Apparatus and Method for Evaluating the Performance of a Heat Exchanger

1. Field of the Invention

The present invention relates to apparatus and a method for evaluating the performance of a heat exchanger. More particularly, the present invention concerns evaluating the heat transfer performance of a tube in tube type heat exchanger.

2. Description of the Prior Art

In many vapor compression refrigeration systems it has been found advantageous to exchange heat energy between refrigerant and water. One efficient type of heat exchanger for accomplishing this heat exchange is a tube in tube coaxial heat exchanger wherein an inner tube is mounted within an outer tube. When used in a refrigeration circuit the heat exchanger is connected to allow water to flow through the inner tube and refrigerant to flow through the outer tube such that heat energy may be transferred between the two.

An application for such a device may be found in the use of a water source heat pump wherein the water acting as a heat source in the heating mode is supplied through the outer tube of the heat exchanger or acting as the cooling sink in the cooling mode is supplied through the same portion of the heat exchanger. Another application could be the heating or chilling of water for either a hydronic cooling system or for a hot water heating system. The utilization of this type of heat exchanger as a combination refrigerant desuperheater and hot water preheater is also known in the industry. Such an application allows superheat energy contained in the refrigerant to be utilized for preheating hot water for a domestic hot water supply when an air conditioner is operating in the cooling mode.

It is particularly efficient to have tube in tube heat exchangers mounted within foamed insulation such that heat transfer between the outer tube and the ambient is prevented. A problem identified with such a heat exchanger is that it is difficult to evaluate the performance of the heat exchanger after foam insulation is supplied to surround the heat exchanger. In order to evaluate the heat transfer between the inner tube and the outer tube a specific scheme as set forth herein was developed.

The invention as set forth herein concerns supplying water at a predetermined temperature and flow rate to the inner tube of the heat exchanger. The water is then circulated through the inner tube of the heat exchanger and discharged into a separate heating unit at the other end of the heat exchanger. This separate heating unit contains heaters which act to supply thermal energy to the water at a constant rate. The now heated water is supplied to the outer tube of the heat exchanger and is circulated through the outer tube of the heat exchanger. Since the entering water temperature to the inner tube is controlled and the heat energy supplied between the inner tube and the outer tube is known, the overall amount of heat energy transferred between the inner and outer tubes may be calculated as a function of known constants and the temperature of the water being discharged from the inner tube. Hence, by the measurement of a single temperature it is possible to determine whether or not the heat transfer performance of the heat exchanger is suitable.

It is an object of the present invention to provide apparatus for measuring the performance of a heat exchanger.

It is a further object of the present invention to provide a method of determining the heat transfer rate between the inner and outer tubes of a tube in tube heat exchanger.

It is a yet further object of the present invention to provide means for determining whether or not a tube in tube heat exchanger is adequate to meet a particular heat transfer requirement.

It is another object of the present invention to provide a method and apparatus serving as in incoming quality control procedure for evaluating heat exchangers.

Another object of the present invention is to provide a safe, economical, simple and reliable method of evaluating the performance of a heat exchanger.

Other objects will be apparent from the description to follow and from the appended claims.

These and other objects are achieved according to a preferred embodiment of the invention wherein there is disclosed a method of measuring the heat transfer performance of a tube in tube heat exchanger having an inner tube defining a first flow path and an outer tube defining a second flow path. Heat transfer fluid is supplied to a tube of the heat exchanger at a known mass flow rate. The heat transfer fluid is received from the tube and heat energy is added to the fluid at a known rate. The fluid is then directed into the other tube of the heat exchanger in heat exchange relation with the first tube such that heat energy is transferred between the two. The temperature change of the fluid between the inlet and the outlet of one of the tubes is then determined, said temperature being indicative of the heat transfer rate between the two tubes.

Apparatus for use in evaluating the performance of a tube in tube heat exchanger having a first flow path and a second flow path is further disclosed. The apparatus includes means for connecting a water supply to the first flow path of the heat exchanger and means for receiving water from the first flow path of the heat exchanger. Said means for receiving water including means for directing said water into the second flow path of the heat exchanger and including a heat source for supplying heat energy at a known rate to the water between the first and second flow paths. Temperature detection means is provided for ascertaining the temperature change of the water along a flow path of the heat exchanger, said temperature change being indicative of the heat energy transferred between the first and second flow path of the heat exchanger. A heat source for heating the water between the first and second paths of the heat exchanger includes a heating unit within a water flow path including electric immersion heaters for supplying heat energy to the water at a known rate, said heating unit being insulated to reduce heat transfer to the ambient from the heating unit.

FIG. 1 is a schematic view of a tube in tube heat exchanger connected to appropriate equipment pursuant to the method and apparatus set forth herein.

FIG. 2 is a graph indicating acceptable and unacceptable heat transfer performance according to the discharge temperature of the water from a tube of the heat exchanger.

The embodiment described herein will be directed to a coaxial tube in tube heat exchanger. The drawing shows a simple U-shaped heat exchanger but it is to be understood that these heat exchangers are typically helical or otherwise circuitous to include a plurality of loops forming a single flow path. It is further to be understood that although the preferred embodiment described herein tests a tube in tube heat exchanger that the method and apparatus described would have like applicability to any heat exchanger having two fluid flow paths in direct heat exchange relation with each other such that the mass flow through each would be constant.

Referring now to FIG. 1 there may be seen heat exchanger 10 having inner tube 22 surrounded by outer tube 20. Urethane insulation 12 surrounds the entire heat exchanger. Water supply 26 acts to supply water through preheater unit 27 having electric immersion heaters 28 and being insulated with insulation 29 such that the supply water is preheated to a certain temperature. The water is then conducted through connecting line 30 from preheater unit 27 to flow meter 82. Flow meter 82 is used to determine the flow rate of water through the system. Conduit 34 connects flow meter 82 to inner tube inlet 32. From there the water flows through inner tube 22 of the heat exchanger and is discharged through inner tube outlet 62 to connecting line 72. Connecting line 72 is connected between inner tube outlet 62 and heating unit 50 to conduct water into heater inlet 54. Water then flows to the heating area 56 with immersion heaters 57 before being discharged through heater discharge 58 to connecting line 70. From connecting line 70 the water is directed to outer tube inlet 60.

Heating unit 50 contains heaters 57 as well as insulation 52 to prevent heat energy transfer between the heaters and the ambient. Heating unit 50 further includes heating area 56, heater inlet 54 and heater discharge 58.

Water flows into outer tube 20 through outer tube inlet 60 and travels back through outer tube 20 surrounding inner tube 22 such that there is heat transfer between the water flowing through the inner and outer tubes. The water is then discharged from outer tube 20 through outer tube outlet 34 into conduit 39. From conduit 39 the water flows through regulating valve 40 for controlling the mass flow rate of the water through the heat exchanger and is directed to water discharge 42. Temperature sensor 36 (T.sub.cin) is mounted in heat exchange relation with the water flowing through conduit 34 prior to inner tube inlet 32. Temperature sensor 64 (T.sub.co) is connected to measure the temperature of the water flowing through connecting line 72 which is the water discharged from inner tube outlet 62. Temperature sensor 66 (T.sub.hin) is connected to measure the temperature of the water flowing through connecting line 70, just prior to the water entering outer tube inlet 60. Temperature sensor 38 (T.sub.ho) is connected to measure the temperature of the water flowing through conduit 39 just after the water exits outer tube outlet 34.

FIG. 2 is a graph of the equation

where the ordinate of the graph is UA and the abcissa of the graph is T.sub.co. UA, the heat transfer coefficient times the area of the heat exchanger measures the heat transfer performance of a heat exchanger. T.sub.co is the fluid exit temperature from the inner flow path.

A portion of the graph is shaded and labeled both 100 and 104. This portion is indicative of an exit water temperature from the inner tube exceeding a predetermined value being indicative that the heat exchanger has sufficient heat transfer between the inner and outer tubes to be acceptable for the desired application. Portion 100 is that portion where the exit temperature as measured by sensor 64 is equal to or greater than a predetermined value. The portion labeled 104 is that portion where the heat transfer rate, UA, is equal to or exceeds a predetermined value. The portion of the graph labeled 102 is that portion where the exit temperature (T.sub.co) is unacceptably low and the heat exchanger would be rejected.

In order to determine the effective rate of heat transfer between the inner and outer tubes it may be calculated by the equation

where Q is the overall heat transfer rate, U is the overall coefficient of heat transfer, A is the heat transfer area and .DELTA.T is the temperature change measured as the mean temperature difference between the inner and outer tubes. The heat transfer rate may also be expressed as: ##EQU1## where q is the heat energy added by heating unit 50, m is the mass flow rate and C.sub.p is the specific heat of water. This equation may be rewritten as ##EQU2## This equation may be rearranged as: ##EQU3## Using the above formula and since the following factors are known: q is equal to the energy input of heating unit 50.

m is the mass flow rate of the water through the heat exchanger.

T.sub.cin is the incoming temperature of the water controlled by preheater unit 27 to a known constant.

C.sub.p is the known specific heat of water which is one BTU per pound per degree Fahrenheit.

Hence, the only unknown to determine the performance of the heat exchanger, UA, is temperature T.sub.co (the temperature sensed at sensor 64).

Assuming a specific example where the energy input to heaters 57 is 3000 watts (10,239 BTU per hour), C.sub.p equals one BTU per pound per degree Fahrenheit and further assuming that T.sub.cin is 80 degrees Fahrenheit and that the volume flow rate is one half gallon per minute of water which is equal to a mass flow rate of 250 pounds per hour. Based on these assumptions the temperature rise T.sub.cin -T.sub.co equals 10,239 BTU per hour divided by 250 pounds per hour and divided by one BTU per pound per degree Fahrenheit. Pursuant to these assumptions the temperature drop equals 41 degrees Fahrenheit. Additionally, using these same values in equation 4 it is determined that UA equals 6.1 times the quantity (T.sub.co -T.sub.cin). Since T.sub.cin is a constant then solving equation 4 with the measured value of T.sub.co will give the value of UA representative of the performance of the heat exchanger.

When using this apparatus as an incoming quality control tool the appropriate water connections are made and by simply measuring the temperature at temperature sensor 64 to determine T.sub.co it may be determined whether or not the heat exchanger is acceptable. If T.sub.co falls to the right within the area labeled 100 or 104 in FIG. 2 then the heat exchanger is acceptable. If the T.sub.co is less than that amount and falls within the region labeled 102 then it is unacceptable.

It is to be understood that the temperature change over both flow paths should be equal. Hence, the temperature change could be determined by subtracting T.sub.ho from T.sub.hin. The inner flow path temperatures are referred to in all of the examples but the outer flow path temperatures could serve equally for this purpose. Additionally, pressure drop measurements over the flow path may be taken to verify the flow resistance along the flow path.

The invention has been described herein with reference to a particular embodiment. It is to be understood by those skilled in the art that variations and modifications can be effected within the spirit and scope of the invention.