Heat Recovery

Saving energy made easy!

04/04/2016

Due to high energy costs and increasing environmental awareness, companies are forced to optimise their processes and to use their resources more economically and efficiently. From an economic point of view, the application of heat recovery by using process heat is getting more and more attractive. Now, AERZEN offers innovative solution concepts, in order to reduce energy costs on a sustained basis. Savings of more than 90% are easily possible!

Decreasing fossil sources of energy and increasing energy consumption do not only require more investments in technologies for renewable energies, such as wind, sun, geothermal energy and biologically renewable resources, they also entail growth of future strategic markets. Especially in case of process air generation, energy savings are very important, as the lion’s share of the life-cycle costs is the consumption of electrical energy. For thermo-dynamical reasons, in case of compressed air applications, heat is generated (electrical energy of the drive power is converted into heat). This heat arises as waste heat from motor, compressor and silencer, but it is also generated when process gas temperature increases.  However, in many cases, this heat is lost without being used. 

In principle, there are two possibilities of heat recovery upon operation of positive displacement blowers, screw and rotary lobe compressors or turbo machines. 

Cooling air for space heating

It can be made good use of the cooling air for the stage, the silencer and the piping under the acoustic hood as well as of the exhaust air of the oil cooler for space heating. Exhaust air channels of the packaged unit bundle waste heat (30° C to 60° C). Then, this air is conveyed as heating air to places where heating is needed. A temperature-controlled outlet ensures a controlled ambient temperature. Depending on the heating period, excess air is released to the outside via an exhaust air channel.

Application of heat exchangers

Another possibility of heat recovery is to benefit from the stored energy of the compressed medium by the application of a heat exchanger. The advantage of this is that the heat is taken from the place where it is generated. Energy losses only arise on the way to the end user. In case of this operating principle, the heated medium passes a heat exchanger that heats the water flowing through to target temperature (only applies to wastewater treatment plant technologies), or which cools the gas down to 50 °C and below (only applies to pneumatics). However, both possibilities require engineering with optimised heat transfer. Consequently, the heated amount of water and its temperature depend on the following: throughput speed of medium in the heat exchanger, pipe bundle length, pipe surface, used materials as well as size of heat exchanger.  Depending on the design of the heat exchanger, the heated water can be used as process water. Also with this method of heat recovery, the hot water is fed to a large extent into the heating system. In this way, more than 85% of the heat energy will be recovered.

The generation of process air is energy-efficient and cost saving. However, many plant operators seldom recognise these potential savings. Thanks to low investment costs, expenditures for installation or integration in the company’s process cycle pay off after only a few months. As application specialist, AERZEN offers a pinpoint design of heat exchangers with maximum amount of heat transmitted and at lowest possible pressure loss. Even older compressed-air systems can be energetically optimised and retrofitted. Another advantage: AERZEN offers everything from one source.

Heat Recovery with Technologies from AERZEN

Rising energy costs, stricter environmental regulations, and scarcer resources demand innovative solutions from industry. The energy generated for industrial processes should be used as sustainably as possible, conserving resources along the way. AERZEN has long recognised  and transposed this requirement. In many applications, the exhaust heat produced by blowers, compressors, and turbo compressors are already recovered and recycled. There are many possibilities for heat recovery. Heat exchangers can convert the heat energy generated by industrial air compression, and use it for a variety of applications: such as water heating, drying, or preheating burner air. Another simple way of heat recovery is redirecting the heated medium. With a cooling flow through the compressor and a sophisticated piping system, the warm air generated in the engine room can be redirected with insulated exhaust ducting, and used for heating other rooms in the facility.

This makes it possible to save heating costs for hot water production, room or hall warming, and warming of industrial and process water. By onward transporting respectively further use of the generated thermal energy by means of exhaust ducting or heat exchangers, also an unnecessarily overheating of engine rooms can be prevented. This way, efficiency losses of the machines due to high ambient temperatures are inhibited. The acquisition of a heat recovery system contributes doubly to sustainable, efficient and conserving applications with high energy savings. The changes that are needed are relatively simple and easy to handle.

Operating Principle of Compressors

AERZEN machines, such as screw compressors and rotary lobe compressors, compress the ambient air sucked in by means of screws.  In this thermodynamic process a large part of the electrical energy is converted into thermal energy. On one hand, the heat energy obtained in the compression can be used in the air or gas. On the other hand, the air is heated below the acoustic hood  of a machine through radiation from the motor, the compressor stage, the piping, or the silencer. The resulting exhaust heat  says nothing about the efficiency of the machine.

In the past, this heat was often lost and was vented out to the open air as wasted heat. The potentials resulting from this large amount of energy are enormous: 85% of the energy that would otherwise be wasted could easily be used for heat recovery. AERZEN, as the market leader in the field of compressor technology, has long been developing energy-efficient solutions for the utilisation of this waste heat for its customers.  AERZEN machines are used in a wide variety of applications. Two main applications are pneumatic conveying and wastewater treatment.

Heat Recovery in the Field of Pneumatic Conveying

In pneumatic conveying (e.g. In the industries of food, bulk goods, chemical, or work/industrial materials), the goal is to transport bulk materials such as flour, dusts, sand, or cement by air. AERZEN compressors, blowers, and rotary lobe compressors are often used to generate the needed air flow. During this process exceptionally high air outlet temperatures can be attained. The atmospheric air at a temperature of about 20°C (80°F)  is sucked in and can be brought to a temperature of up to 280°C (540°F)  during compression, for example, in a screw compressor application. For the conveying of bulk materials, it often must be cooled down again, because the transported materials can otherwise be damaged from the high temperatures. Using a heat exchanger, the compressed air can be extracted from this heat and be used for basic heating, hot water production, or process heat. Depending on the design, the pressure loss due to the installation of a heat exchanger is 1-3% relative to the absolute inlet pressure in the aftercooler. The resulting increased drive power of the motor  is disproportionate to the energy saved by the heat exchangers.

Heat Recovery in Wastewater Treatment

AERZEN is a specialist in water and wastewater treatment. The AERZEN machines are often used in wastewater treatment plants for generating compressed air flow, which ensures biological wastewater treatment in aeration tanks. Naturally, this generation of air flow requires a larger portion of the total energy costs of the plant. Therefore, wastewater treatment plants are generally among the largest consumers of electricity in communities and municipalities. This is reason enough for AERZEN to develop energy-efficient and environmentally friendly solutions for customers. In times of increasing environmental regulations and demands in energy efficiency, it is a matter of course for AERZEN to rely on the resource-conserving technology of heat recovery to offer even more sustainable solutions for customers. The use of the right machine technology offers many possibilities and tremendous potential. Even a compressor with a capacity of 22 kW can cover the heat demands of a single-family home. AERZEN can boast about many examples of successful implementations of heat recovery:

Example of WWTP  Filderstadt-Bonlanden in Baden-Württemberg

As part of a modernisation  and refurbishment project in order to comply with stricter environmental regulations, four Delta Hybrid Rotary Lobe Compressors were equipped with a heat recovery system in the sewage treatment plant, which was constructed as early as the 1960s. This assembly system  provides 100% oil and absorption-free compressed medium for aeration tanks efficiently in a very energy conserving way. Free of oil  and absorbents not only means benefits in maintenance and service, but also process reliability for the plant. The atmospheric air warmed up by the machines is used by an exhaust system to heat additional technical rooms. However, the substantial savings can be seen by installing a shell-and-tube heat exchanger in the main line of the system. Heat is extracted from the process air stream in the heat exchanger and used for hot water preparation.

Essen-Kupferdreh

In the WWTP  Essen-Kupferdreh, AERZEN, as a project partner, achieved highly efficient heat recovery with the plant through a shell-and-tube heat exchanger. The treatment plant uses four positive displacement blowers for the air supply to the aeration tank. The new heat exchanger significantly benefits this application. This built-in heat exchanger supplies the heating and the buffer tank for hot water with heat energy. Altogether, heat recovery in Essen-Kupferdreh saves more than €30,000 ($36,000) annually.

Wertach in Bavaria (Allgäu)

In the wastewater treatment plant  Wertach in Bavaria the installation of a new burner could be abandoned. In Wertach, two fixed-speed Delta Blowers are used for basic loads. For higher demands (e.g. for many hikers or winter sports guests) a third blower with variable speed is added .

The 68°C (154°F) air produced by the compression process is cooled to below 30°C (86°F) using an air-to-water plate heat exchanger. This allows the resulting delta of almost 40°C (104°F) to be used to supply the boiler water to the boiler itself. The pressure loss of the process air in the heat exchanger is considerably low. If no heating is required (say, during the summer), switching is easy. Through a mechanical valve in the piping the air can be passed directly into the ventilation tank instead of going through the heat exchanger -- this way, a needs-based control and heat recovery is always possible. With heat recovery, the plant reached a heating oil savings of about 1,850 liters (490 gallons) per year.

When is Heat Recovery Worthwhile?

Before deciding to purchase anything for heat recovery, several factors should be considered.

For example, the structural conditions must be taken into account:   the spatial proximity of the waste heat source and the heat sink is important.  If these are too far apart, transport losses and increased costs for piping must be taken into account.  If the cost of storing and transporting heat is high, higher investment costs are also to be expected.

In order to ensure that the installation of a heat recovery system pays off, there should also be a T of at least 5-10 k between the waste heat source and the heat sink.

Simply, the use of heat recovery is particularly valuable for larger units and continuous production processes. But even with low levels of heat waste and high capacity utilisation  at the same time, a heat recovery system would be worthwhile to have. In principle, a heat recovery system is the more worthwhile as more continuously the assemblies  are in operation.

Résumé:
The high-energy demand, which is used in production of compressed  air, can be recovered and used in many ways. With AERZEN technologies and know-how, new projects in wastewater treatment or Pneumatic Conveying can be designed to use heat energy for rooms, hall heating, or water warming (processing or service water) or for production processes. The examples of possible uses are diverse:

  • Feed of central heating systems
  • Warm  air heating
  • Sludge drying
  • Hot medium for process tasks (e.g. drying processes in production)
  • Production of hot water for shower and washrooms
  • Service water heating for canteens
  • Construction of hot air locks
  • Preheating  of burner air
  • Pool heating

Existing plants can also be modernised  and made more efficient by basic means. AERZEN offers customers a wide range of services in this area (e.g. a 24-hour telephone information service, a machine diagnosis or the AERaudit), where service employees develop a plan for the modernisation of a plant.
With the aid of the AERZEN room ventilation calculator, technicians and wastewater treatment plant owners can also find the appropriate heat exchanger for their plant themselves.