Patent Publication Number: US-7213349-B1

Title: Heat recovery system for clothes dryer

Description:
BACKGROUND 
   1. Field of Endeavor 
   The information disclosed in this patent relates to heat recovery systems and more particularly to recycling heat by recovering clean heat from a clothes dryer, where that heat subsequently may be utilized in heating a home. 
   2. Background Information 
   Just about every house in the United States includes a clothes dryer and millions more of them are manufactured each year. Clothes dryers are very reliable, and very cheap to build. 
   In general, clothes dryers utilize hot air to dry clothes. The basic components of a clothes dryer include a rotating tumbler and a heater. The rotating tumbler holds the clothes and an electric or gas powered heater heats the air that is drawn through the wet clothes as they tumble. The hot air passing through the clothes heats up the clothes and the water in them. Typical clothes dryer additionally include an exhaust vent that passes out of the dryer and out of the house to permit the water to exit the dryer in the form of steam. 
   With temperatures dropping and electricity and home oil heating prices rising, many consumers are asking, “Why can&#39;t I vent my clothes dryer into my house so the heat that currently goes outside stays inside instead?” Presently, this is not a good idea for a variety of reasons. 
   Hot air coming out of an electric clothes dryer is full of lint, moisture, and dust. Breathing all that lint and dust just is not healthy. In addition, the excess moisture will condense on cold surfaces such as expensive wood tables and electronic equipment not in use. This free roaming moisture from the clothes dryer will cause problems other than damaging household items, including causing mold and aiding termites to damage wood. For gas clothes dryers, small amounts of gas vapor and carbon monoxide may be present in heat exhaust. Breathing in gas vapor and carbon monoxide could cause serious health problems. 
   For the above reasons, typical clothes dryers are connected to an exhaust duct that is open to the outdoors through a duct hole in a wall of the home. Porting outside the home heat generated by the clothes dryer is waste of valuable heat. An electric dryer consumes 220 volts of electricity to generate great volumes of heat for the drying process. By way of comparison, electric baseboard heaters also utilize 220 volts of electricity to generate heat for the home. As a result, every time that an electric clothes dryer is run, the consumer is venting to the outside an equivalent amount of heat as that generated by an electric baseboard heater over the same period of time. 
   As another example, a 4–5 person household may average eight clothes dryer cycles per week and thus eight hours dryer utilize per week. Over a typical six months of winter-like cold, this works out to 192 hours of dryer use. Even a two person household may utilize about 96 hours of clothes dryer time over the same period. These figures represent a huge amount of energy use. 
   In a typical home, the household heating system uses the most energy. However, other home services contribute to heating the home and help reduce the household heating system costs. For example, the water heater is second in energy use, whether the water heater is electric or gas. The water heater adds to heating the home as the water heater&#39;s heat largely is conducted into the home. Moreover, a water heater and the pipes servicing the water heater may be made more energy efficient through insulating blankets. The kitchen is third in energy use, but the kitchen too contributes to the heating of the home, with almost 30,000 British Thermal Units (BTUs)/hour. When utilized, kitchen appliances may provide a substantial gain to the home temperature. 
   The clothes dryer is a lone exception to the list of major home services contribute to heating the home. Except for some incidental radiant heat from the clothes dryer, the heat generated by the close dryer is almost completely lost outside of the home. Recouping the heat generated and presently lost by typical household clothes dryer configurations may provide substantial savings. 
   In view of the above, what is needed is an apparatus and method to overcome these and other problems. 
   SUMMARY 
   While venting clothes dryer heat outside may be acceptable during the summer months, it is desirable to reuse such heat once the outside temperature starts to fall. The disclosed heat recovery unit for a clothes dryer works towards meeting this need. The heat recovery unit may include a housing having a support box and a cover attached to the support box by a hinge. The cover may include an exhaust register and an indicator light cover through which a ‘clean filter now’ light, a ‘low flow’ light, and a ‘high temperature warning’ light may be viewed when lit. The heat recovery unit also may include a controls box positioned in the support box, where the controls box maintains electronic intelligence of the recovery unit. A recovery unit fan positioned in the support box to blow room temperature air up through a heat exchanger in the support box. An internal bypass channel may channel clothes dryer air around and outside of the heat exchanger during hot days. A filter in the support box on an inlet side of the heat exchanger may filter out lint and dust and a catch pan located in the support box may catch condensate from a drip pan located in the support box on an exhaust side of the heat exchanger to catch condensate. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is an isometric view of a heat recovery system for a clothes dryer (heat recovery system  100 ); 
       FIG. 2  is an isometric, partially exploded view of heat recovery system  100  in a position different than that illustrated in  FIG. 1 ; 
       FIG. 3  illustrates a schematic  200  illustrating various air flow paths for recovery unit  104 ; 
       FIG. 4  illustrates heat exchanger  224  of  FIG. 2  in more detail; and 
       FIG. 5  is an isometric, exploded, partial view of heat exchanger  224  illustrating heat exchanger core  220 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is an isometric view of a heat recovery system for a clothes dryer (heat recovery system  100 ). Heat recovery system  100  may include a clothes dryer  102  and a recovery unit  104 . Recovery unit  104  may recycle heat by recovering heat passed from clothes dryer  102 . The recycled heat subsequently may be utilized in heating a home  108 . 
   In describing heat recovery system  100 , it may help to first have a basic understanding of how a typical clothes dryer operates. With a basic understanding of how a typical clothes dryer operates, it may be clearer to see how the recovery unit  104  interacts with clothes dryer  102 . 
   Clothes dryer  102  may utilize hot air to dry clothes. The basic components of clothes dryer  102  may include a frame  110  supporting a door  112 , a cycle control knob  114 , heat setting buttons  116 , a dryer vent hose  118 , and a dryer power cord  120 . Frame  110  may have material removed to form a large hole  122  in a front portion of clothes dryer  102 . Hole  122  may permit room temperature air  124  to pass into an interior of clothes dryer  102 . 
   In operation, room temperature air  124  is sucked into clothes dryer  102  through large hole  122  by a dryer fan  126 . Dryer fan  126  drives all of the air through clothes dryer  102  but is one of the last components to be reached by the air. After arriving inside clothes dryer  102 , room temperature air  124  then is sucked past a heating element  128  to form hot air  130 . The heating element  128  may be electrical or gas. 
   Hot air  130  is passed into a tumbler  132 . Tumbler  132  is where clothes  134  are loaded for the drying process. As hot air  130  makes its way through clothes  134  in tumbler  132 , hot air  130  will picked up moisture, lint, dirt, and any gases released from clothes  134 . For a gas dryer, hot air  130  additionally may include gas vapor and carbon monoxide. This new mixture of air—dirty hot air  136  composed of heat  138  and dirty air  140 —is then drawn into holes  141  in door  112 . 
   Dirty hot air  136  enters door  112  and is directed down through a lint screen  142 . As dirty hot air  136  passes through lint screen  142 , lint screen  142  filters out larger lint particles. It then passes through a fan inlet duct  144  in the front of clothes dryer  102  and into dryer fan  126  that is being moved by dryer motor  146 . Dryer fan  126  centrifugally forces dirty hot air  136  into a fan exit duct  148  duct leading out the back of clothes dryer  102  to dryer vent hose  118 . At this point, both heat  138  and dirty air  140  typically exit home  108  as dirty hot air  136 . 
   It is rather surprising at how much energy heating element  128  of  FIG. 1  consumes. Electric heating elements consume 4,000 to 6,000 watts on most dryers. Gas heating elements consume a similar amount of energy. By way of comparison, a 100-watt light bulb uses 100 watts, a typical desktop computer uses 65 watts, and a central air conditioner uses about 3500 watts. What even more surprising is that hot air  130  only makes one pass through clothes  134 , whether or not the utilize of hot air  130  to dry clothes  134  has been maximized. Typically porting heat  138  outside of home  108  is a huge waste of energy and the consumer&#39;s money. 
   As noted above, recovery unit  104  may recycle heat by recovering heat  138  passed from clothes dryer  102 . As illustrated in  FIG. 1 , recovery unit  104  may include a housing  150  and an exhaust register  152 . Housing  150  may encase many of the contents of recovery unit  104  and exhaust register  152  may vent heated room air  308  ( FIG. 3 ) into home  108  as discussed in more detail below. 
   Recovery unit  104  additionally may include a recovery vent hose  154 , a power cord  156 , and indicator light cover  158 . Recovery vent hose  154  may port dirty air  140  out home window  160  and power cord  156  may be inserted into home power outlet  162  to provide power to recovery unit  104 . Further, visible through indicator light cover  158  when lit may be a ‘clean filter now’ light  164 , a ‘low flow’ light  166 , and a ‘high temperature warning’ light  168 . 
     FIG. 2  is an isometric, partially exploded view of heat recovery system  100  in a position different than that illustrated in  FIG. 1 . Housing  150  may divided into a support box  202  and a cover  204  attached to support box  202  by a hinge  205 . Support box  202  may include a first shelf  206  and a second shelf  207 . 
   First shelf  206  of support box  202  may support a first male joint  208 , a controls box  210 , a recovery unit fan  212 , a drip pan  214 , and a bypass channel  216 . First male joint  208  may be utilized to port dirty hot air  136  into recovery unit  104 . Conveniently, clothes dryer  102  need only be connected to recovery unit  104  at two locations. Dryer vent hose  108  may be attached to first male joint  208  and power cord  120  from clothes dryer  102  may be connected to a power outlet  209  of controls box  210 . Thus, a consumer utilizing recovery unit  104  need not make more connections for clothes dryer  102  than already required for clothes dryer  102 . 
   Controls box  210  may contain the electronic intelligence of recovery unit  104  as well as various electronic connections. Room temperature air  124  to be heated may be drawn into recovery unit  104  by recovery unit fan  212 . Drip pan  214  may serve as a basin for condensation from dirty hot air  136 . Bypass channel  216  may ordinarily resides internal to support box  202 . However, bypass channel  216  is illustrated in  FIG. 2  apart from support box  202  to reveal more features in  FIG. 2 . During the summer months when there is no need to heat home  108 , bypass channel  216  may permit a user to bypass the operations of recovery unit  104  and port hot dirty air  136  directly outside through recovery vent hose  154 . 
   Second shelf  207  of support box  202  may support a filter  218 , a heat exchanger core  220 , and a catch pan  222 . Filter  218  may be utilized to filter out additional lint and dust from hot dirty air  136  that may have been missed by lint screen  142  ( FIG. 1 ). Heat exchanger core  220  may be part of a heat exchanger  224 . Importantly, both filter  218  and heat exchanger core  220  may be removably secured in support box  202  to permit easy cleaning of these components. Catch pan  222  may catch condensate drip from heat exchanger core  220  and port the condensate to drip pan  214  through drain tube  226 . Alternatively, drain tube  226  may be connected to a drain portion (not illustrated) of a clothes washer. 
   Along with support box  202 , cover  204 , and second shelf  207 , filter  218  may be positioned in housing  150  to form a dryer air inlet compartment  225  as an entrance staging location for hot dirty air  136 . Filter  218  may remove additional lint and dust from hot dirty air  136  and heat exchanger core  220  may provide a method to remove heat  138  from hot dirty air  136  to create cool dryer air  228 . Moreover, along with support box  202 , cover  204 , and second shelf  207 , heat exchanger core  220  may be positioned in housing  150  to form a dryer air outlet compartment  230  as an exit staging location for cool dryer air  228 . 
     FIG. 3  illustrates a schematic  300  illustrating various air flow paths for recovery unit  104 . Hot dirty air  136  from clothes dryer  102  may enter recovery unit  104  through male joint  208 . During hot days where interior heating of home  150  is not desired, a gate flap  302  ( FIG. 3 ) of recovery unit  104  may be positioned so that hot dirty air  136  travels through bypass channel  216  rather than through heat exchanger  224 . Bypass channel  216  may channel hot dirty air  136  through a second male joint  304  ( FIG. 3 ) of recovery unit  104  to recovery vent hose  154 . 
   During cold days where interior heating of home  150  may be desired, gate flap  302  may be positioned so that hot dirty air  136  travels into dryer air inlet compartment  225 . From dryer air inlet compartment  225 , hot dirty air  136  may pass through filter  218  and into heat exchanger  224  as hot dryer air  306 . At the same time, recovery unit fan  212  may draw room temperature air  124  into heat exchanger  224 . A cross flow movement of hot dryer air  306  relative to room temperature air  124  may draw heat  138  from hot dryer air  306  into room temperature air  124 , crating cool dryer air  228  out of hot dryer air  306  and creating hot temperature air  308  out of room temperature air  124 . Hot temperature air  308  may then be ported through exhaust register  152  to heat home  108 . Further, cool dryer air  228  may be passed to dryer air outlet compartment  230  and out of home  108 . 
     FIG. 4  illustrates heat exchanger  224  of  FIG. 2  in more detail. Included with heat exchanger  224  may be a heat exchanger core frame  402 , a handle  404 , and a plurality of panels  406  that may be included as part of heat exchanger core  220 . Core frame  402  may fix panels  406  in position. For example, core frame  402  may be a ¼ inch to ⅛ inch rubber/neoprene angle gasket applied to complete a seal around panels  406  and hold panels  406  in place as heat exchanger  224 . 
   Handle  404  may aid in removing heat exchanger  224  from housing  150 . For example, handle  404  may be attached to a core exterior  408  and lay flat in a non-utilize position and extend outward in a utilize position. Handle  404  may include plastic, metal, or other material. 
     FIG. 5  is an isometric, exploded, partial view of heat exchanger  224  illustrating heat exchanger core  220 . Heat exchanger  224  may be thought of as a recuperative cross-flow heat exchange. In a recuperative heat exchange, fluids may exchange heat on either side of a dividing wall. In a cross-flow heat exchanger, the direction of the fluids may travel perpendicular to each other. 
   As noted above, heat exchanger core  220  may include a plurality of panels  406 , such as a panel  501 , a panel  502 , a panel  503 , a panel  504 , and a panel  505 . A cross sectional area of heat exchanger core  220  (or dryer air inlet compartment  225  ( FIG. 2 )) may be twice a cross sectional area of the dryer vent hose  118  ( FIG. 2 ). This increase in area may slow down the hot dryer air  306  ( FIG. 4 ) to permit better a heat exchange. Experiments have illustrated that a heat exchanger core  220  including forty-eight plates, each having a width of approximately one quarter inch, may present twice the cross sectional area of a four inch diameter dryer vent hose  118 . 
   Each of the panels  406  may include fins secured to a plate. The fins may be straight, corrugated, or a combination thereof to increase surface area, channel fluid flow, and/or induce turbulence. For example panel  501 , panel  503 , and panel  505  each may have horizontally arranged fins  507 , fins  509 , and fins  511  respectively. Panel  502  and panel  504  each may have vertically arranged fins  506  and fins  508 , respectively. Panels  501 ,  502 ,  503 ,  504 , and  505  may have plates  512 ,  513 ,  514 ,  515 , and  516 . 
   By placing a free end of the fins of a particular panel  406  against an adjacent plate and interleaving those panels  406  having horizontal arranged fins with those panels  406  having vertically arranged fins such as illustrated in  FIG. 5 , the arrangement may form a recuperative cross-flow heat exchanger core  220  where hot dryer air  306  may flow horizontally and room temperature air  124  may flow vertically. Joints of heat exchanger core  220  may be sealed to prevent leaks from one flow channel to another. Heat exchanger core  220  may be manufactured from metal alloy extrusion. 
   For efficiency, heat exchanger  224  may be designed to maximize the surface area of the plates between the two fluids, while minimizing resistance to fluid flow through heat exchanger  224 . Also, those panels  406  having horizontal arranged fins (such as panels  512 ,  514 , and  516 ) may have their fins arranged to tilt slightly downward towards catch pan  222  ( FIG. 2 ) to aid in removal of condensation. 
   Recovery unit  104  may be a rectangular wall-mounted cabinet that may measure approximately 30 inches long by 20 inches high by 12 inches deep. Most of the upper interior of recovery unit  104  may be dominated by a multi-plate heat exchanger. The heat exchanger may consist of a series of vertically oriented sheets, the one side of each of which may feature a series of closely spaced fins. The orientation of the fins may alternate 90 degrees for each successive sheet. A lower left-hand corner of recovery unit  104  may feature an attachment fitting for a standard dryer vent line. Internally, this may lead to a horizontal flow path through every other plate. Air may flow upwards through the other gaps by means of a 125-volt AC, elongated, centrifugal blower mounted in the lower center of the housing. The underside of the housing may contain the unit&#39;s room air intake, while the exhaust may be mounted on the upper end of the unit. The former may be equipped with a lint filter that could be accessed for cleaning via a hinged panel. 
   The heat exchanger may be mounted at a slight angle to facilitate the drainage of condensation from the dryer side of the heat exchanger into a bottom-mounted drip pan. The dryer side of the airflow path may terminate on the upper right-hand end of the unit. It could be linked to a standard vent line that could be directed out of the structure. The condensation drip pan could be linked to a downward section of tubing. This could be linked at the bottom end of the cabinet to a flexible tubing run that could be diverted into a laundry basin or floor drain. 
   The inlet end of the dryer side of the system may be equipped with a damper that could be utilized to direct flow to a bottom-mounted bypass route during the summer. Operation of the unit may be regulated by an inductive coil type sensor that may detect when the dryer was turned on. The unit could also feature an airflow sensor or related means that could be utilized to illuminate a “CLEAN FILTER” indicator light. A temperature sensor and high temperature warning light also may be included with the system. 
   Recovery unit  104  may include an in unit circuit breaker to prevent overheating. Since power for clothes dryer  102  may be first passed through controls box  210  of recovery unit  104 , a shutdown function may kill power to both clothes dryer  102  and recovery unit  104  to minimize dryer fire. Recovery unit  104  may include a temperature range setting sensor to automatically activate gate flap  302 . 
   Recovery unit  104  works towards fulfilling a market need to recover heat from a clothes dryer. The appealing features of Recovery unit  104  may include its ease of installation, convenience, efficiency, simplicity, and reliability. Recovery unit  104  may be installed in a relatively short period of time using common hand and power tools and may be adapted for utilize in conjunction with both gas and electric dryers. In operation, recovery unit  104  may provide an effective means of recovering most of the heat normally vented outside by a clothes dryer. The intake filter and condensate recovery system of recovery unit  104  may endow recovery unit  104  with a good level of reliability and safety, while its inductive control may make recovery unit  104  automatic in operation. 
   The information disclosed herein is provided merely to illustrate principles and should not be construed as limiting the scope of the subject matter of the terms of the claims. The written specification and figures are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Moreover, the principles disclosed may be applied to achieve the advantages described herein and to achieve other advantages or to satisfy other objectives, as well.