Indirect Contact Condensation Recover

Indirect contact condensation recovery units cool gases to 38­43°C. In this range, the water vapor in gases will condense almost completely. Indirect contact exchangers consist of a shell and tube heat exchangers. They can be designed with stainless steel, glass, Teflon, or other advanced materials.

Direct Contact Condensation Recover

Direct contact condensation recovery involves direct mixing of the process stream and cooling fluid. Since these systems do not involve a separating wall across which heat must be transferred, they avoid some of the challenges of large heat transfer surfaces required for indirect contact units. As flue gases enter the heat exchanger, they are cooled by cold water introduced at the top of the unit. The heated water stream exits through the bottom of the exchanger and provides heat to an external system. A challenge with direct contact condensation is that the water can be contaminated by substances in the flue gas.

Transport Membrane Condenser

Transport Membrane Condensers (TMCs) are a developing technology for capturing water (along with water’s latent heat) from the water vapour in gas exhaust streams. Water is extracted from the flue gas at temperatures above dew point by employing capillary condensation and recycled into the boiler feedwater. Like direct contact heat recovery units, TMCs extract hot water directly from the flue gas; however, since TMCs recover the water via transport thorough a membrane, the recovered water does not become contaminated as in a direct contact unit. The technology has been demonstrated for clean exhaust streams in a natural gas­fired boiler; however, TMCs require more research in advanced materials before widespread implementation for dirtier waste streams is possible.

Heat Pumps (Upgrading Low­Temperature Waste Heat)

Heat exchange technologies described above involve flow of energy “downhill” from a high temperature to a lower­temperature end­use. This can place limitations on opportunities for heat recovery when the waste heat temperature is below the temperature needed for a heating load. (For example, waste heat may be available in the form of hot water at 32°C, while hot water at 82°C is needed elsewhere in the facility). In such cases, a heat pump may provide opportunities for “upgrading” heat to the desired end­use temperature. Heat pumps use external energy inputs to drive a cycle that absorbs energy from a low­temperature source and rejects it at a higher temperature. Depending on the design, heat pumps can serve two functions: either upgrading waste heat to a higher temperature, or using waste heat as an energy input for driving an absorption cooling system. Heat pumps are most applicable to low­temperature product streams found in process industries including chemicals, petroleum refining, pulp and paper, and food processing.

Upgrading heat can be economical in some cases depending on the temperature differential required and the relative costs of fuel and electricity. If a facility has a heat load at a slightly higher temperature than the waste heat source, the heat can sometimes be provided more efficiently by a heat pump than if it were obtained from burning additional fossil fuels.

Coefficient of performance (COP) is a measure of heat pump performance, determined from the heat output and work input:

, where Q is the useful heat output from heat pump, and W is the work input.

An important consideration in determining the feasibility of heat pumps is the waste heat temperature and the desired “temperature lift.” The type of cycle used and the type of working fluid chosen will influence the temperatures at which the heat pump can receive or reject heat, as well as determine the maximum temperature lift achievable. The efficiency of a heat pump decreases as the desired temperature lift increase. Research to develop advanced cycles and novel fluids to increase heat pump performance and flexibility in different temperature ranges could enhance the use of heat pumps for waste heat recovery.

Closed Compression Cycle

Closed compression cycle is used to recover heat from cooling water leaving a sterilizer in a dairy plant. The sterilizer in the plant discharges cooling water at 53°C. A heat pump is used to lower the temperature of the cooling water, while using the heat extracted to increase the temperature of process water used elsewhere in the plant. The heat pump consists of an evaporator, compressor, condenser, and expansion valve. In the evaporator, energy is transferred from the waste heat source to the refrigerant. Then the refrigerant enters the compressor, where its temperature increases. Superheated refrigerant then enters the condenser and transfers heat to the heat sink. Finally, refrigerant is throttled in an expansion valve before returning to the evaporator.

Open Cycle Vapour Recompression

These systems use compression to increase the pressure (and the temperature) of waste vapour. Mechanical vapour recompression (MVR) uses a mechanical compressor, while thermal vapour recompression (TVR) uses a steam ejector, and therefore is heat-driven rather than mechanical driven.

Absorption Heat Pumps

Absorption heat pumps are very similar to the closed compression cycle, except the compressor is replaced by a more complex, heat­driven absorption mechanism. Depending on the plant needs, the system can be configured in multiple ways. A “Type I” heat pump can use a lower­ and a higher­temperature heat input to reject heat at an intermediate level (e.g., upgrade the low­temperature heat). A “Type II” heat pump can use a medium­temperature input to reject heat in one lower­temperature stream and one higher­temperature stream. This second application can be used for air conditioning and/or refrigeration. Chilling cycles can be valuable for applications such as food refrigeration or for cryogenic processes in various industries.