Abstract:
A frost protection system is described for a condensate drain pipe for a boiler, air-conditioning unit etc to protect an outdoors run of the pipe. The system comprises an outdoors temperature sensor, a heater, and a controller for activating the heater in dependence upon the temperature sensed by the sensor. The heater is in the form of a flexible line (for example an electrical cable with a resistive filament). The heater (or a connection for it) passes through a fitting in the pipe indoors, and the heater extends along the inside of the outdoors run of the pipe. In cold weather, the controller activates the heater to produce heat inside the drain pipe and thus reduce the risk of the condensate freezing.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a frost protection system for an apparatus having a condensate drain pipe. 
         [0003]    2. Description of the Related Art 
         [0004]    Various apparatuses produce condensate during normal operation, examples being condensing boilers, freezers, refrigerators and air-conditioning units. It is necessary to dispose of the condensate. Some small apparatuses are able to do this by evaporation. However, for larger apparatuses, it is necessary to drain the condensate away through a drain pipe. In some situations, the drain pipe can be routed internally of the building to the drainage system of the building in which the apparatus is sited. However, in other situations, for example when replacing a non-condensing boiler with a condensing boiler, it can often be very inconvenient and costly to access the building&#39;s drainage system internally of the building, and it is far more convenient to route the condensate drain pipe through a hole in an external wall of the building and then either to the building&#39;s drainage system externally of the building or to a soak-away. 
         [0005]    When the condensate drain pipe is routed externally of the building, the condensate can, in cold weather, freeze in the drain pipe and block it. The condensate can then back up in the drain pipe to the apparatus. This can result in reduced efficiency of the apparatus, tripping-out of the apparatus and/or damage to the apparatus. It can also result in the drain pipe splitting or having its joints forced apart. Some plumbers carefully route the drain pipe externally of the building so that it always has a fall and/or use a pipe with an excessively large internal diameter in an attempt to prevent freezing of the condensate and blocking of the pipe. However, these attempts are rarely successful and/or they result in the appearance of the outside of the building being spoiled and/or they result in an increased cost of the drain pipe. It is, of course, possible to lag the drain pipe. Although lagging may help during a brief cold snap, it has little effect during prolonged cold spells. Also, lagging adds to the cost and makes the pipe more obtrusive. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    An aim of the present invention, or at least of specific embodiments of it, is to prevent freezing in cold weather of condensate in such drain pipes and to do so in a convenient and unobtrusive way. 
         [0007]    In accordance with one aspect of the present invention, there is provided a frost protection system for an apparatus having a condensate drain pipe, the system comprising: a temperature sensor; a heater (for example an electrical heater); and a controller for activating the heater in dependence upon the temperature sensed by the sensor (for example to activate the heater when the sensed temperature falls below a threshold value). The heater is in the form of a flexible line (for example an electrical cable with a resistive filament) for disposal in the drain pipe, and the system further comprises a fitting for the drain pipe to enable the flexible line, or a connection between the controller and the flexible line, to pass from the controller to the inside of the pipe. 
         [0008]    In accordance with a second aspect of the invention, there is provided a building having: an external wall; an apparatus (such as a boiler, a refrigerator, a freezer and an air-conditioning unit) disposed inside the building and having a condensate drain pipe that passes through a hole in the wall; and a frost protection system according to the first aspect of the invention. The sensor is disposed outside the building; the fitting is fitted to the condensate drain pipe inside the building; and at least part of the heater is disposed inside the condensate drain pipe outside the building. 
         [0009]    Accordingly, in cold weather, the controller can activate the heater to produce heat inside the drain pipe and thus reduce the risk of the condensate freezing. Because the heat is produced inside the pipe, it can act more directly on the water and less directly on the air surrounding the pipe than if the pipe were heated from the outside, and thus the amount of heat required is less. It is expected that, with the invention, it will even be possible to reduce the diameter of the drain pipe compared with the size that is typically used absent the invention. In addition to reducing the pipe cost, reducing the pipe size results in less air being heated in the pipe and possibly convecting the heat away. 
         [0010]    In a specific embodiment of the invention, the fitting is in a form of a tee or coupler having first and second portions for connection to a first upstream portion and a second downstream portion, respectively, of the condensate drain pipe and a third portion to enable entry of the flexible line or the connection therefor. The tee or coupler may also have a fourth portion arranged to be fitted to a further pipe having a larger internal diameter than the external diameter of the second portion of the condensate drain pipe so that the second portion of the condensate drain pipe can run inside the further pipe. It has been found that less heat is required to prevent freezing of the condensate in a smaller diameter pipe (for example of 10 mm diameter compared to 22 mm diameter). Furthermore, the air gap that remains between the inner second portion of the condensate drain pipe and the further outer pipe provides thermal insulation which reduces heat loss from the inside of the inner second portion of the condensate drain pipe. 
         [0011]    The system is preferably provided in combination with the second portion of the condensate drain pipe. It has been found that less heat is required to prevent freezing of the condensate the fewer the number of cold joints in the condensate drain pipe. Therefore, the second portion of the condensate drain pipe is preferably flexible (for example a length of polyethene hose). Accordingly, despite any elbows that may be required in the further pipe, the second portion of the condensate drain pipe can wind its way along the inside of the further pipe without any joints. 
         [0012]    The second portion of the tee or coupler is preferably provided as a separate element which is arranged to be connected to the remainder of the tee or coupler when the further pipe is fitted to the fourth portion of the tee or coupler. This enables, for example, a conventional 22 mm equal tee to be employed having (i) one limb to receive the 22 mm upstream portion of the condensate drain pipe, (ii) another limb to receive the flexible heater line (or the connection between the controller and the flexible heater line), and (iii) a further limb to receive both the separate element (to which the 10 mm flexible hose is attached) and the 22 mm further pipe. 
         [0013]    In the case where the heater is electrical and has a resistive filament extending along the line, the line is preferably a coaxial cable, and the resistive filament preferably forms a central core of the coaxial cable. 
         [0014]    The controller may be operable to control the current flow through the resistive filament or the voltage applied across the resistive filament. 
         [0015]    It may be desirable to cut the length of the filament to suit the installation. The resistance of a resistive filament is proportional to its length. With a particular voltage applied between the ends of the filament, the heat production per unit length is inversely proportional to the square of the length of the filament. If voltage control alone is employed, there is a risk that with short lengths of filament, the filament may become excessively hot and damage its insulation. To avoid this, if voltage control is employed, the system may further include a limiter for limiting the flow of electrical current through the resistive element. For a particular limited current flowing through the filament, the heat production per unit length is independent of the length of the filament. 
         [0016]    The controller is preferably operable to control the heater in dependence upon the amount by which the sensed temperature is below a threshold value, for example, increasing the heating effect linearly the greater the temperature difference. The controller preferably employs pulse-width modulation to control the heater. 
         [0017]    The system will typically include a sensor line for connecting the sensor to the controller. In this case, the first-mentioned fitting may be arranged also to enable the sensor line to pass from the controller to the inside of the condensate drain pipe. Alternatively, the system may further include a second fitting to enable the sensor line to pass from the controller to the inside of the condensate drain pipe. In either case, it becomes unnecessary to drill a further hole through the wall so that the sensor line can pass through the wall. If the sensor line passes along the inside of the condensate drain pipe, the sensor may be sited in the pipe, but preferably beyond the far end of the heater, or the sensor may be sited beyond the far end of the drain pipe. Alternatively, such a second fitting may be used to enable the sensor line to pass, outside the building, from the inside of the condensate drain pipe to the sensor outside the pipe. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0018]      FIG. 1  is a schematic diagram of a first embodiment of the invention; 
           [0019]      FIG. 2  is a schematic diagram of a second embodiment of the invention; 
           [0020]      FIG. 3  is a schematic diagram of a third embodiment of the invention; 
           [0021]      FIG. 4A  is a sectioned side view of a first example of a fitting for the drain pipe; 
           [0022]      FIGS. 4B  &amp; C are isometric views of a hung of the fitting of  FIG. 4A  in relaxed and compressed states respectively; 
           [0023]      FIG. 5A  is a sectioned side view of a second example of a fitting for the drain pipe; 
           [0024]      FIGS. 5B  &amp; C are isometric views of a gland member of the fitting; of  FIG. 5A  in relaxed and compressed states respectively; 
           [0025]      FIG. 6A  is a sectioned side view of a third example of a fitting for the drain pipe; 
           [0026]      FIGS. 6B  &amp; C are isometric views of a bung of the fitting of  FIG. 6A  in relaxed and compressed states respectively; 
           [0027]      FIG. 7  is a sectioned side view of a fourth example of a fitting for the drain pipe; 
           [0028]      FIG. 8  is a sectioned side view of a fifth example of a fitting for the drain pipe; 
           [0029]      FIG. 9  is an exploded sectioned side view of a sixth example of a fitting and an alternative drain pipe; and 
           [0030]      FIG. 10  is similar to  FIG. 9 , but after the component parts have been assembled. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Referring to  FIG. 1 , in the first embodiment of the invention, a condensing boiler (not shown) is sited in a building, and a condensate drain pipe  10  leads from the boiler and through a hole  12  in a wall  14  of the building to the outdoors side of the wall. 
         [0032]    A frost protection unit  16  has a housing  18  mounted indoors on or adjacent the wall  14  and the drain pipe  10 . The housing  18  contains an electrical circuit including a power supply unit  20  which is fed with mains electricity via a thermal cut-out  22 . The connections from the power supply unit  20  to the other components of the circuit are, for simplicity, not shown in the drawing. 
         [0033]    A temperature sensor  24  (such as a thermistor or semiconductor sensor) is sited outside the building and is connected by a cable  26  passing through a further hole  28  in the wall  14  to the frost protection unit  16 . The sensor  24  produces a signal dependent on the temperature T O  outside the building, which is supplied to the inverting input of a differential amplifier  30 . On its non-inverting input, the differential amplifier  30  receives a signal from a threshold temperature setting device  32  (such as a fixed resistor of a chosen value or a variable resistor) representing a threshold temperature T T . Typically, the threshold temperature T T  will be set to 5 Celsius. The differential amplifier  30  also receives a signal from a gain setting device  34  (such as a fixed resistor of a chosen value or a variable resistor) representing a gain G 1 . The differential amplifier produces an output control signal having a voltage V C  representing a desired output current from the unit  16 . If the outside temperature T O  is greater than the threshold temperature T T , the control voltage V C  is zero. However, if the outside temperature T O  is less than or equal to the threshold temperature T T , the control voltage V C  is generated according to V C =G 1 .(T T −T O ). The control voltage V C  is supplied to a current regulator  36  having a preset gain G 2  which produces a short-term-average output current I H  given by I H =G 2 .V C . The output current I H  is therefore zero if the outside temperature T O  is greater than the threshold temperature, but is given by I H =G 2 .G 1 .(T T −T O ) if the outside temperature T O  is less than or equal to the threshold temperature T T . 
         [0034]    The current regulator  36  may regulate the output current I H  by progressively varying the output current I H  with progressive variations in the control voltage V C . Alternatively, the current regulator  36  may pulse-width-modulate the output from the PSU  20  at a suitable frequency with a varying mark:space ratio so as to vary the short-term RMS value of the output current I H . The latter option is preferred for reasons of efficiency. 
         [0035]    A tee  38  is fitted into the drain pipe  10  adjacent the frost protection unit  16 . Upstream and downstream portions  10 U, 10 D of the drain pipe  10  are connected to two of the limbs  38 A, 38 B of the tee  38  using conventional plumbing methods, and a flexible heater cable  40  enters the drain pipe  10  through a seal  42  in the other limb  38 C of the tee  38 . The heater cable  40  passes along the downstream portion  10 D of the pipe  10  almost as far as its far end  10 E. 
         [0036]    The heater cable  40  is a coaxial cable having a central resistive core  40 C of, for example, constantan, and a braided conductive shield  40 S of copper, with an intermediate insulator between the core  40 C and shield  40 S and an insulating sheath around the shield  40 S. At the distal end  40 D of the heater cable  40 , the core  40 C and shield  40 S are connected together. At the proximal end  40 P of the heater cable  40 , the core  40 C and shield  40 S are connected to the cores of a two-core cable  44  which are connected at their opposite ends to the output from the current regulator  36  and to the electrical ground of the unit  16 . The heater current I H  therefore passes through the heater cable  40 . 
         [0037]    If the heater cable  40  has a resistance per unit length ρ Ω/m the heating power H per unit length of the cable  40  is given by H=I H   2 .ρ. When the outside temperature T O  is greater than the threshold temperature T T , the heating power H per unit length of the cable  40  is zero. When the outside temperature T O  is less than or equal to the threshold temperature T T , the heating power H per unit length of the cable  40  is given by H=[G 2 .G 1 .(T T −T O )] 2 .ρ. The values of G 2 , G 1  and ρ are chosen by trial and error so that the heating power H per unit length of the cable  40  is sufficient to prevent condensate in the drain pipe  10  freezing in all but the most extreme cold weather. 
         [0038]    It will be noted that the heating power H per unit length of the cable is independent of the length of the heating cable  40 . The heating cable  40  can therefore be cut in length, if desired, and have its cut end reconnected without it substantially affecting how hot the cable  40  will become. 
         [0039]    Referring now to  FIG. 2 , the second embodiment of the invention is similar to the first embodiment described with reference to  FIG. 1 , except in the following three respects. 
         [0040]    First, the drain pipe  10  is connected to the condensate outlet of a freezer (not shown), rather than a boiler. 
         [0041]    Second, the sensor cable  26  does not pass through a separate hole  28  in the wall  14 . Instead it passes into the drain pipe  10  through the seal  42  of the fitting  38  and extends along the downstream portion  10 D of the pipe  10  to its far end  10 E where the temperature sensor  24  is disposed, sufficiently spaced from the heater cable  40  so that the sensor is substantially unaffected by heat from the heater cable  40 . Alternatively, the sensor cable  26  could extend beyond the far end  10 E of the pipe  10 . 
         [0042]    Third, the differential amplifier  30  performs a square root operation on its output V C  so that, if the outside temperature T O  is less than or equal to the threshold temperature T T , the control voltage V C  is generated according to V C =√[G 1 .(T T −T O )]. As a result, if the outside temperature T O  is less than or equal to the threshold temperature T T , the heating power H per unit length of the cable  40  is given by H=G 2   2 .G 1 .(T T −T O ).ρ and is linearly related to the temperature difference rather than the square of the temperature difference. Again, the current regulator  36  may employ amplitude modulation of the output current I H , or more preferably pulse-with modulation. 
         [0043]    Referring now to  FIG. 3 , the third embodiment of the invention is similar to the second embodiment described with reference to  FIG. 2 , except in the following four respects. 
         [0044]    First, the drain pipe  10  is connected to the condensate outlet of an air conditioning unit (not shown), rather than a freezer. 
         [0045]    Second, the sensor cable  26  does not extend to the far end  10 E of the drain pipe  10 , but instead exits from the drain pipe  10  through a seal  42  at a second tee  46  immediately on the outdoors side of the wall  14 . The second tee  46  is similar to the first tee  38 , but the seal  42  is provided in one of the aligned limbs  46 A of the tee  46 , and the portions of the drain pipe  10  are connected to the other two right-angled limbs  46 B,C of the tee  46 . The sensor  24  may be mounted on the wall  14  adjacent the tee  46 . 
         [0046]    Third, a voltage regulator  48  is employed in place of the current regulator  36  of  FIG. 2 . The voltage regulator  36  has a preset gain G 3  and produces an output voltage V H  given by V H =G 3 .V C . The output voltage V H  is therefore zero if the outside temperature T O  is greater than the threshold temperature, but is given by V H =G 3 .√[G 1 .(T T −T O )] if the outside temperature T O  is less than or equal to the threshold temperature T T . The heating power per unit length H of the cable  40  is given by H=V H   2 /(L 2 .ρ), where L is the length of the heater cable  40 . When the outside temperature T O  is greater than the threshold temperature T T , the heating power H per unit length of the cable  40  is zero. When the outside temperature T O  is less than or equal to the threshold temperature T T , the heating power H per unit length of the cable  40  is given by H=G 3   2 .G 1 .(T T −T O )/(L 2 .ρ). The values of G 3 , L and ρ are chosen by trial and error so that the heating power H per unit length of the cable  40  is sufficient to prevent condensate in the drain pipe  10  freezing in all but the most extreme cold weather. Again, voltage regulator  48  may employ amplitude modulation of the output voltage V H  and therefore of the output current I H , or more preferably pulse-with modulation. 
         [0047]    Fourth, be noted that, in the third embodiment of  FIG. 3 , the heating power H per unit length of the cable is not independent of the length of the cable  40 , but instead is inversely proportional to the square of the length L of the cable  40 . Therefore, if the heating cable  40  is cut in length, it may become excessively hot in use. To prevent this, the voltage regulator  48  has an upper limit on its output current TH set by a current limit setting device  50  (such as a fixed resistor). 
         [0048]    The tee  38  and seal  42  shown in  FIG. 1  are shown in greater detail in  FIGS. 4A-C . The tee  38  is of conventional form. The seal  42  for the tee  38  is provided by a resilient tapered bung  52  having a central hole  54  for the heater cable  40  and a slit  56  extending from the hole  54  to the edge of the bung  52  to enable the bung  52  to be forced sideways onto the cable  40 . When the hung  52  is inserted into the socket  58  of the limb  38 C of the tee  38 , the slit  56  closes up and the bung  52  is compressed between the cable  40  and the socket  58  to seal the cable  40  to the tee  38 . 
         [0049]    An alternative tee  38  and seal  42  are shown in  FIGS. 5A-C . The seal  42  is in the form of a squash gland having a resilient annular gland member  60  and a gland nut  62 . The limb  38 C of the tee  38  has a cavity  64  and an internal thread  66  to form the housing for the squash gland. The gland member  60  and nut  62  are slid along the heater cable  40  to the required position. The gland member  60  is inserted into the cavity  64  and the nut  62  is screwed into the gland housing so as to compress the gland member  60  so that it seals with the cable  40  and the tee  38 . The gland member  60  may be slit, similarly to the bung  52  of  FIGS. 4A-C , so that it can be fitted sideways onto the cable  40 . 
         [0050]    The tee  38  and seal  42  shown in  FIG. 2  are shown in greater detail in  FIGS. 6A-C . The seal  42  is provided by a bung  68  similar to the bung  52  of  FIGS. 4A-C , except that the bung  68  has two holes  70 , 72  for the heater cable  40  and sensor cable  26 , respectively, a slit  74  extending between the holes  70 , 72  and from the hole  70  to the edge of the bung  68  to enable the hung  52  to be forced sideways onto both cables  40 , 26 . 
         [0051]      FIG. 7  shows an alternative fitting  74  for sealing the heater cable  40  to the drain pipe  10 . The fitting  74  is generally in the form of a moulded plastics straight coupler having sockets  74 A,B for connecting the upstream and downstream portions  10 U,D of the pipe  10 . However, inner and outer contacts  76 I,O of a phono plug are connected to the core and shield  40 C,S of the cable  40  and are moulded into a bulge  74 C in the wall of the fitting  74  during moulding of the fitting  74  so as to form a phono plug  76  projecting from the wall of the fitting  74 . The end of the cable  44  from the frost protection unit  16  is fitted with a complementary phono line socket  78  which is connected to the phono plug  76 . Similar principles may be employed for connecting the sensor cable  26  through the wall of the pipe  10 , and for connecting both the heater cable  40  and the sensor cable  26  through the wall of the pipe at the same fitting. 
         [0052]      FIG. 8  shows an alternative method of sealing the heater cable  40  (or sensor cable  26 ) to the drain pipe  10 . In this case, a hole  80  is drilled through the wall of the pipe  10 . The cable  40 , 26  is then threaded through the hole to the required position. A bead  82  of room-temperature-vulcanizing (RTV) silicone rubber is then applied around the cable  40 , 26  and allowed to cure so as to seal the cable  40 , 26  to the pipe  10 . For the purposes of this specification, a bead  82  of rubber or the like is intended to be covered by the term “fitting”. 
         [0053]      FIGS. 9 and 10  show a further fitting and arrangement of condensate drain pipe. The fitting includes a conventional 22 mm equal tee  38  and a reducer  84 , both of rigid plastics material. The 22 mm upstream portion  10 U of the condensate drain pipe, leading from a boiler, is of rigid plastics material which is solvent-welded into the central limb  38 A of the tee  38 . The reducer  84  is moulded from plastics material and has a flange portion  86  which is a snug fit in the lower limb  38 D of the tee  38  and a downwardly-projecting spigot portion  88  to which the upper end of a length of flexible polythene hose  10 H is fitted. The hose  10 H runs through a further 22 mm pipe  90  of rigid plastics material which may have various elbows and couplers (not shown) and which passes through the wall of the building (not shown) from the inside to the outside. The upper end of the further pipe  90  is solvent-welded into the lower limb  38 D of the tee  38  after the reducer  84  has been fitted. The connection between the heater cable  40  and its supply cable  44  is moulded into a bung  42  which is solvent-welded into the upper limb  38 C of the tee  38 . The heater cable  40  therefore extends from the bung  42 , through the tee  38  and spigot portion  88  of the reducer  84  and then runs along the inside of the hose  10 H so that the distal end  40 D of the heater cable  40  is adjacent the distal end  10 E of the hose  10 H. 
         [0054]    The cross-sectional diameter of the heater cable  40  is substantially less than the inner diameter of the spigot portion  88  of the reducer  84  and the inner diameter of the hose  10 H so that condensate from the boiler can flow from the upper portion  10 U of the condensate drain pipe  10 U, through the tee  38 , spigot portion  88  and hose  10 H, to the open distal end  10 E of the hose  10 H. The hose  10 H therefore forms the downstream portion of the condensate drain pipe. The hose  10 H is preferably provided as one continuous length so that, outside the building, there are no joints or very sharp bends in the hose  10 H to promote the seeding of ice crystals. Also, the provision of two layers material (i.e. of the hose  10 H and the further pipe  90 ) and the air gap  92  between them increase the thermal insulation between the heater cable  40  and the outside air, as compared with the heater cable  40  simply running through a 22 mm pipe, so that the heater cable  40  needs to produce less heat in order to prevent the condensate freezing in the hose  10 H. Furthermore, the further pipe  90  provides physical protection for the flexible hose  10 H. 
         [0055]    The sensor  24  and sensor cable  26  are not shown in  FIGS. 9 and 10 . The sensor cable  26  may be routed to the outside of the building independently of the condensate drain pipe, as in  FIG. 1 , or it may be fed through the hose  10 H as in  FIGS. 2 and 3 , or it may be fed through the air gap  92  between the hose  10 H and the further pipe  90 . 
         [0056]    It will be appreciated that many modifications and developments may be made to the embodiments of the invention described above. For example, the amplifier  30  may have a transfer function other than linear or square root. Also, any of the embodiments described above may be modified to employ the method of any of the other embodiments (i) of connecting the heater cable  40  to its supply cable  44 , (ii) of passing the heater cable  40  and/or supply cable  44  from the outside to the inside of the condensate drain pipe  10 ; and (iii) of passing the sensor cable between the outside and the inside of the condensate drain pipe  10 . 
         [0057]    It should be noted that the embodiments of the invention have been described above purely by way of example and that many other modifications and developments may be made thereto within the scope of the present invention.