How to conduct compressed air audits on a budget

Low-cost, low-tech ideas to improve compressed air system operation and efficiency.

By Ron Marshall, Compressed Air Challenge

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Is your budget so tight you can’t afford to bring in a compressed air auditor? There are a number of low-tech and low-cost ways to assess your system to get a good idea of your current operating costs, system efficiency, and level of system waste. Once you know your numbers you can even predict possible savings for some common compressed air efficiency measures. Why wait when you can do it now and start the process of harvesting some low hanging fruit?

A definite first step in understanding your system is to understand the supply side and how the compressed air is produced. Effective assessment methods should look at both the supply and the demand parts of the system. A very good way to learn about compressed air systems is to take part in one of Compressed Air Challenge’s Fundamentals of Compressed Air Systems seminars somewhere near you. Some of the methods discussed in this article are also presented in this seminar.

Drawing a block diagram

The first step in assessing your system is to draw out a block diagram of your supply side and some elements of the demand side so you can start to understand how things are connected and how the compressed air flows through to the end uses. On your block diagram you will need to collect and record relevant information about your compressors, air dryers, filters, and storage receivers (Figure 1). This will give you a resource from which you can do calculations or ask questions of your compressor supplier or service provider. One important piece of information is compressor nameplate data. Through this information you can learn the type of air compressors you have, their pressure ratings, their rated power consumption, and how much air they can produce.

cac1
Figure 1. The first step in assessing your system is to draw out a block diagram of your supply side and some elements of the demand side so you can start to understand how things are connected and how the compressed air flows through to the end uses. (Source: Compressed Air Challenge)

Control mode

One of the things students of the CAC’s Fundamentals seminar learn is the differences in the various compressor control modes. These modes of operation and the way each compressor is set up to operate within a system are often the most important elements in producing compressed air efficiently. In fact, one of the key ways to optimize your compressed air system is to produce the compressed air in the most efficient manner possible.

For the purposes of this article we will deal with lubricated rotary screw compressors, the most common type of compressor in the industrial market. This leaves out centrifugal compressors and multi-stage reciprocating units. Lubricated rotary screw compressors can operate in one of five capacity control modes:

  • start/stop
  • inlet modulation
  • load/unload
  • variable displacement
  • variable speed.

To complicate matters some compressors can operate in a number of these modes at the same time. Let's assume for our purposes that any compressors used in examples will be in one mode only.

An air compressor has to be controlled because, if you think about it, it is very rare that a fully loaded compressor will exactly match the plant load in a facility. If the compressor were left uncontrolled it obviously would not start when there was low pressure; if it were manually started and left to run at full load it would push the pressure up to extreme values until something blew up. Of course this is not what we want, so the compressor manufacturers have figured out various ways to automatically control the compressors by limiting the output in some way to match the compressed air load. Determining which control mode you're using is important because each has different characteristics. If you don’t know, you should ask your compressor supplier.

Calculating baseline energy and cost

Making a block diagram tells you what you have; the next items you need to calculate are how much energy the system is consuming and how much it's costing. The second step in the road to improvement involves creating a baseline, determining your energy consumption. This step involves taking basic electrical measurements or estimating the electrical consumption. In addition, annual operating hours need to be determined. For more accurate cost estimate results, you’ll also need a copy of your most recent electrical bill.

For most operating modes, it's fairly easy to get a rough idea of each compressor’s energy consumption. The tricky part is estimating how much compressed air flow each compressor is producing so you can estimate the supply system efficiency, expressed as specific power (kW per 100 cfm produced). The method of measurement depends on how accurate you want to be, with the highest accuracy costing the most money.

Measuring the compressor power consumption is best done using a three-phase kW meter. If measuring an air compressor for baselining without a kW meter there is a standard formula to use to estimate the power consumption from measured Amps and Volts:

fig1

 

 

Where:
A = average Amps of all three phases
V = average line-to-line voltage
PF = measured or estimated power factor (Power factor at full load can often be taken from the main compressor nameplate. If not known, use 0.85 at full load and 0.6 at in the unload position.)

Measuring electrical parameters shall always be done by qualified personnel using the appropriate personal protective equipment and approved safety procedures.

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