, etc going out, and it will have heat going out.

The compressor will have certain operating characteristics, a "map" shall we say, of its pressure ratio and its flow rate. The system that it is connected to will also have a map of that sort. Let's say that the pressure at the system is known. (It may be variable with time ... but known at a given point in time.) Go into the operating characteristics map of the compressor with that pressure (or pressure ratio that corresponds), and find the flow rate.

If the compressor has a check valve at the discharge (inside the box that you drew), and the pressure downstream is above what the compressor can do, the flow rate will be zero. The compressor can't make enough pressure to make that check valve open because the downstream pressure is higher.

Suppose the system that it's connected to is a storage tank which is starting out at atmospheric pressure. Then you switch on the compressor. It starts running. The discharge pressure has to match the pressure of the downstream system ... which starts out at atmospheric pressure. The compressor is running, it is drawing little shaft power because it isn't doing much compressing, but the flow rate is high. Obviously that flow rate is going into raising the pressure of the downstream vessel. The pressure gauge starts going up ... the compressor audibly starts working harder ... the flow rate starts going down but at least for a while, it's still non zero. If you leave it running (and the maximum discharge pressure of the compressor isn't enough to blow up the tank!), eventually the tank will reach the maximum possible pressure. Then the compressor cannot deliver any flow. The compressor sits there spinning but no longer can deliver any flow. It's trying ... each stroke will compress the air within the cylinder ... but the outlet valve can't open because the inlet pressure still doesn't reach the outlet pressure ... then the little bit of compressed air in the cylinder pushes the piston back down again.

There is an air compressor in my workshop where you are welcome to see and hear this happen. Your local industrial supply shop where you could do the same thing, is probably closer.

 

[zeusfaber]

To look at it a different way...

There are two distinct parts to each compression stroke.

During the first part, when the (pressure in the outlet line + the small pressure it takes to overcome the plate valve spring) is higher than the pressure above the piston. The plate valve is closed, the machine compresses the gas and the pressure rises steadily.

Eventually, the pressures equalise and the plate valve opens.

For the rest of the stroke, the piston is effectively pumping gas at virtually constant pressure (how constant depends on how much bigger the system is than the remaining volume above the piston).

When the pressure in the receiving system is low, the machine spends most of every stroke in pumping mode.

When the system is fully charged, by the time the plate valve opens, there's no stroke left so, with no time to spend pumping, the machine doesn't actually deliver any air to the system.

A.

 

[FAIZANYOUSUF]

Thank you very much for your replies. So by reading your post, I must realize that compressor do make pressure and that pressure has to be greater than downstream to make flow rate happen.
Thanks

Actually one of the guy tells in the post that compressor does not make pressure but the system pressure is the sole reason behind compression and delivery of a gas i.e. more the resistance the system offers, the higher the discharge pressure will be.