Heat Recovery in Compressed Air Systems

Up to 94% of the electrical energy consumed by an air compressor is converted into heat. In most installations, this heat is simply vented to atmosphere and wasted. Heat recovery systems capture that energy and put it to productive use, reducing your overall energy costs and improving the sustainability of your operations.

How Heat Recovery Works

During the compression process, air temperature rises significantly. In an oil-lubricated rotary screw compressor, the oil absorbs much of this heat before passing through a cooler. In an oil-free machine, the heat is carried in the compressed air itself and removed by an aftercooler.

A heat recovery system intercepts this thermal energy before it is lost. Instead of rejecting the heat to atmosphere through the standard cooler, the system redirects it to a useful purpose, such as space heating, process heating, or hot water generation.

The amount of recoverable energy depends on the compressor size, running hours, and the temperature differential required. As a rule of thumb, for every 100 kW of electrical input to a compressor, approximately 90 kW of thermal energy can be recovered.

Air-to-Air Heat Recovery

Air-to-air heat exchangers transfer heat from the compressor's cooling circuit directly into the surrounding air. This is the simplest form of heat recovery. In practice, it means ducting the warm exhaust air from the compressor room into an adjacent space that needs heating, such as a warehouse, workshop, or loading bay.

This approach requires minimal capital investment. Many compressor rooms already vent warm air to outside. By redirecting that ductwork indoors during colder months (and back outside in summer), you gain free heating at virtually no cost. It is especially effective in facilities that run compressors year-round and have large spaces to heat.

Air-to-Water Heat Recovery

Air-to-water heat exchangers transfer the heat generated by the compressor into a water circuit. The hot water can then be used for central heating, pre-heating boiler feed water, sanitary hot water, or industrial process heating.

This method typically recovers heat at temperatures between 50 and 80 degrees Celsius, depending on the compressor type and the heat exchanger design. It is well suited to sites with significant hot water demand, such as food and beverage manufacturers, laundries, and facilities with large welfare provisions.

Air-to-water systems require a heat exchanger unit (often plate or shell-and-tube type) and connection to the site's water circuit. The payback period is usually short, often under two years, particularly on larger compressor installations running extended hours.

Heat Recovery Wheels

Heat recovery wheels (also called thermal wheels or rotary heat exchangers) are rotating devices that transfer heat from one air stream to another. The wheel rotates slowly between the warm exhaust air stream and the cooler incoming air stream, absorbing heat from the exhaust and releasing it into the supply air.

Heat recovery wheels are most commonly found in building ventilation systems but can be applied to compressor installations where the warm exhaust air needs to pre-heat ventilation supply air. They are effective at recovering heat at moderate temperature differentials and are a good option when the recovered heat needs to be delivered as warm air rather than hot water.

Benefits of Heat Recovery

• Reduced energy bills: Recovering waste heat to supplement your heating system directly reduces your gas or electricity spend on space and water heating.
• Lower carbon emissions: By displacing fossil fuel heating, heat recovery reduces your site's carbon footprint. This supports corporate sustainability targets and regulatory compliance.
• Fast payback: Heat recovery retrofits typically pay for themselves within one to three years, depending on compressor size and running hours.
• Improved compressor performance: In some configurations, heat recovery can also help to stabilise compressor operating temperatures, contributing to longer component life and fewer breakdowns.

Retrofitting Heat Recovery

Heat recovery systems can be retrofitted to most existing compressed air installations. The process involves assessing the compressor's thermal output, identifying the most practical use for the recovered heat, and installing the appropriate heat exchanger.

The feasibility and payback depend on several factors: the compressor's rated power, its average loading, running hours, proximity to the point of heat demand, and the current cost of the heating fuel being displaced. A site survey is the best starting point.

Airmech has 45 years of experience in compressed air systems and can advise on the most effective heat recovery solution for your site. Whether you are looking to offset gas heating costs or improve your energy efficiency rating, we can help you quantify the savings and manage the installation. Contact us to arrange a site assessment.