When a business looks at ways to improve its compressed air system’s energy efficiency, there are a number of solutions available, from datalogging and leak detection to layout and design. For businesses looking to take the energy efficiency of their compressed air systems a step further, heat recovery offers a potential opportunity. However, businesses must look closely at the calculations to ensure it is a truly viable option. Here, Andy Jones from Mattei weighs up the pros and cons
When considering heat recovery in the overall equation, businesses are essentially coming at energy and cost saving from a different direction. Datalogging, fixing leaks and identifying the optimum compressor solution for a company’s precise compressed air needs is all about maximising efficiencies within the process of producing compressed air.
Heat recovery effectively adds a new dimension to the compressed air system to achieve an outcome that is not directly related to its primary function. In a nutshell, businesses don’t invest in a compressed air system for the purpose of generating heat, but as heat is a by-product of the process, there are circumstances in which heat recovery can add value to the compressor system.
Heat recovery is by no means a new technology, but in relation to compressors it has come to the forefront as part of the general focus on reducing carbon emissions by conserving energy
In a compressed air system, around 90 per cent of the electrical energy used by a compressor is converted to mechanical energy, which in turn is converted to heat. Around 80 per cent is transferred to heat to the lubricant oil and a further 10 to 12 per cent is transferred to the compressed air, again in the form of heat.
Integrated into the compressor’s cooling system, a heat recovery unit is capable of recovering up to 90 per cent of the available thermal energy. In a typical heat recovery situation, the system can transfer the heat generated by the compressor’s oil and air circuits, to heat water up to 90°C.
In some industries, this water can readily be used as process water in a production plant, which is common practice in the food, pharmaceutical, textile and chemical industries. Other uses include sanitary purposes, pre-heating boiler water or space heating.
This capability leads firstly to the question of whether heat recovery capability should dictate the choice of compressor type. With some oil-free compressors, it is said that it is possible to reclaim 100 per cent of the total electrical energy input in the form of hot water. However, that is countered by the fact that these compressors are in themselves inefficient when compared to oil-lubricated machines because there is no cooling effect during the actual compression.
To assess whether the additional investment required for an oil-free compressor would be justified, it is necessary to compare the energy costs of running the compressor with the savings that heat recovery would offer. The average heat recovery potential for an oil flooded compressor is 85 per cent, compared to 100 per cent with some types of oil free compressor. However, the running and maintenance costs of an oil-lubricated compressor are lower, and, even taking into account heat recovery, total energy costs are lower with an oil-lubricated compressor. So, in principal, heat recovery potential should not govern the decision on compressor type.
The next question is whether there will be sufficient payback from additional investment in heat recovery? For businesses without a need to heat process water, the primary use for the recovered heat is likely to be space heating, which is ideal when the compressor is within the workspace being heated. Where the compressor is remote from the space(s) to be heated, a ducting system will be required.
When considering the option of heat recovery, a number of factors need to be taken into account. How much air will be available, when and where from? What are the business’ requirements for hot air, how much can be used and when will it be needed? Companies also need to look at the current source of energy for heating. This information needs to be compared with the average operating hours of the compressed air system. In general the compressor will be running all year round whereas the need for heating may only be seasonal.
Heat recovery does offer potential for considerable energy savings by making positive use of otherwise waste heat. However, payback has to take into account the cost of investment. As a general rule, the larger the compressor system and the longer the running hours, the shorter the payback on heat recovery will be. In the right application, heat recovery is a positive measure, but businesses do have to consider it as part of a wider approach to energy management rather than as an energy-saving solution in itself.