Posted 15 February 2008 - 09:52 AM
Further on our subject of compressor speeds, a colleague stepped into my office after I wrote my last comments to you and started in on the same subject. He brought in a copy of a book: "Compressors - Selection and Sizing", 2nd Edition, by Royce N. Brown and published by Gulf Publishing Co. I thought you might be interested in what Mr. Brown says about the subject and I quote him directly:
"For a single-acting cylinder compressing at the outer end of the cylinder,
Pd = (St) (N) ( Π D2/4)....................3.1
Pd = (St) (N) [ Π (D2 –d2)/4)............................3.2
Pd = Piston displacement, cubic inches
St = Piston stroke, inches
N = Compressor rotative speed, rpm
D = Cylinder internal diameter, inches
d = Piston rod diameter, inches
Another value to be determined is piston speed, PS. The average piston speed may be calculated by:
PS = 2 x St x N .....................................3.8
The basis for evaluation of piston speed varies throughout industry. This indicates that the subject is spiced with as much emotion as technical basics. An attempt to sort out the fundamentals will be made. First, because there are so many configurations and forms of the reciprocating compressor, it would appear logical that there is no one piston speed limit that will apply across the board to all machines. The manufacturer is at odds with the user because he would like to keep the speed up to keep the size of the compressor down, while the user would like to keep the speed down for reliability purposes. As is true for so many other cases, the referee is the economics. An obvious reason to limit the speed is maintenance expense. The lower the piston speed, the lower the maintenance and the higher the reliability. The relationship given by Equation 3.1 defines the size of the cylinder. Therefore, if the speed is reduced to lower the piston speed, then the diameter of the cylinder must increase to compensate for the lost displacement if one is to maintain the desired capacity. As cylinder size goes up, so does the cost of the cylinder. It is not difficult to see why the user and manufacturer are at somewhat of a cross purpose. If the user's service requires a high degree of reliability and he wants to keep cylinder and ring wear down, he must be aware of the increase in cost.
To complicate the subject of piston speed, look at Equations 3.1 and 3.8. Note the term St (stroke). The piston speed can be controlled by a shorter stroke, but because of loss of displacement, the diameter and/or the speed must be increased, If only speed is increased, the whole exercise is academic as the piston speed will be back up to the original value.
If, however, diameter alone or both diameter and speed are increased, the net result can be a lower piston speed. This is another factor that comes to bear at this point because valve life decreases with the increase in the number of strokes and this can negate the apparent gain in maintenance cost by shortening the stroke (but increasing the RPM). It would appear that the engineer trying to evaluate a compressor bid just can't win. The various points are not tendered just to frustrate the user but rather are given to help show that this is another area that must have a complete evaluation. All facets of a problem must be considered before an intelligent evaluation can be made.
After all the previous statements, it would seem that it is very difficult to select piston speed. For someone without direct experience, the following guidelines can be used as a starting point. Actual gas compressing experience should be solicited when a new compressor for the same gas is being considered. These values will apply to the industrial process type of compressor with a double-acting cylinder construction. For horizontal compressors with lubricated cylinders, use 700 feet per minute (fpm) and for non-lubricated cylinders use 600 fpm. For vertical compressors with lubricated cylinders, use 800 fpm and for non-lubricated cylinders use 700 fpm. Another factor to consider is the compressor rotative speed relative to valve wear. The lower the speed, the fewer the valve cycles, which contribute to longer valve life. A desirable speed range is 300 to 600 rpm."
I think Mr. Brown has been reading some of my threads in the past. His comments are almost identical to mine - as well as his experience. This does not surprise me.