- Energy recovery from compressed air installations doesn’t always result in heat being available exactly when it’s required and sometimes not in sufficient quantities.
- Most new compressors from the major suppliers can be adapted to or be supplemented with standard heat recovery equipment.
- The temperature of recovered energy determines the possible application areas and thereby the value of it.
- Heat recovery for space heating requires an extra stage of heat exchange and the temperature possibilities are lower.
- Compressed air systems represent an excellent source for heat recovery and could improve overall system efficiency.
When air is compressed, it heats up – it’s simple physics. The heat energy is concentrated in a decreasing volume of air. To maintain proper operating temperatures, the compressor must transfer the excess heat to a cooling medium before the air enters the distribution system. As much as 90% of that heat can be recovered for use in your operation. If you can supplement or replace the electricity, gas or oil needed to heat water for washrooms, or direct warm air into a workspace, warehouse, loading dock or entryway, the savings can add up. The possibilities to recover this waste heat via hot air or hot water are good. The return on the investment for energy recovery typically is as short as one to three years. In addition, if the energy is recovered by means of a closed-loop cooling system, for water-cooled compressors, the compressor runs cooler, increasing reliability and service life because of better operating conditions such as equal temperature levels and high cooling water quality.
Did you know?
Figure 1. A compressed air motor consumes eight times more energy than an electric motor. (Source: Compressed Air Challenge)
The power input for a 1 hp “air” motor requires almost 8 hp versus 1 hp required for a direct-drive electric motor (Figure 1). Which makes more sense?
As participants of Compressed Air Challenge’s Fundamentals of Compressed Air seminars learn, the energy cost comparison is $1,349 using compressed air or $168 using an electric motor for five days/week in a two-shift operation at $0.05/kWh.
The losses that produce this difference occur on both the supply and demand sides — heat of compression losses on the supply side and wasted air on the demand side. Compressed air is a necessary part of most plant operations, but it isn’t the most efficient source of energy in a plant. Recovering and redirecting the heat normally shed from compressing air can offset some of the waste and operating costs associated with compressed air. A central compressor plant in a large industry that consumes 500 kW during 8,000 operating hr/year represents a yearly energy consumption of 4 million kWh. The possibilities for recovering substantial amounts of waste heat via hot air or hot water are real.
Having a fundamental understanding of how your plant’s compressed air system works and which forces influence it can help you improve its performance and increase its reliability. The overall efficiency of a compressed air system can be as low as 10% to 15%. Figure 2 shows two main components of inefficiency; one is from the wasted air by losses through leaks, artificial demand and inappropriate uses. The other is from heat of compression. In most systems, production will see only 50% of the total air compressed from the supply side. With some basic understanding in the physics of how air is compressed, as much as 90% of that heat can be recovered for use in your operation.
Figure 2. Heat of compression represents 85% of the energy in compressed air production. (Source: Draw Professional Services.
Heat recovery case study
An electronics manufacturer has an electroplating operation that requires 20 gpm to 40 gpm of deionized (DI) water at a temperature of 131° F (55° C)) for a final rinsing process. It has 16 lines that require this flow. Insufficient water temperature leads to process problems. It takes 550 kW of energy in the form of steam and electric elements to heat this quantity of water.
The plant purchased a heat recovery package for three of its 450-hp oil-free rotary-screw compressors. With 80% to 90% heat of compression recovery, it now can heat 21 gpm per compressor at 90° C (194° F). This is more than enough water flow and temperature to handle the 16 plating lines. By adding in the heat recovery system, there’s no longer any need for the steam boiler or electrical heaters (550 kW), which had been required to heat this water. The plant has been using the system for more than two years and documented total energy savings of about $230,000 per year. The system cost about $100,000, plus installation.
Whether you run rotary-screw, centrifugal or reciprocating compressors, whether water-cooled or air-cooled, you can recover a good percentage of the waste heat it generates. The temperature of the recovered energy determines the possible application areas and thereby the value.