Today, more than ever, we face demands to increase energy efficiency – above all in industry.
Plant managers and directors are constantly looking for new ways to cut production costs, improve production processes with the highest possible return on investment and keep the cost per unit of product as low as possible.
Of all the systems that production managers look after, the compressed air supply system offers the greatest opportunities for improvement and overall savings.
At the same time, among the many manufacturers of compressor equipment, you have to choose between compression technologies.
Piston or screw compressor? Fixed-speed or variable-speed? Oil-lubricated or oil-free? Single-stage or two-stage screw compressor? Anyone facing this many questions while trying to make the best decision for their production feels like running away and hiding!
This article introduces single-stage and two-stage screw compressor technologies.
01How efficiency is measured
Before moving to concrete comparisons, let us recall the most important parameters that show how efficient (or inefficient) an existing system is.
In compressor systems we have the compressor capacity, expressed in l/min or m³/h; the working pressure, in bar (8/10/13 or more); and the power of the compressor, in kW. Taken together over a unit of time, these give the efficiency of the compressor system – the specific power, expressed as the kW of energy used to produce 1 m³ of air per minute or per hour.
The lower the specific energy consumption, the more efficient the system – and the cheaper the product in terms of consumption.
02Single-stage or two-stage screw compressors?
A single-stage screw compressor has one set of rotors in a single housing, driven directly by an electric motor, via gears, or via belts and pulleys.
Two-stage screw compressors have two sets of synchronised rotors, either in a common housing (“above/below” design) or in two separate stator housings connected in tandem (“end-to-end” design).
The difference between single-stage and two-stage screw compressors lies not so much in the end result (capacity) as in the amount of energy needed to achieve it.
A simple analogy: if someone asks you to push a car into a parking space, it will take a certain amount of your energy. But if a friend helps you, the result will be the same – yet it will take FAR LESS TOTAL ENERGY to get there.
03The “energy cost” factor in the purchase decision
The cost of electricity needed to run a compressor makes up as much as 75% of the total cost of ownership of a screw compressor, so the energy savings from a two-stage screw compressor can be significant.
For example, suppose your plant runs 24 hours a day, 7 days a week (8,736 operating hours), needs 28 m³/min at 8 bar, and you pay €0.12 per kWh of electricity.
Option A
- 250 HP single-stage screw compressor
- Capacity 29 m³/min at 8 bar
- Consumption 212.6 kW at full load
- Motor efficiency 96%
- Price €65,000
Option B
- 200 HP two-stage screw compressor
- Capacity 30 m³/min at 8 bar
- Consumption 188 kW at full load
- Motor efficiency 96%
- Price €78,000
Option A may be an efficient solution compared with other single-stage screw compressors, but a simple formula shows why the advantages of two-stage screw compressors deserve serious consideration:
kW × operating hours × price per kWh / motor efficiency
- Option A (250 HP single-stage): 212.6 × 8,736 × 0.12 / 0.96 = €232,163 a year
- Option B (200 HP two-stage): 188.0 × 8,736 × 0.12 / 0.96 = €205,296 a year
Both compressors deliver the required amount of compressed air at the required pressure, but the two-stage screw compressor is more energy-efficient – saving around €26,900 in a single year of operation.
Add to these significant savings the technology of “shared” work between two sets of rotors, and for large compressed air users two-stage screw compressors are the natural and logical choice for achieving energy efficiency.
