Buying a Compressor? Looking at saving energy in your plant? - Here are some steps you should consider before paying out a good sum of money into a machine that will have a legacy spanning 15 to 20 years!
Know your DEMAND (Present and Future)
Every Plant Engineer knows this but they assume they know the plant's CFM requirement and don't need to measure it. Conducting a study or an audit on compressed air usage will help by optimizing sizing based on data, historical and present – There is the need to know the existing demand and
future required demand. Without an audit, i.e. flow and pressure measurement existing and future demand is guesswork -
engineering should have less guessing.
Request for Equipment Matrix from Consumer, i.e. your plant production engineer! – They will inform you on the
number of machines and required parameters!
DESIGN - You have to be one step ahead of suppliers!
A Supplier's first priority is sales- period. He or she has been given the task to sell the product and they have to always highlight the good points. That is their job! Yours is to make the best decision based on your requirements and limits! Do your research; for example, check on maintenance complexity. You dont's want to be stuck with a special equipment that needs John or Justin to fly in 1000 km away to "fine-tune" the machine! Its a compressor and not a space shuttle! Check on actual performance. Learn about SCFM and ACFM. Google it and read.
You know your sizing, know the pressure required and the dryness factor - do a matrix of the different type of compressors available, their price, their maintenance issues, cost and benefit and your decision making is made much easier. We have met plant engineers who limited their selection by selecting the same compressor brand which has been running in their plant for the last 20 years! Such myopic views is sad because the engineer is not only doing injustice to him or her but generally to the company and the environment!
Your design does not just stop at selecting the equipment
Ensure piping is correctly sized and with less bends as possible – explore layout and seek best solution. Check sizing on Receiver Tanks – calculate usage, estimate frequency of demand. Receiver tank is meant to cater for small changes in the system – i.e. correctly sized receiver tank will excessive cyclic time for the compressor will be greatly reduced!
Some points:
- Use refrigerant dryer to pre-dry and dry compressed air before using high energy waste type dryers, i.e. desiccant dryers. Pre-Drying is the smart way to dry compressed air!
- Select a desiccant dryer that has large settings limits, i.e. from -4 Deg to – 40 Deg.
- Explore the possible use of VSD Compressors if the loading is going to have large variation. Take note that if load is to be constant and in the range of 80 to 100%, VSD type compressors are not optimal.
- The best efficiency compressors are usually centrifugal type and a CFM/ kW for the given pressure should be used as the comparison
MONITORING – You cannot design and manage what you don’t monitor (or know)!
When you spending a few hundred thousand dollars on a new compressor, budget in a basic monitoring system - you need flowmeters, pressure sensors before and after filters, dryers and at the end user point and power meters to track efficiency of the compressors. It will be better to capture the data in one minute intervals and chart real-time!
Further Optimization
- Heat Recovery for boiler feedwater, other hot water use
- Ensuring cooler ambient air is channeled to the compressors. If possible use cool (and clean) exhaust air to precool the ambient air for example.
The diagram below shows a schematic design of the optimized compressor plant with single drying and pressure requirement.
Further refinement can be done by
segrating based on required pressure and dewpoint - i.e.
subsystems where
a) Drying of air can be done closer to production and where
required - only drawback is possibly noise (can be handled)
b) Low and High Pressure loop - if there is a large
variation in the pressure requirement, the system can be
split into 2 loops, say 8 bar and 5 bar loops