Patent Publication Number: US-2023164886-A1

Title: Heating Units for Heating Enclosures and Methods of Heating Enclosures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. § 371 U.S. National Phase entry of, and claims priority to, PCT/US2021/024019 filed Mar. 24, 2021, and entitled “Heating Units for Heating Enclosures and Methods of Heating Enclosures,” which claims benefit of U.S. provisional patent application Ser. No. 62/994,666 filed Mar. 25, 2020, and entitled “Heating Units for Heating Enclosures and Methods of Heating Enclosures,” each of which is hereby incorporated herein by reference in its entirety for all purposes. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     Enclosure heaters may be used to heat a volume of fluid within a container or housing. For example, enclosure heaters may be used in petrochemical plants to maintain a fluid stream at or above a particular temperature. In some applications, hardware associated with the enclosure heater may be limited to a predetermined maximum surface temperature, as established for example, by hazardous area safety regulations. 
     BRIEF SUMMARY 
     Embodiments of heating units for heating an enclosure are described herein. In an embodiment, a heating unit comprises a first base having a central axis, a first end, a second end axially opposite the first end, and a cavity extending axially from the first end. In addition, the heating unit comprises a first heater disposed in the cavity of the first base. Further, the heating unit comprises a first heat sink mounted to the first base, wherein the first heat sink has a central axis oriented parallel to the central axis of the first base, a first end proximal the first end of the first base, and a second end proximal the second end of the first base. The first heat sink includes a plurality of laterally spaced fins and a plurality of laterally spaced channels. Each channel is laterally positioned between a pair of laterally adjacent fins of the plurality of fins. Still further, the heating unit comprises a manifold coupled to the first end of the first base. The manifold has a central axis, a first end, a second end axially opposite the first end, and an outer surface. The outer surface of the manifold includes a first surface extending axially from the first end to the second end. The first surface of the manifold faces the first base and the first heat sink. The manifold includes a first flow passage and a first plurality of orifices in fluid communication with the first flow passage. Each orifice of the first plurality of orifices has an outlet at the first surface that is aligned with one of the channels of the first heat sink. The first flow passage and the first plurality of orifices are configured to flow a fluid into and through the channels of the first heat sink. 
     In another embodiment, a heating unit for heating an enclosure comprises a base having a central axis, a first end, a second end axially opposite the first end, and a cavity extending axially from the first end. In addition, the heating unit comprises a positive temperature coefficient (PTC) heater disposed in the cavity of the base. The PTC heater slidingly engages the base and is configured to conductively transfer thermal energy to the base. Further, the heating unit comprises a heat sink mounted to the first base. The heat sink has a central axis oriented parallel to the central axis of the first base, a first end, and a second end axially opposite the first end of the base. The heat sink includes a plurality of laterally spaced fins and a plurality of laterally spaced channels. Each fin and each channel extends axially from the first end of the heat sink to the second end of the heat sink. Each channel is laterally positioned between a pair of laterally adjacent fins of the plurality of fins. The base is configured to conductively transfer thermal energy to the heat sink. Still further, the heating unit comprises a manifold coupled to the first end of the first base. The manifold has a central axis, a first end, a second end axially opposite the first end, and an outer surface. The outer surface of the manifold includes a first surface extending axially from the first end to the second end. The first surface of the manifold is adjacent the first end of the base and the first end of the heat sink. The manifold includes a flow passage and a plurality of orifices in fluid communication with the first flow passage. Each orifice of the first plurality of orifices has an outlet at the first surface in fluid communication with one of the channels. The first flow passage and the first plurality of orifices are configured to flow a fluid into and through the channels of the first heat sink along the plurality of fins. 
     Embodiments of methods for heating an enclosure with a heating unit are disclosed herein. In an embodiment, a method comprises (a) heating a first base of the heating unit with a first positive thermal coefficient (PTC) heater. In addition, the method comprises (b) transferring thermal energy from the first base to a plurality of fins of a first heat sink coupled to the first base during (a). The plurality of fins of the first heat sink are oriented parallel to each other. Further, the method comprises (c) flowing a fluid into a manifold coupled to the first base during (a) and (b). Still further, the method comprises (d) flowing the fluid through a first plurality of orifices of the manifold and into a plurality of channels of the first heat sink during (c). Each channel of the first heat sink is positioned between a pair of adjacent fins of the plurality of fins of the first heat sink and each orifice of the first plurality of orifices is aligned with one of the channels of the first heat sink. 
     Embodiments described herein comprise a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical characteristics of the disclosed embodiments in order that the detailed description that follows may be better understood. The various characteristics and features described above, as well as others, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the disclosed embodiments. It should also be realized that such equivalent constructions do not depart from the spirit and scope of the principles disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of various exemplary embodiments, reference will now be made to the accompanying drawings in which: 
         FIG.  1    is a schematic view of an embodiment of a system for heating an enclosure; 
         FIG.  2    is an isometric view of the heating unit of  FIG.  1   ; 
         FIG.  3    is an exploded isometric view of the heating unit of  FIG.  2   ; 
         FIG.  4    is an isometric cross-sectional view of the heating unit of  FIG.  2   ; 
         FIG.  5    is an isometric view of an embodiment of a heating unit in accordance with principles described herein; 
         FIG.  6    is an isometric cross-sectional view of the heating unit of  FIG.  5   ; 
         FIG.  7    is an isometric view of an embodiment of a heating unit in accordance with principles described herein; 
         FIG.  8    is an exploded isometric view of the heating unit of  FIG.  7   ; 
         FIG.  9    is a front view of an embodiment of a heating unit in accordance with principles described herein; 
         FIG.  10    is an isometric view of an embodiment of a heating unit in accordance with principles described herein; 
         FIG.  11    is an exploded isometric view of the heating unit of  FIG.  10   ; 
         FIG.  12    is a cross-sectional view of the manifold of the heating unit of  FIG.  10   ; 
         FIG.  13    is an isometric view of an embodiment of a system for heating an enclosure; 
         FIG.  14    is an exploded isometric view of the heating unit of  FIG.  13   ; and 
         FIG.  15    is an isometric view of an embodiment of a heating unit in accordance with principles described herein. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY DISCLOSED EMBODIMENTS 
     The following discussion is directed to various exemplary embodiments. However, one of ordinary skill in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. 
     The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection of the two devices, or through an indirect connection that is established via other devices, components, nodes, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a given axis (e.g., central axis of a body or a port), the terms “radial” and “radially” generally mean perpendicular to the given axis, and the terms “lateral” and “laterally” generally mean to the side of the given axis (e.g., to the left or right of the given axis). For instance, an axial distance refers to a distance measured along or parallel to the axis, and a radial distance means a distance measured perpendicular to the axis. 
     As previously described above, enclosure heaters may be used to heat a volume of fluid within a container or housing. In some petrochemical gas sampling systems, it may be desirable to maintain a stream of a sampled gas at or above a particular temperature such as the dewpoint of the sampled gas. However, in some jurisdictions, regulations limit the maximum allowable surface temperatures of hardware in enclosure heaters. Accordingly, embodiments described herein are directed to enclosure heaters that offer the potential to maintain surface temperatures below a particular set point, while maximizing the rate of heat transfer to the local environment and enclosure. 
     Referring now to  FIG.  1   , an embodiment of a system  10  for heating an enclosure  2  is shown. Enclosure  2  is a housing or container defining an inner chamber or volume  4  disposed within enclosure  2  and an outer volume  6  outside and external to enclosure  2 . A fluid conduit  14  extends through ports  8 ,  12  in enclosure  2  and traverses inner chamber  4 . As will be described in more detail below, in this embodiment, a sample fluid  16  to be heated within enclosure  2  flows periodically or continuously through conduit  14 . System  10  also includes a heating unit  100  positioned within inner chamber  4  of enclosure  2 . In general, heating unit  100  heats the fluid (e.g., air) within chamber  4 , which in turn heats conduit  14  traversing chamber  4  and sample fluid  16  flowing through conduit  14 . 
     Referring still to  FIG.  1   , in this embodiment, heating unit  100  includes an inlet  18 , a choke  20  disposed along inlet  18 , and an outlet  22 . Inlet  18  is a conduit that supplies air or another working fluid to heating unit  100 . As will be described in more detail below, a working fluid (e.g., air) flows through inlet  18  and choke  20  into heating unit  100 , then flows through heating unit  100  to outlet  22 , and then flows through outlet  22  and exits heating unit  100 . In some embodiments, inlet  18  supplies the working fluid from within inner volume  4 , whereas in other embodiments, inlet  18  supplies the working fluid from a source external enclosure  2  such as, for example, compressed air. Similarly, in some embodiments, outlet  22  exhausts the working fluid into chamber  4 , whereas in other embodiments, outlet  22  exhausts the working fluid external to enclosure  2  such as, for example, into outer volume  6 . 
     During operations, heating unit  100  supplies thermal energy into system  10  to increases the temperature of inner chamber  4 , conduit  14  extending through chamber  4 , and sample fluid  16  flowing through conduit  14 . More particularly, heating unit  100  supplies a first heat transfer Q 1  into inner volume  4 , thereby heating conduit  14  contained therein. As conduit  14  increases in temperature, a second heat transfer Q 2  transfers thermal energy from conduit  14  to heat sample fluid  16  flowing through conduit  14 . Some thermal energy may be transferred across enclosure  2  as a third heat transfer Q 3 , and thus, in some embodiments, insulation may be added to the outside of enclosure  2  to reduce and minimize the third heat transfer Q 3 . 
     Referring now to  FIGS.  2  and  3   , heating unit  100  has a central or longitudinal axis  105  and includes a main body or base  110 , a heat sink  120  coupled to base  110 , a manifold  130  coupled to base  110 , a fitting  150  coupled to manifold  130 , a plurality of heating elements or heaters  160  removably disposed in body  110 , and a temperature sensor  170  coupled to heaters  160  within body  110 . In embodiments described herein, heaters  160  are positive temperature coefficient (PTC) heaters, and thus, may also be referred to as PTC heaters  160 . However, in other embodiments, one or more of the heaters (e.g., heaters  160 ) can be other types of heaters such as resistive heaters, capacitive heaters, dielectric heaters, inductive heaters, etc. 
     Referring now to  FIGS.  2 - 4   , base  110  has a central or longitudinal axis  115  oriented parallel to axis  105 , a first or open end  110   a , a second or closed end  110   b  axially opposite first end  110   a , and a rectangular prismatic body  112  extending axially between ends  110   a ,  110   b . Body  112  has a first or upper planar surface  114  extending axially from end  110   a  to end  110   b , and a second or lower planar surface  116  extending axially from end  110   a  to end  110   b . Surfaces  114 ,  116  are oriented parallel to each other. In addition, base  110  includes a recess  118   a  extending axially from first end  110   a  into body  112  and a plurality of parallel pockets or cavities  118   b  extending axially from recess  118   a  toward second end  110   b . It should be appreciated that neither recess  118   a  nor cavities  118   b  extend to or through second end  110   b . Thus, recess  118   a  defines an opening in end  110   a , however, there is no opening in end  110   b  (i.e., end  110   b  is closed). In this embodiment, recess  118   a  has a generally rectangular cross-section with semi-cylindrical rounded ends in a plane oriented perpendicular to axis  115 , and each cavity  118   b  is an elongate cylindrical bore extending axially from recess  118   a . In this embodiment, cavities  118   b  are laterally spaced apart and positioned with their central axes oriented in a common horizontal plane. As will be described in more detail below, in other embodiments, one or more cavities (e.g., cavities  118   b ) may have different geometries to accommodate heaters with different geometries. 
     Referring now to  FIGS.  2  and  3   , heat sink  120  has a central or longitudinal axis  125  oriented parallel to axes  105 ,  115 , a first end  120   a  proximal first end  110   a  of base  110 , and a second end  120   b  axially opposite first end  120   a  and proximal end  110   b  of base  110 . In addition, heat sink  120  includes a base plate  122  extending axially between ends  120   a ,  120   b , and a plurality of laterally spaced (relative to axis  125 ) elongate, parallel heat fins  124  extending from base plate  122 . Due to the lateral spacing of parallel heat fins  124 , a plurality of laterally spaced (relative to axis  125 ) channels  126  are defined between heat fins  124 . Namely, one channel  126  is laterally positioned between each pair of laterally adjacent fins  124 . Fins  124  and channels  126  extend axially from end  120   a  to end  120   b , and extend perpendicularly from base plate  122 . Each fin  124  has the same geometry and extends from the same side of base plate  122 . In this embodiment, fins  124  are uniformly laterally spaced relative to central axis  125  and extend from base plate  122  parallel to a second axis  127  oriented perpendicular to axis  125 , base plate  122 , and surface  114 . 
     Manifold  130  has a central or longitudinal axis  135  disposed in a plane oriented perpendicular to axes  105 ,  115 ,  125 , a first end  130   a , a second end  130   b  axially opposite first end  130   a , and a body  132  extending axially between ends  130   a ,  130   b . In this embodiment, body  132  of manifold  130  has a rectangular prismatic geometry with a first planar face or surface  134  extending axially between ends  130   a ,  130   b  and a second planar face or surface  136  extending axially between ends  130   a ,  130   b . Surfaces  134 ,  136  are oriented parallel to each other, parallel to axis  135 , and perpendicular to axes  105 ,  115 ,  125 . 
     As best shown in  FIG.  4   , manifold  130  include a primary flow passage  138  extending axially from first end  130   a  to second end  130   b , thereby defining openings in both ends  130   a ,  130   b . The opening in first end  130   a  formed by passage  138  defines inlet  18  in first end  130   a  of manifold  130 , and the opening formed by passage  138  in second end  130   b  defines an inlet  19  in second end  130   b  ( FIG.  3   ). Thus, in this embodiment, passage  138  extends through both ends  130   a ,  130   b ; however, in other embodiments, passage  138  may only extend through one end of the manifold (e.g., end  130   a  or end  130   b  of manifold  130 ). As will described in more detail below, in this embodiment, inlet  18  is open and used to supply fluid into manifold  130 , whereas inlet  19  is plugged and is not used to supply fluid into manifold  130 . However, in general, inlet  18 , inlet  19 , or both inlets  18 ,  19  can be used to supply fluid into manifold  130 . 
     For most sample gas heating applications, main passage  138  has a diameter ranging from about 0.125 in. to about 0.50 in., alternatively from about 0.25 in. to about 0.50 in., and alternately from about 0.375 in. to about 0.50 in. A plurality of orifices  140  extend from passage  138  to surface  136 . In particular, orifices  140  are uniformly axially spaced relative to axis  135 , and extend laterally and radially (relative to axis  135 ) from passage  138  to surface  136 . Thus, orifices  140  are generally disposed in a plane oriented perpendicular to surface  136  and parallel to axes  115 ,  125 ,  135 . In this embodiment, the axial spacing of orifices  140  is the same as the lateral spacing of channels  126  of heat sink  120  such that an outlet of each orifice  140  at surface  136  is aligned with one of the channels  126  positioned between each pair of laterally adjacent fins  124  when manifold  130  and heat sink  120  are mounted to base  110 . As will be described in more detail below, a working fluid flows through choke  20  into inlet  18 , through inlet  18  into passage  138 , and then from passage  138  through orifices  140  and exits orifices  140  at surface  136  into channels  126  between fins  124 . For most sample gas heating applications, each orifice  140  has a diameter ranging from about 0.003 in. to about 0.075 in., alternatively from about 0.010 in. to about 0.050 in., and alternately from about 0.020 in. to about 0.040 in. 
     As best shown in  FIGS.  2  and  3   , a fitting  150  is coupled to body  132  at end  130   a  and is in fluid communication with inlet  18  and passage  138 . Thus, fitting  150  flows the working fluid into passage  138  via inlet  18 . In this embodiment, choke  20 , which may be used to control flow of working fluid into manifold  130 , is disposed in fitting  150 . In some embodiments, choke  20  is selectably adjustable between a fully open position and a partially closed position, while other embodiments include a non-adjustable internal orifice (not shown). In other embodiments, fitting  150  may be coupled to body  132  along end  130   b  in fluid communication with open inlet  19  (while inlet  18  is plugged), or a fitting  150  may be coupled to each open inlet  18 ,  19  (with neither inlet  18 ,  19  plugged). 
     Referring now to  FIG.  3   , each heater  160  has a central or longitudinal axis  165  oriented parallel to axes  105 ,  115 , a first end  160   a , a second end  160   b  axially opposite first end  160   a , and an elongate cylindrical body  162  extending axially from first end  160   a  to second end  160   b . In this embodiment, heating unit  100  includes two PTC heaters  160 , however, in general, any suitable number of heaters  160  may be used depending on the heat output requirements of heating unit  100 . 
     As previously described, in this embodiment, each heater  160  is a positive temperature coefficient (PTC) heater. The use of PTC heaters for heaters  160  may be particularly advantageous in embodiments described herein as no feedback temperature control system may be required to ensure heaters  160  remain below a predetermined set temperature. While not specifically required, in this embodiment, heating unit  100  includes a temperature sensor  170 . In general, temperature sensor  170  can be any device or sensor that measures and communicates temperature including, without limitation, a resistance temperature detector (RTD) or thermocouple. Temperature sensor  170  may be used to alarm a user when heating unit  100  falls below a particular predetermined temperature (e.g., in the event that PTC heaters  160  fail to operate as intended) to provide temperature feedback for a control system (not specifically shown) that monitors the heaters (e.g., heaters  160 ), function as an over temperature switch that may disconnect the heaters if the temperatures rises above a predetermined set point temperature, detect failure of the heater(s), or combinations thereof. 
     As described above, each heater  160  is a PTC heater. In general, PTC heaters are made of a positive temperature coefficient material (PTC material), which has a resistance that increases with a rising operating temperature. The PTC material can be selected to have a sharp increase in resistance at a particular “Curie temperature” or “set point temperature” such that PTC heater  160  will reach but not exceed the Curie temperature when exposed to a constant voltage. For regulatory purposes, in some embodiments, a maximum permissible temperature along outer surfaces of heating unit  100  may be dictated and required. In such embodiments, a self-temperature regulating PTC heater  160  using a PTC type material offers the potential for a fail-safe system that reliably maintains heating unit  100  below the maximum temperature. For an added level of security, temperature sensor  170  may also be used to further ensure that the outer surfaces of heating unit  100  are maintained below the maximum temperature setting. 
     As previously described, in other embodiments, heaters  160  may be other types of heaters (other than PTC heaters) such as resistive heaters, capacitive heaters, dielectric heaters, inductive heaters, etc. In such embodiments, a temperate feedback control may be used to regulate the set point temperature. More particularly, a signal from temperature sensor  170  may be used by a control system to selectively apply power to heaters  160  to maintain the set point temperature. Alternatively, or in addition, thermocouples  172  may be placed on any portion of heating unit  100  to provide temperature feedback, which may be used to regulate heaters  160  (e.g., to maintain a desired temperature of heating unit  100 , limit the maximum temperature of heating unit  100 , etc.). Additional thermocouples, which are not used for heater  160  control, may also be placed on any portion of heating unit  100  for monitoring purposes and or as part of addition safety systems, which may be required for example by particular regulations. 
     Referring again to  FIGS.  2  and  3   , to assemble heating unit  100 , PTC heaters  160  are axially advanced through recess  118   a  and into corresponding cavities  118   b , base plate  122  of heat sink  120  is fixably attached to body  112  of base  110 , and body  132  of manifold  130  is fixably attached to body  112  of base  110 . In particular, the planar surface of base plate  122  directly engages and is compressed against surface  114  of body  112  to promote efficient conductive heat transfer therebetween, and surface  136  of body  132  directly engages and is compressed against end  110   a  of base  110 . In this embodiment, bolts may be used to attach and compress bodies  112 ,  132 , and base plate  122 . Heaters  160  are positioned within cavities  118   b  of base  110  via close sliding fit to promote efficient conductive heat transfer between PTC heaters  160  and base  110 . In some embodiments, a press fit is may be used, with or without a thermally conductive paste. When heater unit  100  is assembled, fins  124  are disposed on the side of base plate  122  opposite base  110  and extending outward and away from body  112  and base plate  122 . With manifold  130  attached to end  110   a  of base  110 , recess  118   a  and cavities  118   b  are closed off at end  110   a  and PTC heaters  160  are captured within corresponding cavities  118   b . Fitting  150  is coupled to end  130   a  of manifold  130  in fluid communication with inlet  18 . The outlet ends of orifices  140  along surface  136  are positioned above body  112  and base plate  122  with each outlet end being aligned and in fluid communication with one of the channels  126  positioned between each pair of the laterally adjacent fins  124 . 
     Referring now to  FIGS.  1  and  4   , as previously described, during heating operations, heating unit  100  transfers thermal energy into system  10 , thereby increases the temperature of inner volume  4 , conduit  14 , and sample fluid  16  within conduit  14 . More particularly, heating unit  100  supplies a first heat transfer Q 1  into inner volume  4 , thereby heating conduit  14  extending therethrough. As conduit  14  increases in temperature, a second heat transfer Q 2  transfers thermal energy through conduit  14  to sample fluid  16 , thereby heating sample fluid  16  disposed therein. A third heat transfer Q 3 , may transfer thermal energy across enclosure  2  from inner volume  4  to outer volume  6 . As previously described, thermally insulating layers (not shown) may be provided on the outside enclosure  2  to minimize third heat transfer Q 3  such that a larger percentage of thermal energy from heating unit  100  is transferred into sample fluid  16 . Thermal energy may also be further transferred via inlet  18  and outlet  22 . For example, a heated working fluid supplied via inlet  18  may deliver additional thermal energy into heating unit  100 , which may be delivered into inner volume  4  directly via mass transfer as the working fluid fills inner volume  4 , or without the working fluid filling inner volume  4 . In addition, outlet  22  may transfer thermal energy away from system  10  and into outer volume  6 , for example in embodiments where outlet  22  physically passes through enclosure  2 . 
     As used herein, the term “heat transfer” and the term “thermal energy transfer” (e.g., first heat transfer Q 1 ) may include conductive heat transfer, convective heat transfer, radiative heat transfer, and combinations thereof. Unless otherwise specified, the total heat transfer at each location discussed herein may be increased or decreased by increasing or decreasing one or more of the conduction, convection, and radiation heat transfer components of the total heat transfer. For example, heating unit  100  may be placed in abutting contact with conduit  14  to increase the conductive heat transfer therebetween, thereby increasing first heat transfer Q 1  and second heat transfer Q 2 . Additionally, the materials and/or surface finishes of the components of heating unit  100  (e.g., fins  124 , base  110 , etc.) can be selected to increase or decrease the emissivity coefficient, and thus, increase or decrease the radiation heat transfer component of the total heat transfer. Further, the convective heat transfer component of the total heat transfer may be increased or decreased, for example, by flowing higher velocity working fluids across heating unit  100 , by increasing or decreasing surface areas, and by varying the spacing between components, such as fins  124  of heat sink  120 . For example, in an embodiment with a decreased fin  124  spacing along heat sink  120 , a greater number of fins will be used for a given sized heat sink  120 , and thus heat sink  120  will present a larger overall surface area, which may in some embodiments tend to increase the convective heat transfer. However, with less space between fins  124 , less flow area is available to accommodate the working fluid flow. In some embodiments, a reduced flow area between fins  124  may result in higher working fluid flow velocities through orifices  140 , which again may tend to increase the convective heat transfer component of first heat transfer Q 1 . However, in some other embodiments, a reduced flow area between fins  124  may result in a decreased working fluid flow velocity, due to pressure losses between inlet  18  and outlet  22 , and thus may reduce the convective heat transfer component of first heat transfer Q 1 . 
     Referring to  FIG.  4   , the conductive heat transfer component of a fourth heat transfer Q 4  relies on contact between body  112  of base  110  and PTC heater  160  (heater  160  not shown in  FIG.  4   ), and relies on a temperature gradient between PTC heater  160  and first surface  114  to transfer thermal energy across body  112  to heat sink  120 . Therefore, body  162  of PTC heater  160  will be maintained at a higher temperature than the maximum temperature along outer surfaces of heating unit  100 . Maximizing the rate of fourth heat transfer Q 4 , allows a maximum rate for second heat transfer Q 2  into sample  16 , however, maximizing the rate of fourth heat transfer Q 4 , may be limited by the maximum temperature allowed along outer surfaces of heating unit  100  (e.g., as required by particular regulations). Therefore, in embodiments described herein, a fifth heat transfer Q 5  and a sixth heat transfer Q 6  are maximized using convection, so that the rate of fourth heat transfer Q 4  can be maximized, while also satisfying the maximum temperature allowed along outer surfaces of heating unit  100 . More specifically, during operations, a pressurized working fluid is supplied to main passage  138  via inlet  18 . The flow rate of the working fluid into and through main passage  138  is controlled and limited by choke  20  ( FIG.  3   ) and the number and diameters of orifices  140 . The working fluid flows through main passage  138  and into the orifices  140 , which emit the working fluid into channels  126  as represented by arrows  142  in  FIG.  4   . The flow  142  of the working fluid within channels  126  generally moves axially (relative to axes  105 ,  125 ) along the length of the channels  126 . Flow  142  in the axial direction in channels  126  between fins  124  may induce lower pressure regions within channels  126 , which in turn may draw fluid in chamber  4  surrounding heating unit  100  (e.g., air flow) into channels  126  as represented by arrows  144  in  FIG.  4   . In general, the flow  142  of working fluid into channels  126 , along with the flow  144  into channels  126 , offers the potential to increase the convective heat transfer associated with fifth heat transfer Q 5  and sixth heat transfer Q 6 , thereby enhancing the transfer of thermal energy from heating unit  100  to conduit  14  and the sample fluid  16  therein, while maintaining the surface temperature of heating unit  100  relatively low (e.g., below the maximum permissible surface temperature). 
     As previously described, PTC heaters  160  transfer thermal energy to base  110  via conduction, base  110  transfer thermal energy to heat sink  120  via conduction, and thermal energy moves through heat sink  120  from base plate  122  into and through fins  124  via conduction. To enhance conductive heat transfer through and between base  110  and heat sink  120 , base  110  and heat sink  120  are made of thermally conductive materials such as metals and metal alloys. For example, in some embodiments, base  110  and heat sink  120  are made of aluminum. 
     Referring now to  FIGS.  5  and  6   , an embodiment of heating unit  200  is shown. In general, heating unit  200  can be used within system  10  in place of heating unit  100  previously described. Heating unit  200  is similar to heating unit  100  previously described, and thus, components of heating unit  200  that are the same as those in heating unit  100  are identified with like reference numerals, and the description below will focus on features that are different. 
     In this embodiment, heating unit  200  has a central or longitudinal axis  205  and includes a plurality of bases  110  coupled together, a plurality of heat sinks  120  coupled to bases  110 , a manifold  230  coupled to bases  110 , and a plurality of PCT heaters  160  disposed in each base  110 . Bases  110  and heat sinks  120  are each as previously described with respect to heating unit  100 . Manifold  230  has a central or longitudinal axis  235 , a first end  230   a , a second end  230   b  axially opposite first end  230   a , and a body  232  extending axially between ends  230   a ,  230   b . In this embodiment, body  232  has a rectangular prismatic shape including a first planar face or surface  234  and a second planar face or surface  236  opposite first surface  234 . Surfaces  234 ,  236  extend axially between ends  230   a ,  230   b . Inlets  18 ,  19  as previously described are provided at ends  230   a ,  230   b , respectively. In this embodiment, inlet  18  is open and used to supply fluid into manifold  230 , whereas inlet  19  is plugged and is not used to supply fluid into manifold  230 . However, as previously described, in general, inlet  18 , inlet  19 , or both inlets  18 ,  19  can be used to supply fluid into manifold  130 . 
     As best shown in  FIG.  6   , a pair of radially spaced main passages  238  extend axially (relative to axis  235 ) through body  232  from first end  230   a  to second end  230   b . In this embodiment, inlet  18  is positioned between passages  238 . Each passage  238  has a central axis  239 ,  241 , respectively, oriented parallel to axis  235 . A gas passage  248  also extends axially (relative to axis  235 ) through body  232  from first end  230   a  to second end  230   b . Passage  248  is positioned between main passages  238 , is oriented parallel to main passages  238 , and defines inlets  18 ,  19  at ends  230   a ,  230   b , respectively. In this embodiment, passage  248  has a smaller diameter than inlets  18 ,  19  and passages  238 . Main passages  238  and passage  248  are in fluid communication with each other via a cross drilled passages extending from passage  248  to each passage  238 . 
     A plurality of gap orifices  249  extend from passage  248  to face  236 , and a plurality of orifices  240  extend from each passage  238  to face  236 . Gap orifices  249  are uniformly axially spaced relative to axes  235 ,  239 ,  241 , and extend radially and laterally relative to axis  235  from passage  248  to face  236 . Orifices  240  are uniformly axially spaced relative to axes  235 ,  239 ,  241 , and extend radially and laterally relative to axis  239 ,  240  of the corresponding passage  238  to face  236 . In this embodiment, orifices  249  generally lie in a plane oriented perpendicular to surface  236  and parallel to axes  205 ,  215 , and orifices  240  extending from the same passage  238  generally lie in a plane oriented perpendicular to surface  236  and parallel to axes  205 ,  215 . In this embodiment, the axial spacing of orifices  240  is the same as the lateral spacing of channels  126  and fins  124  of the corresponding heat sink  120  such that an outlet of each orifice  240  along surface  236  is aligned with one channel  126  positioned between a pair of laterally adjacent fins  124  of the corresponding heat sink  120  when manifold  230  and heat sinks  220  are mounted to bases  110 . As will be described in more detail below, a working fluid flows into inlet  18  into and through passages  248 ,  238 , and then flows from passages  248 ,  238  through orifices  249 ,  240 , respectively, and exits orifices  249 ,  240  at surface  236 . 
     For most sample gas heating applications, each main passage  238  has a diameter ranging from about 0.125 in. to about 0.50 in., alternatively from about 0.25 in. to about 0.50 in., and alternately from about 0.375 in. to about 0.50 in.; and passage  248  has a diameter ranging from about 0.062 in. to about 0.25 in., alternatively from about 0.125 in. to about 0.25 in., and alternately from about 0.188 in. to about 0.25 in. For most sample gas heating applications, each orifice  240  has a diameter ranging from about 0.003 in. to about 0.075 in., alternatively from about 0.010 in. to about 0.050 in., and alternately from about 0.020 in. to about 0.040 in.; and each orifice  249  has a diameter ranging from about 0.003 in. to about 0.075 in., alternatively from about 0.010 in. to about 0.050 in., and alternately from about 0.020 in. to about 0.040 in. 
     Referring again to  FIGS.  5  and  6   , to assemble heating unit  200 , PTC heaters  160  are positioned within corresponding cavities  118   b  of each base  110 , base plate  122  of each heat sink  120  is fixably attached to body  112  of one base  110 , and body  232  of manifold  230  is fixably attached to both bodies  112  of bases  110  at ends  110   a . In this embodiment, bolts may be used to attach and compress body  232  with both bodies  112 , and to compress each body  112  with the corresponding base plate  122 . The bases  110  are positioned radially adjacent to each other (relative to axes  115 ) with surfaces  116  facing each other and oriented parallel to each other with a gap  246  disposed therebetween. Base plates  122  engage surfaces  114  of bodies  112 , and thus, fins  124  generally extend from bases  110  away from each other. Manifold  230  is attached to bases  110  with surface  246  engaging ends  110   a , thereby closing recesses  118   a  and cavities  118   b  at ends  110   a  and capturing PTC heaters  160  within cavities  118   b . Fitting  150  is coupled to end  230   a  of manifold  230  in fluid communication with inlet  18 . Bodies  112  and base plates  122  are positioned between the two rows of the outlet ends of orifices  240  along surface  236  with each outlet end aligned with one channel  126  positioned between each pair of the laterally adjacent fins  124  of the corresponding heat sink  120 . As best shown in  FIG.  6   , the outlet ends of orifices  249  along surface  236  are positioned between bodies  112  in alignment with and in fluid communication with corresponding gap  246 . 
     Referring still to  FIGS.  5  and  6   , heating unit  200  transfers thermal energy into a system (e.g., system  10 ) in a similar manner as heating unit  100  previously described. In particular, during operations, a pressurized working fluid is supplied to gap passage  248  via inlet  18 . The pressurized fluid flows through gap passage  248  and into and through main passages  138 , which are in fluid communication with gap passage  248 . The flow rate of the working fluid into and through passage  238 ,  248  is controlled and limited by choke  20 , as well as by the number and diameters of orifices  240 ,  249 . The working fluid flows through passages  238 ,  248  and into the orifices  240 ,  249 , respectively. Orifices  240  emit the working fluid into channels  126  between each pair of laterally adjacent fins  124  of the corresponding heat sink  120  (as represented by flow  142 ) in the same manner as previously described with respect to heating unit  100 . Orifices  249  emit the working fluid into gap  246  between opposed surfaces  116  of bases  110  as represented by flow  252  in  FIG.  6   . Flow  252  generally progresses axially (relative to axes  205 ,  215 ) through gap  246  and results in seventh heat transfer Q 7 . In the embodiment shown in  FIGS.  5  and  6   , PTC heaters  160  transfer thermal energy to bases  110  via conduction, bases  110  transfer thermal energy to corresponding heat sinks  120  via conduction, and thermal energy moves through each heat sink  120  from base plate  122  into and through fins  124  via conduction. To enhance conductive heat transfer through and between bases  110  and heat sinks  120 , bases  110  and heat sinks  120  are made of thermally conductive materials such as metals and metal alloys. 
     Referring now to  FIGS.  7  and  8   , an embodiment of heating unit  300  is shown. In general, heating unit  300  can be used within system  10  in place of heating unit  100  previously described. Heating unit  300  is similar to heating unit  100  previously described, and thus, components of heating unit  300  that are shared the same as those in heating unit  100  are identified with like reference numerals, and the description below will focus on features which are different. 
     In this embodiment, heating unit  300  has a central or longitudinal axis  305 , and includes a base  310 , a heat sink  320  coupled to base  310 , and a manifold  330  coupled to base  310 . Base  310  is the same as base  110  previously described with the exception that base  310  has a width measured perpendicular to axis  115  that is less than the width of base  110 , and further, only one cavity  118   b  is provided in base  310  to accommodate one heater  160 . In addition, heat sink  320  is the same as heat sink  120  previously described with the exception that heat sink  320  has a width measured perpendicular to axis  125  that is less than the width of heat sink  120 , which results in fewer fins  124  on base  310  as compared to base  110 . Manifold  330  has a central or longitudinal axis  335 , a first end  330   a , a second end  330   b  axially opposite first end  330   a , and a body  332  extending axially between ends  330   a ,  330   b . In this embodiment, body  332  has a rectangular prismatic shape including a first planar face or surface  334  and a second planar face or surface  336  opposite first surface  334 . Surfaces  334 ,  336  extend axially between ends  330   a ,  330   b . Inlet  18  is disposed at end  330   a  and inlet  19  is disposed at end  330   b . A main passage  338  extends axially (relative to axis  335 ) through body  332  from first end  330   a  to second end  330   b . Passage  338  has a central axis  337  oriented parallel to axis  335  and defines inlets  18 ,  19  at ends  330   a ,  330   b , respectively. In this embodiment, inlet  18  is open and used to supply fluid to main passage  338  of manifold  330  while inlet  19  is plugged and is not used to supply fluid into main passage  338 . However as previously described, in other embodiments, inlet  18 , inlet  19 , or both inlets  18 ,  19  can be used to supply fluid to main passage  338  of manifold  330 . A plurality of axially spaced orifices  140  as previously described (not shown) extend radially and laterally from passage  338  to face  336 . Inlet  18 , passages  338 , and orifices  140  direct a pressurized working fluid into channels  126  between fins  124  in the same manner as previously described with respect to heating unit  100 . 
     To assemble heating unit  300 , PTC heater  160  is positioned within cavity  118 , base plate  122  of heat sink  320  is fixably attached to body  112  of base  310 , and body  332  of manifold  330  is fixably attached to body  112  of base  310 . In particular, the planar surface of base plate  122  directly engages and is compressed against surface  114  of body  112  to promote efficient conductive heat transfer therebetween, and surface  336  of body  332  directly engages and is compressed against end  110   a . In this embodiment, bolts may be used to attach and compress body  332  with both bodies  112  and to compress each body  112  with the corresponding base plate  122 . PTC heater  160  is advanced through recess  118   a  and into cavity  118   b  of base  310  via close sliding fit to promote efficient conductive heat transfer between PTC heaters  160  and base  110 . With manifold  330  attached to end  110   a  of base  310 , recess  118   a  and cavity  118   b  are closed off at end  110   a  and PTC heater  160  is captured within cavity  118   b . Fitting  150  is coupled to end  330   a  of manifold  330  in fluid communication with inlet  18  in this embodiment, however fitting  150  may also be coupled to end  330   b  and inlet  19 . The outlet ends of orifices  140  along surface  336  are aligned with channels  126  of the corresponding heat sink  320 . Generally speaking, heating unit  300  operates in the same manner previously described for heating unit  100 . 
     Referring now to  FIG.  9   , an embodiment of heating unit  400  is shown. In general, heating unit  400  can be used within system  10  in place of heating unit  100  previously described. Heating unit  400  is similar to heating unit  100  previously described, and thus, components of heating unit  400  that are the same as those in heating unit  100  are identified with like reference numerals, and the description below will focus on features which are different. 
     In this embodiment, heating unit  400  has a central axis  405  and includes a base  110  as previously described, a heat sink  420  coupled to base  110 , and a manifold  430  coupled to base  110 . Heat sink  420  includes a base plate  422  and a plurality of laterally spaced fins  424  extending from base plate  422  parallel to a second axis  427  oriented perpendicular to axis  405  and surface  114 . In this embodiment, each fin  424  includes serrations  428 , which in some embodiments are formed as a wavy or undulating surface. 
     Manifold  430  includes a main passage  438  defining inlets  18 ,  19  and a plurality of laterally spaced orifices  440 . In this embodiment, inlet  18  is open and is used to supply fluid to main passage  438  of manifold  430 , whereas inlet  19  is plugged and is not used to supply fluid to main passage  438  of manifold  430 . However, as previously described, in other embodiments, inlet  18 , inlet  19 , or both inlets  18 ,  19  can be used to supply fluid into main passage  438  of manifold  430 . Inlet  18 , passage  438 , and orifices  440  are in fluid communication with each other. In this embodiment, orifices  440  are laterally spaced such that each orifice  440  is aligned with a channel  426  laterally positioned between each pair of laterally adjacent fins  424 . During heating operations, flow  142  as previously described passes between each pair of adjacent fins  424  and induced flow  144  as previously described may also occur along the distal free ends of fins  424 . The geometry of serrations  428  may be adjusted to control the flow directions of flow  142  and induced flow  144  and to optimize the overall heat transfer from heating unit  400 . In addition, the plurality of orifices  440  may include at least one orifice  440  with a different diameter, as the flow rate of flow  142  may be balanced or separately “tuned” between each pair of fins  424 . 
     In the embodiments of heating units  100 ,  200 ,  300  previously described, heaters  160  having elongate cylindrical bodies  162  that are seated in mating cavities  118   b  extending axially from corresponding recesses  118   a  in ends  110   a  of bases  110 ,  310 . However, in other embodiments, the heaters (e.g., heaters  160 ) have geometries other than cylindrical and/or the heaters may be installed in a different manner. 
     Referring now to  FIGS.  10  and  11   , an embodiment of heating unit  500  is shown. In general, heating unit  500  can be used within system  10  in place of heating unit  100  previously described. Heating unit  500  is similar to heating unit  100  previously described, and thus, components of heating unit  500  that are the same as those in heating unit  100  are identified with like reference numerals, and the description below will focus on features that are different. 
     In this embodiment, heating unit  500  has a central or longitudinal axis  505  and includes a base  510 , a heat sink  120  coupled to base  510 , a manifold  530  coupled to base  310 , a fitting  150  coupled to manifold  530 , and a plurality of PCT heaters  560  disposed in base  510 . Heat sink  120  is as previously described with respect to heating unit  100 . In this embodiment, heaters  560  are positive temperature coefficient (PTC) heaters, and thus, may also be referred to as PTC heaters  560 . However, in other embodiments, one or more of the heaters (e.g., heaters  560 ) can be other types of heaters such as resistive heaters, capacitive heaters, dielectric heaters, inductive heaters, etc. 
     Base  510  has a central or longitudinal axis  515  oriented parallel to axis  505 , a first or open end  510   a , a second or closed end  510   b  axially opposite first end  510   a , and a rectangular prismatic body  512  extending axially between ends  510   a ,  510   b . Body  512  has a first planar surface  514  extending axially from end  510   a  to end  510   b , and a second planar surface  516  extending axially from end  510   a  to end  510   b . Surfaces  514 ,  516  are oriented parallel to each other and face away from each other. In addition, base  510  includes a recess  518   a  extending axially from first end  510   a  into body  512  and a pocket or cavity  518   b  extending axially from recess  518   a  toward second end  510   b . It should be appreciated that neither recess  518   a  nor cavity  518   b  extends to or through second end  510   b . Thus, recess  518   a  defines an opening in end  510   a , however, there is no opening in end  510   b  (i.e., end  510   b  is closed). In this embodiment, recess  518   a  has a generally rectangular cross-section with semi-cylindrical rounded ends in a plane oriented perpendicular to axis  515 , and cavity  518   b  is a rectangular recess that extends axially from recess  518   a  and laterally (relative to axis  515 ) from surface  514 . Thus, cavity  518   b  can be accessed through recess  518   a  and through surface  514 . 
     Manifold  530  has a central or longitudinal axis  535 , a first end  530   a , a second end  530   b  axially opposite first end  530   a , and a body  532  extending axially between ends  530   a ,  530   b . In this embodiment, body  532  has an L-shaped cross-sectional shape (as opposed to rectangular) in any plane oriented perpendicular to axis  535 . Accordingly, as best shown in  FIG.  12   , body  532  has a first planar face or surface  534 , a second planar face or surface  536  opposite and parallel to first surface  534 , a third planar face or surface  537  extending perpendicularly from surface  536 , and a fourth planar face or surface  538  extending perpendicularly from surface  537 . Surfaces  534 ,  536 ,  538  are oriented parallel to each other, whereas surface  537  lies in a plane oriented perpendicular to surfaces  534 ,  536 ,  538 . Surface  537  may be described as a step that extends between surfaces  536 ,  538 . Each surface  534 ,  536 ,  537 ,  538  extends axially from first end  530   a  to second end  530   b . Inlets  18 ,  19  as previously described are provided at ends  530   a ,  530   b , respectively. In this embodiment, inlet  18  is open and used to supply fluid into manifold  530 , whereas inlet  19  is plugged and is not used to supply fluid into manifold  530 . However, as previously described, in general, inlet  18 , inlet  19 , or both inlets  18 ,  19  can be used to supply fluid into manifold  530 . 
     Referring again to  FIG.  12   , a main passage  539  extends axially (relative to axis  535 ) through body  532  from first end  530   a  to second end  530   b  and defines inlets  18 ,  19  at ends  530   a ,  530   b , respectively. Passage  539  is positioned within body  532  proximal surface  538 . A plurality of orifices  540  extend from passage  539  to face  538 . Orifices  540  are uniformly axially spaced relative to axis  535 , and extend laterally from passage  539  to face  538 . In this embodiment, orifices  540  extending from passage  539  generally lie in a plane oriented perpendicular to surfaces  534 ,  536 ,  538  and parallel to axis  505  and surface  537 . In this embodiment, the axial spacing of orifices  540  is the same as the lateral spacing of channels  126  and fins  124  of heat sink  120  such that an outlet of each orifice  540  along surface  538  is aligned with one channel  126  positioned between a pair of laterally adjacent fins  124  of heat sink  120  when manifold  530  and heat sinks  120  are mounted to bases  510 . As will be described in more detail below, a working fluid flows into inlet  18 , through passage  539 , from passages  539  through orifices  540 , and exits orifices  540  at surface  538 . 
     For most sample gas heating applications, main passage  539  has a diameter ranging from about 0.125 in. to about 0.50 in., alternatively from about 0.25 in. to about 0.50 in., and alternately from about 0.375 in. to about 0.50 in. For most sample gas heating applications, each orifice  540  has a diameter ranging from about 0.003 in. to about 0.075 in., alternatively from about 0.010 in. to about 0.050 in., and alternately from about 0.020 in. to about 0.040 in.; and each orifice  249  has a diameter ranging from about 0.003 in. to about 0.075 in., alternatively from about 0.010 in. to about 0.050 in., and alternately from about 0.020 in. to about 0.040 in. 
     As best shown in  FIG.  11   , each heater  560  has a central or longitudinal axis  565  oriented parallel to axes  505 ,  515 , a first end  560   a , a second end  560   b  axially opposite first end  560   a , and an elongate generally flat body  562  extending axially from first end  560   a  to second end  560   b . Heaters  560  are disposed in cavity  518   b . An elongate thermal pad  561  is disposed on both sides of each heater  560  to facilitate the transfer of thermal energy from heaters  560  to heat sink  120  and body  512 . In particular, thermal pads  561  positioned between heaters  560  and heat sink  120  are compressed therebetween, and thermal pads  561  positioned between heaters  560  and body  512  are compressed therebetween. In this embodiment, heating unit  500  includes two PTC heaters  560 , however, in general, any suitable number of heaters  560  may be used depending on the heat output requirements of heating unit  500 . 
     Referring still to  FIG.  11   , a temperature sensor  570  is provided within cavity  518   b . In general, temperature sensor  570  can be any device or sensor that measures and communicates temperature including, without limitation, a resistance temperature detector (RTD) or thermocouple. Temperature sensor  570  may be used to alarm a user when heating unit  500  falls below a particular predetermined temperature (e.g., in the event that PTC heaters  160  fail to operate as intended) to provide temperature feedback for a control system (not specifically shown) that monitors the heaters (e.g., heaters  560 ), function as an over temperature switch that may disconnect the heaters if the temperatures rises above a predetermined set point temperature, detect failure of the heater(s), or combinations thereof. 
     Referring again to  FIGS.  10  and  11   , to assemble heating unit  500 , PTC heaters  560  are positioned within cavoty  518   b  of base  510 , base plate  122  of heat sink  120  is fixably attached to body  512  of base  510 , and body  532  of manifold  530  is fixably attached to body  512  of base  512  at end  510   a . In this embodiment, bolts may be used to attach and compress body  532  against body  512 , and to compress body  512  with base plate  122 . Base plate  122  engage surface  514  of body  512 , and thus, fins  124  generally extend from base  510  away from base  510 . Heaters  560  are compressed within cavity  518  between thermal pads  561 , body  512 , and base plate  122 . Manifold  530  is attached to base  510  with surfaces  536 ,  537  engaging end  510   a . Fitting  150  is coupled to end  530   a  of manifold  530  in fluid communication with inlet  18 . Body  532  of manifold  530  and base plate  122  are positioned such that the outlet end of each orifice  540  along surface  538  is aligned with one channel  126  between each pair of the laterally adjacent fins  124  of the corresponding heat sink  120 . 
     Referring still to  FIGS.  10  and  11   , heating unit  500  transfers thermal energy into a system (e.g., system  10 ) in a similar manner as heating unit  100  previously described. In particular, during operations, a pressurized working fluid is supplied to passage  539  via inlet  18 . The pressurized fluid flows through passage  539  to orifices  540 . The flow rate of the working fluid into and through passage  539  can be controlled and limited by choke (e.g., choke  20 ), as well as by the number and diameters of orifices  240 . The working fluid flows through passage  539  and into the orifices  540 , which emit the working fluid into channels  126  between each pair of laterally adjacent fins  124  of the corresponding heat sink  120  in the same manner as previously described with respect to heating unit  100 . In the embodiment shown in  FIGS.  10  and  11   , PTC heaters  560  transfer thermal energy to base  510  via conduction, base  510  transfer thermal energy to heat sink  120  via conduction, and thermal energy moves through each heat sink  120  from base plate  122  into and through fins  124  via conduction. To enhance conductive heat transfer through and between base  510  and heat sink  120 , base  510  and heat sink  120  are made of thermally conductive materials such as metals and metal alloys. In the embodiments of heating units  100 ,  200 ,  300 ,  400 ,  500  previously described, the base (e.g., base  110 ,  310 ,  510 ), the heat sink (e.g., heat sink  120 ,  320 ,  420 ), the manifold (e.g., manifold  130 ,  230 ,  330 ,  530 ), and heaters (e.g., PTC heaters  160 ,  560 ) are distinct and separate components that are coupled together during assembly to form the corresponding heating unit (e.g., heating unit  100 ,  200 ,  300 ,  400 ,  500 ). However, in other embodiments, any two or more of the base, heat sink, manifold, and the heaters may be integral or monolithically formed as a single piece. For example, as shown in  FIGS.  13  and  14   , an embodiment of a heating unit  100 ′ is shown. Heating unit  100 ′ can be used in place of heating unit  100  in system  10  and is substantially the same as heating unit  100  previously described with the exception that heat sink  120  and base  110  are monolithically formed as a single piece that is subsequently coupled to manifold  130  after the heater(s)  160  are positioned in corresponding cavities  118   b . As another example, as shown in  FIGS.  15  and  16   , an embodiment of a heating unit  500 ′ is shown. Heating unit  500 ′ can be used in place of heating unit  100  in system  10  is substantially the same as heating unit  500  previously described with the exception that heat sink  520  and manifold  530  are monolithically formed as a single piece that is subsequently coupled to base  510  after the heater(s)  560  are positioned in corresponding cavity  518   b.    
     In the manner described, embodiments disclosed herein include enclosure heaters which maintain surface temperatures within an enclosure below a particular set point, while also maximizing the heat transfer rate between the heated enclosure and a conduit containing a flowing gas stream. In addition, embodiments disclosed herein are directed to enclosure heaters which may be used with Positive Temperature Coefficient type heaters, which may be reliably controlled below a particular set point temperature, while also allowing the use of redundant controls, which may further increase the system reliability. 
     While exemplary embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. For example, PTC heaters  160  may be provided in any shape (e.g., such as in flat sheets or having an elongated rectangular shape), or may be produced as an integral portion of base  110  and/or heat sink  120 . One method for producing an integrated PTC heater  160  may be to directly deposit the PTC material within cavity  118   b  of base  110 . In addition, in some embodiments, the base (e.g., base  110 ) and the heat sink (e.g., heat sink  120 ) are a single, integral, monolithic structure. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the disclosure. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.