Heat recovery

Heat recovery with a heat recovery rate of 0.8 or 80%

Heat recovery (WRG) is a collective term for processes for making the thermal energy of a mass flow leaving the process usable again. In the simplest case, with relatively continuous heat dissipation and absorption, a heat exchanger is sufficient , otherwise a heat storage device is also necessary, for example a regenerator as a short-term heat storage device.

The aim of heat recovery is to minimize the primary energy requirement . In addition to energy management needs, ecological requirements are also met.

Overview

The process of heat recovery can be used with gases, liquids or solids for heating or cooling processes. Heat recovery systems can be divided according to their heat exchanger into:

Recuperative systems
Regenerative Systems
Regenerators
Heat pumps

Evaluation of the transmission behavior with regard to pollutants and odors based on VDI 2071
with regard to ventilation systems:
WRG system	with heat exchanger	back wärmzahl	Rear moist number	Transfer behavior at
WRG system	with heat exchanger	back wärmzahl	Rear moist number	Smells	Germs	dust	Oil & fat	Gases	operating trouble
Recuperative systems
	Plate heat exchanger	0.4-0.8 (*)	0.0	O	O	O	O	-	-
	Plate heat exchangers (moisture-permeable foils)	0.4-0.8 (*)	0-0.8	O	-	O	-	-	-
	Shell and tube heat exchanger	0.3-0.5 (*)	0.0	O	O	O	O	-	-
Regenerative Systems
Circulatory network systems	Compact heat exchanger	0.3-0.5	0.0	+	+	+	+	+	+
Circulatory network systems	Countercurrent layer heat exchanger	0.7-0.8	0.0	+	+	+	+	+	+
Heat pipes	Gravity heat pipe ( thermosiphon )	0.2-0.4 (*)	0.0	O	O	O	O	O	O
Heat pipes	Capillary heat tube	0.5-0.8 (*)	0.0	O	O	+	O	O	O
Regenerators
Rotors	Rotor with sorption	0.7-0.8 (*)	0.6-0.7	O	-	O	O	-	-
Rotors	Rotor without sorption	0.7-0.8 (*)	0.1-0.2	O	-	O	O	-	-
Others	Capillary blower	0.2-0.4 (*)	0.2-0.4	-	-	-	-	-	-
Others	Switch memory	0.6-0.9 (*)	0.5-0.7	-	-	-	-	-	-
Heat pumps
	Compressor heat pump	./.	0.0	+	+	+	+	+	+
	Adsorption heat pump	./.	0.0	+	+	+	+	+	+
(-) unsuitable, (-) less suitable, (o) only suitable with auxiliary and special construction, (+) suitable (*) Contains leakage and / or co-rotation, (./.) Missing reference scale

Heat recovery and rewetting numbers

The heat recovery coefficient indicates the ratio of the transferred temperature to the temperature difference of the inlet media and is identical to the degree of temperature exchange, efficiency or degree of change. The heat recovery rate can be related to the warm and cold side. With the same heat capacity current, the values of both heat recovery coefficients are the same in the sensitive case. The heat recovery in the picture above results, for example, in a double-sided heat recovery coefficient of (19 ° C - 0 ° C) / (24 ° C - 0 ° C) = (24 ° C - 5 ° C) / (24 ° C - 0 ° C) = 0.8 or 80%. Analogously, the moisture recovery number indicates the transferred absolute humidity to the maximum transferable absolute humidity.

Advantages of heat recovery

Reduction of the connected load [kW] for heating and cooling energy
Reduction of energy consumption [kWh] for heating and cooling
Downsizing or eliminating the boiler, refrigeration machine, recooling system, piping, technical center, chimney, ...
Reduction of investment and operating costs in other trades (e.g. heating, cooling)
Reducing pollutant emissions
Reduction of the temperature difference (elimination or reduction of the reheater for comfortable supply air temp.)

Applications of heat recovery

Air exchange: In air-conditioned buildings as well as in passive houses and Minergie houses, the energy content of the exhaust air is used by a ventilation system to temper the supply air. The supply air is heated in the cold season and cooled in the warm season. Since new buildings are built tightly to protect against outside noise and heat loss, controlled ventilation systems are used in residential buildings to automatically remove sufficient moisture and prevent the formation of mold . Heat recovery can also be integrated here. Recirculation mode is not classified as heat recovery. Heat recovery systems with multifunctional use can be used as refrigeration systems ( adiabatic machines ) and can cool from the evaporative cold of the exhaust air.
sewage: Most of the domestic hot water consumption is used for personal hygiene. When showering, a flow of warm waste water and the consumption of cold water for water heating take place at the same time. In a downpipe heat exchanger , the draining wastewater is cooled and the incoming cold water is heated. The heated cold water then flows into the water heater and needs up to 35% less energy to be heated to 60˚C. Normally, the temperature of the mixed wastewater from residential buildings is so low that the thermal energy it contains can only be used with heat pumps (see wastewater heat recovery ). Heat exchangers for places where there is sufficient warm water (shower, bathtub) are still relatively uncommon, although they are currently economical. An interesting development are shower cubicles, shower trays and downpipe heat exchangers in which the cold water is heated up with the waste water. This preheats the cold water and can be used directly or fed into the water heater. Conclusion: a significant reduction in the energy consumption required for hot water.
Industrial processes: 14,000 kW absorption heat pump for utilizing industrial waste heat in an Austrian district heating plant

Many industrial processes require high temperatures. When cooling the products, but also the heated ambient air or other gases that are heated up during the process, heat can be recovered. This heat can be recuperatively introduced into the process at low temperature levels (i.e. the two fluids run in separate systems) or used for heating purposes. Furthermore, the heat can be converted into electrical energy. Since waste heat usually occurs at a low temperature level, conversion into electrical energy is usually only possible with the help of the ORC process .
Exothermic reactions: In some industrial processes, the mass flow that leaves the process contains combustible gases in addition to the heat and can provide additional heat through their combustion. In the case of a wind heater , the "hot blast " provided for the blast furnace process is generated by recovering the heat from the top gas and by burning the top gas and supplied natural gas. With this technology it became possible in the 19th century to increase the "hot wind" to temperatures of around 1300 ° C. Only then did the production of some alloys become economically successful, such as ferromanganese , which was important for warship construction at the time, and ferrosilicon .

The use of exhaust gases from steam boilers (“ economisers ”) and power plants ( combined heat and power ) is generally not referred to as heat recovery.

literature

VDI Society for Technical Building Equipment: VDI guideline VDI 2071 , heat recovery in ventilation and air conditioning systems, Beuth-Verlag, 1997

Web links

Wiktionary: heat recovery - explanations of meanings, word origins, synonyms, translations

Heat recovery and waste heat recovery, Federal Office for Economic Affairs (Switzerland) PDF file

Individual evidence

↑ Ferdinand Kohn: About the representation of iron manganese alloys and their use for steel production . In: Emil Maximilian Dingler (ed.): Polytechnisches Journal . tape 200 , fourth issue. JW Cotta'sche Buchhandlung, 1871, p. 280 ( books.google.de ).

[1] Ferdinand Kohn: About the representation of iron manganese alloys and their use for steel production . In: Emil Maximilian Dingler (ed.): Polytechnisches Journal . tape 200 , fourth issue. JW Cotta'sche Buchhandlung, 1871, p. 280 ( books.google.de ).