This is a simple question with a complicated answer. It depends on the type of compressors you have and what your goals are in terms of energy efficiency and pressure control. This first blog post deals with small reciprocating compressors; as for the control of screw compressors, we will save for part 2.
If your system consists of a number of small reciprocating compressors, energy efficiency will not be your primary concern. You should control the compressors to maximize the compressor life. Typical small multi-stage air-cooled reciprocating compressors need to run at duty cycles that are lower than about 60%. There are a few manufacturers that offer units that run continuously, but these are rare. The installed cooling systems on these typical compressor types are simple, no units should run at 100% load for long periods of time or overheating damage will occur.
Setting these units up in a cascaded pressure band strategy, where one compressor’s start and stop pressure band is slightly higher than the next, will cause problems because if the system demands that more than one compressor runs, one of the compressors will be fully loaded all the time, this breaks the rule about having less than 60% duty cycle.
Proper control of a combined system of multiple reciprocating compressors requires parallel strategy, where all the compressors more or less start and stop together controlled by a central pressure switch. Of course, if a system of four or five reciprocating units all started at the same instant, it could stress your electrical system. Thus, time delays need to be designed in to prevent this. A controller that might start the compressors one at a time with a time delay between starts would cause less stress on the main supply breaker. Some more sophisticated systems might rotate compressors to share hours, or only run new units, leaving the old units for back up duty.
A simple way to run multiple compressors in a single facility is to separate the compressors into independent zones, one compressor to a zone. The layout of each zone should be such that the load is balanced so no one compressor will run fully loaded for long periods of time at above its maximum rated duty cycle.
Care should be taken in monitoring the compressors in any control strategy to ensure they are not exceeding their rated duty cycle, even with parallel control. If your plant or shop load is such that the compressors must run fully loaded all the time, then you may need to consider purchasing rotary screw compressors which have no problem with full load operation.
Storage receiver capacity and pressure band width is important to protect against damage to the compressor motors due to excessive starts per hour. Typically, very wide pressure bands are applied and significant receiver capacity installed to minimize the starting frequency.
Be aware that if your compressors are all running at full load, and they weren’t doing so before, there may be a problem with your compressors or system. Internal failures of compressors or water drains can false-load your system and keep the compressors loaded. Also, system leakage or inappropriate use may be wasting compressed air and your compressor energy. If this is a recent problem, investigate and fix!
If you are concerned about your compressor control, contact your supplier to come have a look at your system. Your supplier may have a control system solution to help solve your problems.
[…] common type of compressor found in industrial sites. If you have small reciprocating compressors, go back to Part 1 of this […]