Heat Recovery Method and Apparatus

Abstract

A hot water heater or similar heating device includes equipment for pre-cooling hot flue gas while preheating water for the water heater. It further includes a heat and mass exchanger for transferring heat and water from the pre-cooled flue gas to combustion air for the hot water heater. The pre-cooler may comprise a separate device or may be incorporated as part of a condensing water heater. The heat and mass exchanger may use membranes having condensing sides and evaporating sides, which allow water to pass from the condensing sides to the evaporating sides. It may further comprise troughs for wetting the membranes.

What is claimed is: 
     
         1 . A heat recovery method comprising the steps of:
 a) providing a first heat and mass transfer membrane having a condensing side and an evaporating side;   b) wetting the evaporating side of the heat transfer membrane with an evaporative liquid;   c) passing hot humid gas along the condensing side of the heat transfer membrane;   d) passing cool dry air along the evaporation side of the heat transfer membrane;   e) cooling the gas along the condensing side of the membrane by evaporating the evaporative liquid into the gas passing along the evaporation side of the membrane and saturating the gas passing along the evaporation side of the heat transfer membrane; and   f) passing condensed evaporative fluid through the membrane from the condensing side to the evaporating side.   
     
     
         2 . The method of  claim 1  wherein the evaporative fluid is water. 
     
     
         3 . The method of  claim 1 , further including the steps of:
 providing a plurality of heat and mass transfer membranes having condensing sides and evaporating sides, and placing the membranes adjacent to each other such that evaporating sides of membranes face each other and condensing sides of membranes face each other;   applying steps (b)-(f) to the membranes;   providing channels between the membranes to guide air flow.   
     
     
         4 . The method of  claim 3  wherein the evaporative fluid is water. 
     
     
         5 . The method of  claim 3 , further including the steps of providing troughs for liquid at the edges of membranes and wicking liquid from the troughs to the evaporating side of the membranes. 
     
     
         6 . The method of  claim 5 , further including the steps of providing overflow perforations through troughs and draining liquid through the perforations. 
     
     
         7 . The method of  claim 5 , further including the step of collecting excess condensate water. 
     
     
         8 . The method of  claim 4  further including the steps of:
 providing a pre-cooler; 
 passing hot flue gas from a hot water heater through the pre-cooler to provide hot humid gas for step (c); 
 preheating water with the pre-cooler; 
 providing the preheated water to the water heater; 
 providing the saturated gas from step (e) as combustion air for the water heater. 
 
     
     
         9 . The method of  claim 8  wherein the pre-cooler is a pre-condenser. 
     
     
         10 . Apparatus for heat recovery comprising:
 a heat and mass exchanger including—   a plurality of membranes having condensing sides and evaporating sides and placed in a stack with condensing sides facing condensing sides and evaporating sides facing evaporating sides, wherein the membranes allow evaporative liquid to pass through the membranes from condensing sides to evaporating sides;   wetting apparatus for wetting evaporating sides of membranes with an evaporative liquid;   condensing-side channels formed between adjacent condensing sides of membranes;   evaporating-side channels formed between adjacent evaporating sides of membranes;   a flue gas providing element for flowing hot humid gas through condensing-side channels and transferring heat to the membranes;   a combustion air providing element for flowing cool dry gas through evaporating-side channels and cooling membranes, cooling flue gas, and saturating combustion gas.   
     
     
         11 . The apparatus of  claim 10  wherein the membranes include a wicking structure for spreading evaporation fluid over evaporating sides and a capillary transfer structure for passing evaporative fluid through the membranes. 
     
     
         12 . The apparatus of  claim 11 , further comprising troughs at the edges of membranes for providing evaporative fluid to the wicking structures. 
     
     
         13 . The apparatus of  claim 12  wherein the troughs further include overflow perforations for allowing evaporative liquid to drain. 
     
     
         14 . The apparatus of  claim 11 , further comprising:
 a pre-cooler having a exhaust-gas channel for cooling flue gas from a hot water heater and providing it to the flue gas providing element; and a water channel for preheating water and providing it to the hot water heater, and   wherein combustion air from the HMX comprises combustion air for the hot water heater.   
     
     
         15 . The apparatus of  claim 14  wherein the pre-cooler is a pre-condenser. 
     
     
         16 . A hot water heater comprising:
 a tank for water;   a source of cold water for providing water to the tank;   a combustor for heating the water in the tank, the combustor configured to intake combustion air and fuel and output hot flue gas;   a pre-cooling mechanism for heating water and cooling hot flue gas to output pre-cooled flue gas; and   a heat and mass exchanger including—
 a plurality of plates having condensing sides and evaporating sides and placed in a stack with condensing sides facing condensing sides and evaporating sides facing evaporating sides; 
 wetting apparatus for wetting evaporating sides of plates with an evaporative liquid; 
 condensing-side channels formed between adjacent condensing sides of plates; 
 evaporating-side channels formed between adjacent evaporating sides of plates; 
 an element for flowing pre-cooled flue gas from the pre-cooling mechanism through condensing-side channels and transferring heat through the plates to the evaporation side; 
 an element for flowing air through evaporating-side channels, saturating the air flowing through evaporating-side channels and providing it as humid combustion air for the combustor. 
   
     
     
         17 . The hot water heater of  claim 16  wherein the pre-cooler is a pre-condenser. 
     
     
         18 . A device for providing warm humid combustion air to a combustor comprising:
 a pre-cooler;   a conduit for providing hot flue gas from the combustor to the pre-cooler;   a conduit for providing a fluid to the pre-cooler;   wherein the pre-cooler includes apparatus for cooling the hot flue gas with the fluid while warming the fluid and providing warm flue gas;   a heat and mass exchanger (HMX);   a conduit for providing input combustion air to the HMX;   a conduit for providing the warm flue gas from the pre-cooler to the HMX;   a conduit for providing a liquid to the HMX;   wherein the HMX includes apparatus for warming the input combustion air with the warm flue gas and humidifying the atmospheric air with the liquid to produce warm, humid combustion air; and   a conduit for providing the warm, humid combustion air to the combustor.   
     
     
         19 . The device of  claim 18  wherein the pre-cooler is a pre-condenser. 
     
     
         20 . The device of  claim 19  wherein the fluid is water and further comprising a hot water heater having a tank configured for heating water in the tank with the combustor and further including a conduit for providing the warmed water to the hot water heater tank.