POSSIBILITIES AND EFFICIENCY OF WASTE HEAT UTILIZATION IN COMPRESSED AIR PRODUCTION

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Reginald Joel Wilkerson
5 years ago
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1 POSSIBILITIES AND EFFICIENCY OF WASTE HEAT UTILIZATION IN COMPRESSED AIR PRODUCTION Peter Gális Aerzen Slovakia s.r.o. Mariánska Stupava Slovak Republic Phone: Fax: info@aerzenslovakia.sk. Waste heat recovering from screw compressors. In compressed air production, its temperature indrease during the compression process. The final temperature depends on the compressor construction, the inlet air temperature and the final pressure. To make sure the operation of the compressor is efficient and to achieve compressed air of good quality, it s necessary to cool the compressor during compressing the air, either by an air absorbed from the compressor room, or from outside (this is more advantageous) or by cooling water (Fig.1). Fig. 1

2 The cooling warmed air is transported outside again by a ventilator and that way all the energy returned to the air is wasted. Increasing costs of energy force the producers and operators of the compressors to develop and to use efficient devices for heat recovering systems, and use it for premises heating and domestic hot water preparation. If we would like to assume opportunities of heat recovering systems in compressors, we have to know basic principles of thermodynamic, that means to learn how electricity is transforming to the heat in compressor. The energy transported by warmed cooling air or water, contains around 95 % of the electric energy, brought to the compressor (Fig.2). Warming the engine Electric input from network Residual heat in compressed air Oil cooler Final cooler of compressed air Radiation of heat of off taken energy is transported out by cooling medium (water/air) Fig. 2 Only around 4 % of energy remain as a residual heat in compressed air and are transported out to the compressed air distribution system. The rest, 1 % is the energy lost by radiation to environment. The engine changes the power on the heat as well, so we have to take in consideration also the engine input. Proportion of input and output is the engine efficiency, which is moving between %. So for example in the event of engine 45 kw of output, the additional energy loss is appr. 5 kw more.

3 Heating the premises (halls, rooms). If we would like to use the heat contained in the cooling air, we have to make some building adaptations, which could be different, as the local conditions are changing from place to place. At first, it s necessary to lead the cooling air stream directly in the compressor, then catch it again and to transport it out. The anti-noise cover of the screw compressor is well adapted and it s possible to connect air-technical channels to it for input and output of the air to make it possible to import the warmed air in the room during winter time and to export it to the environment outside the building during summer time. But, in this way of heating the cooling air shouldn t be used for heating of such rooms, which are too far from the compressor room. The most of warmed air with temperature O C is lost during long transport or it s necessary to use isolated pipes or channels. Ideal installation of the compressor with waste heat recovering for warm air heating can be seen at the figure (Fig.3). 1 compressor with anti-noise cover, 2 admission pipe, 3 exhaust pipe, 4 additional exhaust ventilator, 5 regulation key (thermostatically controlled), 6 exhaust pipes (warming the room), 7 heat exchanger, 8 exhaust pipe (to the outside for summer season), 9 admission key Fig. 3 Warming the domestic hot water and heating water. If we want to utilize the waste heat more effectively than in previous case or to transport it on longer distances, the plate heat exchangers for heating (Fig. 4) or security heat exchangers for heating of domestic hot water - a bunch of pipes with double walls (Fig. 5), are the most suitable for screw compressors with oil injection.

4 1 absorption filter, 2 absorption regulator, 3 compressor stair, 4 combined tank: the compressed air-oil, 5 oil separator, 6 thermostatic oil regulation valve, 7 oil cooler, 8 oil filter, 9 clack valve of minimal pressure, 10 additional cooler of compressed air, 11 heat exchanger, 12 pressure switch, 13 balanced reservoir Fig. 4 and 5 Mentioned exchangers work separately from a type of cooling system, while the recuperation fore-cooler will come in front of the water and air oil cooling and thus we will get into the cooling circuit of the screw compressor. Except for that, the devices are prepared to heat the rooms directly. After installation of corresponding pipe system and regulation elements (e.g. thermostats, valves, and so on), it is possible to switch on the water heating or the warm air heating. In case of oil-free compressors cooled by water, the above mentioned recuperation is also possible, but it is necessary to solve it in accordance with local needs and conditions. How much energy can we save? Along with calculation of costs we can save, we have to take in consideration, that utilization of waste heat depends on the time of compressor operation in full output, while only in this case the usable waste heat is produced.

5 In accordance with the basic thermodynamic principles or with the tables - see tables (Fig. 6 and 7), it is possible to calculate amount of heat contained in the cooling medium. Drive [kw] 5,5 7, , , ,5 Drive Usable output Performed output [kw] 6,47 8,72 12,29 16,29 19,86 23,35 31,84 39,27 47,77 57,73 68,23 86,69 102,78 125,91 151,44 180,04 226,09 280,75 Amount of heat Amount of heat [kj/h] Table 6 t 25 K K Amount of warm air Amount of water in t 35 K K t 50 K K Equivalent crude oil [l/h] 0,65 0,87 1,23 1,63 1,99 2,34 3,18 3,93 4,78 5,77 6,82 8,67 10,28 12,59 15,14 18,0 22,6 28,1 Costs saving with 1000 Bh [DM] [kw] [kw/h] [MJ/h] 11,0 8,9 32,0 0,305 0,217 0, ,- 15,0 12,3 44,2 0,420 0,300 0, ,- 18,5 14,8 53,2 0,509 0,363 0, ,- 22,0 17,7 63,7 0,609 0,435 0, ,- 30,0 24,4 87,8 0,835 0,596 0, ,- 37,0 30,3 109,0 1,040 0,743 0, ,- 45,0 37,7 135,7 1,295 0,925 0, ,- 55,0 45,5 163,8 1,565 1,118 0, ,- 65,0 54,9 197,6 1,885 1,346 0, ,- 75,0 63,1 227,1 2,170 1,550 1, ,- 90,0 74,0 266,4 2,545 1,818 1, ,- 110,0 90,0 324,0 3,095 2,210 1, ,- 132,0 110,5 397,0 3,800 2,714 1, ,- 160,0 133,5 480,6 4,590 3,278 2, ,- 200,0 168,3 605,8 5,790 4,136 2, ,- 250,0 208,9 752,0 7,180 5,128 3, ,- Table 7

6 Values mentioned in the table are amounts of usable output, which originate from operation of the screw compressors delivered by us (BOGE, HAFI) in a full load. At the end we can state, that the decision, if to utilize the waste heat from compressed air production, eventually in which efficient way, depends on local conditions, i.e. size of installed compressors, amortization time of the devices and utilization time of the compressors in full load. Literature: 1. Doc. Ing. Miroslav Horák, CSc. Ing. Peter Jančura: Technika stlačeného vzduchu 2. HAFI a.s. Bratislava: Efektívna výroba a úprava stlačeného vzduchu Praktická príručka energetika 3. Ulrich Bierbaum (Boge Kompressoren): Druckluft - Kompendium

HEAT RECOVERY WITH AERZEN TURBOS, BLOWERS AND COMPRESSORS. Reduce costs for compressed air generation

HEAT RECOVERY WITH AERZEN TURBOS, BLOWERS AND COMPRESSORS Reduce costs for compressed air generation REDUCE COSTS FOR COMPRESSED AIR GENERATION BENEFIT FROM SAVINGS POTENTIALS. Dwindling resources, stringent