Reverse heat exchanging system for boiler flue gas condensing and combustion air preheating

The combination of a reverse heat exchanging system and a heat pump system employing evaporative heat transfer fluids that continuously circulate in closed heat exchanging circuits to cool and condense acidic water vapors contained in a relatively hot flue gas stream flowing from a fossil fueled boiler and to recover heat energy usually emitted into the atmosphere to preheat the boiler combustion air stream and feed water stream, the combination which comprises:

• a. a flue gas cooling and condensing heat exchanger enclosure containing a first high temperature flue gas cooling evaporative heat exchanger, a second medium temperature flue gas cooling evaporative heat exchanger, a third (above ambient) low temperature flue gas cooling evaporative heat exchanger, and a fourth (below ambient) low temperature flue gas cooling evaporative heat exchanger, a top inlet to receive said boiler hot flue gas stream, a bottom outlet to release cooled flue gas, a mist eliminator and a bottom outlet to drain acidic water vapor condensate;
• b. a combustion air preheater heat exchanger enclosure containing a first low temperature combustion air heating condensing heat exchanger, a second medium temperature combustion air heating condensing heat exchanger and a third high temperature combustion air heating condenser heat exchanger, a bottom inlet to receive a relatively cold combustion air stream from the atmosphere, and a top outlet to release preheated combustion air to said boiler;
• c. said first flue gas high temperature cooling evaporator heat exchanger co-operates with said first high temperature combustion air heating condenser heat exchanger employing a first evaporative working fluid that circulates in a first closed loop piping system, a first working fluid circulating pump, and a temperature control system to respond to changes in the mass flow and temperature of the flue gas and combustion air streams passing though said high temperature heat exchangers and to adjust the mass flow rate of said first working fluid to maintain a preset pressure and temperature range of said first working fluid;
• d. said second flue gas medium temperature flue gas cooling evaporator heat exchanger co-operates with said second medium temperature combustion air heating condenser heat exchanger employing a second evaporative working fluid that circulates in a second closed loop piping system, a second working fluid circulating pump and a temperature control system to respond to changes in the mass flow and temperature of the flue gas and combustion air streams passing through said medium temperature heat exchangers and to adjust the mass flow rate of said second working fluid to maintain a preset pressure and temperature range of said second working fluid;
• e. said third flue gas (above ambient) low temperature cooling evaporator heat exchanger co-operates with said third low temperature combustion air heating condenser heat exchanger employing a third evaporative working fluid that circulates in a third closed loop piping system, a third working fluid circulating pump, and a temperature control system to respond to changes in the mass flow and temperature of the flue gas and combustion air streams passing through said low temperature heat exchangers and to adjust the mass flow rate of said working fluid to maintain a preset pressure and temperature range of said third working fluid;
• f. said fourth (below ambient) low temperature flue gas cooling evaporative heat exchanger employs a forth evaporative working fluid that circulates in a fourth piping system connected to a refrigeration gas compressor, a direct expansion valve and co-operates with a heat pump condenser heat exchanger utilized to preheat the boiler feed and make-up water streams.
• g. an intermittent heat exchanger coil washing system utilizing high pressure water or steam spray nozzles.