Abstract:
An improved and accurate enthalpy tunnel is presented, designed to condition and control airflow patterns within the main tunnel and air sampling subsystem so as to present a homogenous volume immediately upon introduction into the enthalpy tunnel. The air volume velocity is slowed and the air volume is completely mixed in a settling chamber which also serves to maintain the static discharge pressure on the unit under test. The rectangular design in use in the industry is replaced with circular geometry, using a cylindrical tunnel shape as opposed to a rectangular shape. The circular geometry creates a flat, uniform velocity profile. This tunnel improves the design of current art enthalpy tunnels by using a single nozzle instead of a bank of nozzles, presenting to the sampling mechanism a smooth, stable and uniform flow profile. The sampling method used in the prior art is replaced with a sampling tunnel that also conditions its air flow profile, leading to consistent sampling. Means for controlling the air velocity and mass flow rate in both the main tunnel and the sampling tunnel in real time during the performance of a test, using a feedback loop controller for both the sampling and main tunnels, and by providing variable flow rate discharge mechanisms is provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates in general to devices used for measuring the change in enthalpy of an air stream from preconditioned to conditioned states. In particular, this invention is a device for determining the rating of air conditioning or forced air heating units (hereinafter “air-conditioning units”), using the change in enthalpy between entry and exit air streams. 
     SPECIFIC PROBLEMS IN THE PRIOR ART 
     A class of devices for determining the rating of air conditioners and heaters is known in the art as enthalpy tunnels or code testers. The basic design of an enthalpy tunnel is described in the American Society of Heating, Refrigeration and Air Conditioning Engineers, (ASHRAE) Standard 37-1988,  Methods of Testing for Rating Unitary Air - Conditioning and Heat Pump Equipment,  the disclosure therein hereby incorporated by reference. Enthalpy tunnel tests measure wet and dry bulb temperatures of entry and exit air from air-conditioning units and determine the enthalpy of the air mass in both conditions. These measurements allow the difference in enthalpy between the entry and exit air masses to be calculated. The enthalpy difference, combined with the mass flow rate of the air is then used to determine the amount of work the air conditioner or heater has performed. 
     ASHRAE Standard 37-1988 provides the general requirements of an enthalpy tunnel and further provides tolerances and standards for associated instrumentation and ancillary equipment. However, the Standard provides only general guidelines for the structure of an enthalpy tunnel, not a specific design. 
     In general, enthalpy tunnel testing units consist of an indoor room containing the heating or cooling coil and the test equipment, and an outdoor room containing the compressor and diffusor unit, such an arrangement usually referred to as a test cell. The indoor and outdoor rooms are thermally separated; waste heat generated by the air-conditioning unit exhausted to the outside room, and ultimately to the atmosphere. The enthalpy tunnel is connected to the heating or cooling coil under test by means of duct-work, and a controlled, measured mass of air is directed across the coil. The enthalpy tunnel accomplishes three goals: (1) It controls and measures the flow rate of air through the heating or cooling coil; (2) It controls the static pressure drop across the heating or cooling coil, and (3) It measures the exit temperature and humidity of the air volume exiting the heating or cooling coil being tested. 
     The exit temperature and humidity combined with the exit volumetric airflow rate yields the exit mass flow and enthalpy of the conditioned air. The inlet temperature and humidity combined with the same mass flow rate for the exit yields the unit inlet enthalpy of the air. The difference between the two is the work performed by the unit. The same mass flow rate inlet and outlet, along with the energy calculations, is an expression of the First Law of Thermodynamics. 
     The guidelines given in ASHRAE Standard 37-1988 for the construction of an enthalpy tunnel are non-specific as to the physical structure of enthalpy tunnels but are suggestive. State of the art enthalpy tunnels follow this suggestion, and generally consist of a rectangular sheet metal tunnel with a series of diffusion screens and flow control devices. Current art enthalpy tunnels also use a bank of fixed nozzles for flow control, and static pressure control as suggested by the Standard. 
     In an enthalpy tunnel constructed in accordance with the ASHRAE Standard 37-1988, using the design suggested therein, conditioned air enters the tunnel and then passes through a diffusion screen to facilitate mixing and homogeneity of the air stream. The air then passes through a bank of fixed nozzles, used to create a pressure drop and, in enthalpy tunnels of current design, control volumetric flow rates and static pressure at the exit of the air conditioning unit under test. The pressure drop across the nozzle bank is measured by means of a draft-range differential pressure transmitter. The resulting information is used to determine a volumetric flow rate. The air stream is then passed through a second diffusion screen and then into a discharge chamber. The temperature and humidity of the exit air stream are measured by sampling the air at the entrance to or in the discharge chamber. FIG. 5 of ASHRAE Standard illustrates the suggested configuration of this type of enthalpy tunnel. 
     The sampled air is measured to determine wet and dry bulb temperatures. ASHRAE Standard 37-1988 requires that the air velocity over the wet bulb temperature measuring instrument be 1000 feet per minute (fpm.) Velocities above or below 1000 fpm require that the wet bulb measurements be corrected in accordance with ASHRAE Standard 41.1-1986, the disclosure therein hereby incorporated by reference. Current art enthalpy tunnels have no reliable method for insuring that this velocity is maintained. Consequently, a correction calculation is routinely performed, allowing more error to enter the calculated rating of the unit under test. Subsequent calculations using wet and dry bulb temperatures in conjunction with the volumetric flow rate, are used to calculate the mass flow rate and the work performed by the unit. 
     ASHRAE Standard 37-1988 also requires enthalpy tunnels to be equipped with a discharge fan to control the static discharge pressure of the air conditioning unit being tested, and for means to vary the capacity of the fan. Current art enthalpy tunnels use the fixed nozzle bank described above to control the amount of air that passes through the tunnel. Depending on the desired static discharge pressure required, nozzles are plugged or freed to restrict or increase flow, indirectly varying the discharge fan capacity, the flow through the tunnel and the static discharge pressure the unit under test sees. 
     This method of varying flow and controlling static pressure, although widely used, has undesirable effects on the air flow in the enthalpy tunnel, and on the accuracy of the measurements made using this method. The use of a nozzle bank, as suggested by the Standard, creates multiple jets in the downstream air mass. Flow rates could be set using any number of nozzle configurations, and nozzle configurations are not consistent from test to test. The velocity profile across the tunnel after the nozzle bank is extremely irregular. This irregularity is in part corrected by the use of a diffusion screen downstream of the nozzle bank, but testing has shown that the velocity profile is still far from flat even after passing through the downstream diffusion screen. 
     In general, the results of testing air conditioning units using this “plugged nozzle” method of varying air flow have been inconsistent, non-repeatable and inaccurate. Depending on what nozzles are plugged and due to the rectangular shape of the tunnels, unstable zones of re-circulation and varying pressure gradients are created. These unknown quantities cause errors in both flow and pressure readings, and result in non-homogenous temperature profiles within the air stream. As a result, the temperature of the air that is sampled is non-uniform, and results in erroneous calculations. In addition, the “plugged nozzle” method of setting volumetric flow rate and/or static pressure does not allow for modification of these parameters during a test run. Temperature and humidity changes can and do often occur during tests. 
     Measurements and tests conducted by the Applicants have shown that the air flow patterns downstream of fixed nozzle banks, are non-uniform even when no nozzles are plugged. The use of multiple nozzles creates downstream of the nozzle bank unstable laminar and turbulent flow regions. These regions are exaggerated and especially prevalent when nozzles are plugged to control the air flow rate. In addition to the non-uniform flow present in the main tunnel, the air sampling configurations presently being used produce uneven flow and pressure gradients within the sampling tubes themselves. Most current art enthalpy tunnels use a grid array of piping as a sample collection device. Usually the array is constructed using tubing or piping connected with tees and elbows. The tubing is perforated and the grid is attached to a sample fan or blower. The grid arrays generally have a series of holes drilled into the piping or tubing that are uniform in size and regular in placement. No effort is made to ensure that the volume of air sampled from every hole is the same, or that the sample as a whole is truly representative of the enthalpy conditions present in the air mass. 
     Inconsistent and non-repeatable results caused by the design flaws in the current in design of enthalpy tunnels have forced the industry to incorporate correction factors into the calculations using the data from current art enthalpy tunnels. The result of the inaccurate testing and calculations is that air conditioning units are placed into the stream of commerce without a truly accurate representation of their efficiency rating or capacity. 
     It is the current state of the art in enthalpy tunnels to operate them in test cells and in fixed locations. In general, test cells and enthalpy tunnels require that duct-work be routed from the unit under test to the tunnel, and often these routings take circuitous and tortuous paths. A tunnel design imparting some flexiblity in the location of the tunnel with respect to the unit being tested would further condition the air flow prior to its introduction into the tunnel. 
     SUMMARY OF THE INVENTION 
     An improved and accurate enthalpy tunnel is presented. The present invention overcomes limitations in the prior art enthalpy tunnels, whose designs do not attempt to condition and control the airflow patterns within the main tunnel and air sampling subsystem. The present invention replaces the ASHRAE Standard 37-1988 suggested configuration, which is widely used without modification by the industry, with a design that meets the requirements of the Standard and provides consistent, controlled repeatable results. The test results from this enthalpy tunnel may be used without “correction factors” as a true and accurate measure of the capacity of a tested air-conditioning unit. 
     The present invention departs from the current art design of enthalpy tunnels by conditioning the air mass flow rate so as to present a homogenous volume immediately upon introduction into the enthalpy tunnel. The air volume velocity is slowed and the air volume is completely mixed in a settling chamber. 
     The present invention also replaces the rectangular design in use in the industry with circular geometry, using a cylindrical tunnel shape as opposed to the suggested rectangular shape. The circular geometry creates a flat, uniform velocity profile. The present invention improves the design of current art enthalpy tunnels by using a single nozzle instead of a bank of nozzles, presenting to the sampling mechanism a smooth, stable and uniform flow profile and circumventing the need for instrumentation calibration due to changes in nozzle configuration. The sampling method used in the prior art is replaced with a sampling tunnel that also conditions its air flow profile, leading to consistent sampling. 
     The present invention also provides means for controlling the air velocity and mass flow rate in both the main tunnel and the sampling tunnel in real time during the performance of a test, using a feedback loop controller for both the sampling and main tunnels, and by providing variable flow rate discharge mechanisms. These improvements over the current art generate predictable airflow, temperature and pressure profiles within the system, and consistent repeatable test results. 
     It is another improvement of the present invention over the prior art that only one diffusion screen need be used. Multiple screens are required where the fixed nozzle bank is used for flow control and determination because of the significant disruption of the air flow profile. The present invention provides a variable capacity discharge blower eliminating the need for plugged nozzle flow and control, and multiple diffusion screens. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a schematic drawing of the preferred embodiment of the present invention illustrating the major components. 
     FIG. 2 is a isometric drawing of the interface/settling chamber. 
     FIG. 3 is a sectional view of the interface/settling chamber. 
     FIG. 4 is a sectional view of the main tunnel. 
     FIG. 5 is a schematic of the enthalpy tunnel and its support structure. 
     FIG. 6 is a schematic of the sampling tunnel. 
     FIG. 7 is a schematic of the sampling tunnel instrumentation section. 
     FIG. 8 is a schematic of the sampler blower sub-system. 
     FIG. 9 is a block diagram of the control system. 
     FIG. 10 is a block diagram of the control algorithm. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 the present invention, an enthalpy tunnel indicated generally at  10 , may be seen. The enthalpy tunnel  10  comprises five major components: An interface/settling chamber  20 , a main air flow sensor  30 , a main tunnel  40 , a sampling tunnel assembly  50 , and a main blower assembly  80 . It is important in all thermodynamic tests to ensure that heat losses are minimized. Therefore, in general, all duct-work used in conjunction with the present invention is insulated and sealed at the connection points with aluminum duct tape. In addition, all exterior surfaces of the enthalpy tunnel  10  are insulated to prevent heat loss or gain, or constructed from materials with a high R factor and that quickly approach thermal equilibrium. 
     The first major component a conditioned air stream encounters when passed through the enthalpy tunnel  10  is the interface/settling chamber  20 . The interface/settling chamber  20  is a large substantially cylindrical volume having an inlet adapted to receiving an air stream via duct-work, an aerodynamically smooth outlet for presenting air to the main air flow sensor  30  and the main tunnel  40 , and a large central volume. The large central volume allows the entering air stream&#39;s velocity to retard and to be thoroughly mixed. 
     In general, the air entering the interface/settling chamber  20  will either be pushed in by a blower integral to the air conditioning unit being tested, or will be pulled in by the main blower assembly  80 . In either case, the entering air will have a positive velocity. The air stream, upon entering the interface/settling chamber  20  will tend to form a jet, and create areas of backflow and swirling. The purpose of the interface/settling chamber  20  is to condition the air stream so as to make it uniform and suitable for sampling without error. The size and shape of the interface/settling chamber  20  is chosen so that a particular mass flow rate of air will be conditioned to have a flat velocity profile across the cross section of the interface/settling chamber  20 , and will be completely mixed. 
     Referring to FIGS. 2 and 3, the interface/settling chamber  20  is constructed as a series of cylinders of decreasing size, which are in turn constructed from two identical fiberglass molded halves secured together, the halves being bolted or otherwise secured together using flanges integrated into the mold along the edges. The series of decreasing cylinders provides a lip or flange at the connection point between the cylinders. This flange provides a convenient place to install diffusion screens. 
     The interface/settling chamber  20  serves three purposes. First, it acts as a universal interface structure allowing various size duct-work to be attached to the tunnel. As previously indicated, an air-conditioning unit under test will be attached to the enthalpy tunnel  10  via duct-work. The interface/settling chamber  20  comprises an entrance plate assembly  21 , a main body  27 , and an aerodynamically bell shaped exit nozzle  28 . The entrance plate assembly  21  consists of a circular plate  22  having a centrally located opening  23 . The opening  23  is surrounded by a clip assembly  24 . The clip assembly  24  allows the insertion and capture of duct work. 
     The second purpose of the interface/settling chamber  20  is to retard the inlet air velocity to near zero, and conditioning the velocity flow profile. Conditioned air leaving the unit under test exits the unit, enters the connecting duct-work and passes through the opening  23  with a positive velocity. The unit under test may or may not have a discharge blower of its own. The interface/settling chamber  20  provides a volume of sufficient size and shape to retard the air stream velocity immediately after exiting the entrance plate assembly  21  and entering the interface/settling chamber  20 . The purpose of the interface/settling chamber  20  is to allow the air stream to approach zero velocity as close as practical for a given mass flow rate, to completely mix the air volume, and to condition the air stream so it has a flat velocity profile. In the preferred embodiment, the interior volume of the interface/settling chamber  20  is sized to accommodate an air flow up to and including 10,000 SCFM. For larger flow rates, it is anticipated that larger volume settling chambers can be used. 
     In the preferred embodiment, a circular cross sectional area was chosen for the interface/settling chamber  20 . It is essential to create a mass flow rate of air that is completely mixed and that has a flat velocity profile. The use of a circular, or near circular geometry yields the flattest velocity profile. However, other cross-sectional shapes will also provide acceptable results. In general, the smoother the interior surface and the fewer corners the air encounters, the flatter the velocity profile. A circular cross section approaches a flat velocity profile more quickly for a given velocity than other shapes, and it is therefore more efficient to use a circular geometry. 
     After entering the interface/settling chamber  20  through the opening  23 , the air stream first expands and the air velocity nears zero. The air mass then passes through a diffusion screen  29 . The purpose of the diffusion screen is to help ensure a homogenous air volume by disrupting the flow profile of the air entering settling chamber and by enhancing mixing. Diffusions screens such as the ones used in the present invention are well known in the art. ASHRAB Standard 37-1988 provides requirements for the number, placement and open area required of the screens. 
     The third purpose of the interface/settling chamber  20  is to provide a single aerodynamically smooth, bell shaped exit opening for the air entering the main air flow sensor  30  and the main tunnel assembly  40 . Referring to FIG. 3, the bell-shaped exit opening  28  may be seen. The bell-shaped exit opening  28  funnels the air into the main air flow sensor  30  and the main tunnel assembly  40  in a manner that preserves the homogeneity of the air mass. Classical flow analysis shows that a bell shaped entrance into a pipe minimizes system losses with respect to pressure drop and produces a nearly uniform velocity profile at the beginning of the pipe, i.e. the end of the converging cross-section of the bell shape. The flow profiles upon exit are smooth and the pressure losses are minimal. The uniform and regular velocity profile provides for consistent sampling and measurements. 
     The bell shaped exit opening  28  attaches to the main flow sensor  30  by means of a conventional bolted flange, although any type of convenient air-tight fitting may be used. A second diffusion screen may be attached to the interface/settling chamber  20  at the outlet of the bell shaped exit opening  28 , to help ensure a homogenous air stream as the air stream enters the main flow sensor  30 , and the main tunnel assembly  40 . In the preferred embodiment of the invention, a second diffusion screen is not used, but it is contemplated that a second screen could be used and attached here. 
     The method of attachment of any of the parts of the enthalpy tunnel  10  is unimportant to the functionality of the enthalpy tunnel  10  as a whole, although it is necessary that all fittings be leak tight. Using a fastening device that allows for ease of removal of the constituent parts eases the assembly and disassembly process. Although not incorporated into the preferred embodiment of the present invention, it is envisioned that main flow sensor  30 , the main tunnel  40 , and the sampling tunnel assembly  50  may be used with differing interface/settling chamber assemblies to accommodate air conditioning units with flow rates outside the range of 2500 to 10,000 CFM. Using fastening devices such as easily removable bolted flanges will facilitate adaptability. 
     After exiting the interface/settling chamber  20 , the conditioned air stream enters the main flow sensor  30 . The main flow sensor  30  is an integral device pair consisting of a flow sensor and flow transmitter. The main flow sensor  30  is a commercially available sensing unit, comprising a flow-straightener, nozzle/venturi, and array of pilot tubes. In the preferred embodiment the main flow sensor is a NPZ1000 sensor manufactured by Brandt Industries. These devices are well known in the art, but it is critical to effective sampling with the novel enthalpy tunnel to choose a flow element that produces a smooth outlet air flow profile. 
     After exiting the main flow sensor  30 , the conditioned air enters the main tunnel  40 . Referring to FIG. 4, a sectional view of the main tunnel, shown generally at  40  may be seen. The main tunnel  40  comprises an inner core  41 , an outer shell  42 , an inlet flanged end  43 , an outlet flanged end  44 , and a sample tunnel connection  45 . The main tunnel  40  is essentially an elongated hollow cylinder with flanged ends. The main tunnel  40  length is determined by the physical space requirements of the ancillary equipment. The shape and length of the tunnel after the point where the air stream is sampled is unimportant, and its shape and length is dictated by engineering concerns. The width of the unit as a whole is minimized to allow it to pass through equipment doors. In the preferred embodiment, this restriction dictates that the main blower assembly  80  and sampling tunnel assembly  50  cannot be side-by-side. However, alternate configurations are possible where size and shape are not problematic. 
     Referring to FIG. 5, the enthalpy tunnel  10  and its support structure may be seen indicated generally at  200 . It is a point of novelty of the present invention that the enthalpy tunnel  10  is relatively portable and adaptable to various test unit configurations. In the preferred embodiment, the support structure  200  is fabricated from aluminum, but any material providing the necessary structural strength would suffice. The support structure  200  is also mounted on wheels for mobility, and includes vertical adjustment means for aligning the enthalpy tunnel  10  with air conditioning unit outlet ducting of various heights. In the preferred embodiment, the vertical adjustment means are jack screws, but alternate lifting means such as scissors lifts or hydraulic or pnematic cylinders would suffice. 
     The main tunnel  40  length could be lessened by reducing (as possible) the length of the sampler section. The diameter of the main tunnel  40 , while directly impacting the Reynolds Number associated with the air stream in the main tunnel  40 , is derived from the diameter of the main flow sensor  30 . The main flow sensor  30  is sized for the required maximum flow rate, in the case of the preferred embodiment, 10,000 SCFM. The Reynolds Number depends, primarily, on the viscosity and velocity of the air, and the main tunnel  40  diameter. Length of the main tunnel  40  is not a factor. 
     Referring again to FIG. 4, the sample tunnel connection  45  may be seen. The sample tunnel connection  45  consists of an elongated cylindrical extension of the wall of the main tunnel  40  set at approximately 45 degrees to the longitudinal axis of the main tunnel  40 , connecting to the central hollow area of the main tunnel  40  via an opening through the wall of the main tunnel  40 . The angle of the sample tunnel connection  45  to the longitudinal axis of the main tunnel  40  is important in that the shallower the angle, the less perturbation to the air flow stream as it enters the sampling tunnel assembly  50 . The 45 degree angle used in the preferred embodiment is a compromise between the overall length of the enthalpy tunnel  40  and acceptable perturbation to the flow stream. In general, the shallower the angle a sampling connection makes with a tunnel the longer the unit as a whole must be. The construction of the main tunnel  40  following the main flow sensor  30  and the acquisition of sample air do not affect the measurements obtained. It is necessary to accurately measure the air flow in the main tunnel  40 , and to separate a representative sample of air for measurement of the wet and dry bulb temperatures. Perturbation of the air stream after the sample is taken does not affect the accuracy of the measurements. In the preferred embodiment, the sample tunnel connection  45  is constructed integrally with the main tunnel  40  and has at its termination a flanged connection  46 . The flanged connection  46  allows the main tunnel  40  to be connected to the sampling tunnel assembly  50 . Alternative embodiments contemplate a sample tunnel integrally molded with the main tunnel  40 . 
     As the air exits the main sensor  30  and starts down the main tunnel  40  a portion of the air stream encounters a diverter tube  47  that extends into the main tunnel  40 . Referring to FIG. 4, the diverter tube  47  may be seen. The diverter tube  47  is an elongated cylindrical tube that enters the main tunnel  40  with its longitudinal axis parallel to the longitudinal axis of the sample tunnel connection  45 . The diverter tube  47  extends approximately one third of the main tunnel  40  diameter into the main tunnel  40 . In the preferred embodiment of the invention, the end of the diverter tube  47  furthest into the main tunnel  40  is cut at an angle of 67.5 degrees from its longitudinal axis. There is no specific diameter required for the diverter tube  47 . In general, the diameter of the diverter tube  47  is such that the diverter tube outside diameter matches the inside diameter of the sampling tunnel assembly  50 . Because perturbation of the air flow behind the diverter tube  47  in unimportant to the accuracy of the measurements, the diameter of the diverter tube  47  is selected to provide a sufficient sample that can be controlled in velocity to 1000 fpm. 
     A large negative pressure exists behind the main flow sensor  30 , caused by acceleration of the air through the main flow sensor  30 , which has a internal nozzle. The diverter tube  47  is located in the main tunnel  40  just behind the outlet of the main flow sensor  30 . In this region, the local velocity is high, hence a low local pressure exists. The diverter tube  47  acts like a Pitot-static tube. The dynamic pressure in the main tunnel  40 , which is a function of the air velocity squared, is used to generate a local stagnation pressure at the inlet of the diverter tube  47 , which is then overcome by a sampler blower  70  which may be seen by reference to FIG.  6 . 
     The location of the opening of the diverter tube  47  is necessary, as stated above, to prevent a negative static pressure that could not be overcome by the sampler blower  70 . The main flow sensor  30  includes an integral nozzle that accelerates the air. The resulting exit air flow is analogous to small air jet in the middle of a larger area where the flow around the edges of the jet swirl back. This backswirl creates a negative pressure. Inserting the diverter tube  47  opening into this jet overcomes the negative pressure. The geometry of the diverter tube  47  tip is the result of a series of tests -measuring the static pressure induced in the diverter tube. The 67.5 angle used in the preferred embodiment gives the highest (least negative) static pressure at the inlet of the diverter  47 . 
     The diverter tube  47  converts the dynamic pressure in the main tunnel  40  into a local static pressure at the diverter tube  47  inlet that is close to zero gauge. This allows the air to be pulled into the sampling tunnel assembly  50  by the sample blower assembly  70  at a rate sufficient to meet the ASAE requirement of 1000 fpm. Without the diverter tube  47 , the pressure in the main tunnel  40  is low enough that air cannot be pulled into the sampling tunnel assembly  50  over the entire operating range of the machine, 2500-10,000 SCFM. 
     Referring now to FIG. 6, the sampling tunnel assembly may be seen, indicated generally at  50 . The sampling tunnel assembly  50  comprises a transition connector  51 , a sampling air flow sensor  52 , a sampler tube sub-assembly  60 , and the sampling blower assembly  70 . In the preferred embodiment of the present invention, the transition connector  51  is a flanged 45 degree elbow. The shape and dimensions of the transition connector  51  itself are not essential to the overall operation of the enthalpy tunnel  10 , nor of the sampling tunnel  50 . However, it is part of the inventive quality of the present invention to provide a sampling mechanism that conditions the sampled air, creating in the sample volume a uniform and consistent velocity profile. In this regard, the sampling tunnel  50  is designed to minimize the number and severity of piping bends, and use a substantially circular geometry. The sampling tunnel  50  is designed to minimize the disruption to the air flow, and to allow the sampling tunnel assembly  50  to be close to and parallel to the main tunnel  40 . 
     In contrast to the other parts of the enthalpy tunnel  10 , the sampling tunnel assembly  50  is un-insulated, and is maintained at the same temperature as the air being sampled. It is desirable to construct the sampling tunnel  50  from materials that reach equilbrium temperature with the surrounding air quickly. This minimizes the time the tunnel must “soak” prior to taking data. The sampling tunnel assembly  50  is enclosed in an insulated enclosure (not shown) which surrounds the sampling tunnel assembly  50 . Air from the conditioned air volume being tested is drawn from the main tunnel  40 , passed through the sampling tunnel assembly  50 , and is exhausted into the insulated enclosure. At the beginning of a test, the air conditioning unit under test is allowed to run, air is pulled through the sampling tunnel assembly  50  by the sampling blower assembly  70 , exhausting to the insulated enclosure. The sampling tunnel assembly  50  is then allowed to come to an equilibrium temperature, as indicated by a temperature that is stable and within +/−0.1 degrees Fahrenheit with the sampled air, before data is collected. Keeping the sampling tunnel assembly  50  close to and parallel to the main tunnel  40  is an efficient method of keeping the enthalpy tunnel  10  as a whole, compact and easier to cool than a larger unit, and as stated above, capable of being transported through standard door openings. 
     Referring again to FIG. 6, the sampling air flow sensor  52  may be seen. The sampling air flow sensor  52  is similar to the other air flow measuring devices previously described in this specification. A flow transmitter (the two devices, sensor and transmitter, are purchased as a matched pair) is appropriately attached to the sampling air flow sensor  52  so that the air volume passing through the sampling tunnel assembly may be accurately measured. These types of devices are well known in the art and are not critical to the inventive qualities of the present invention except for their exit profile qualities as mentioned above. After exiting the sampling air flow sensor  52 , the air stream enters the sampler tube sub-assembly  60 . 
     Referring to FIG. 7, the sampler tube sub-assembly  60  may be seen. The sampler tube sub-assembly  60  comprises a flow straightening element  61 , a sampling tube  62 , a discharge nozzle  63 , and a water cup  64 . The flow straightening element  61  is an elongated cylindrical thin walled honeycombed structure. In the preferred embodiment the flow straightening element  61  is a honeycomb core of polycarbonate such as a Plascore model PCFR250W6.000-2. This particular flow straightening element has honeycomb channels approximately ¼ inch flat to flat measurement. The flow straightening element  61  approximates an array of long small tubes and serves to mitigate any swirling motion within the air stream. The size of the channels and the length of the element  61  must be chosen so as to avoid excessive back pressure, and still mitigate nearly all swirling. In the preferred embodiment, the flow straightening element  61  is 6 inches in length and 6.25 inches in diameter however, other dimensions may be employed as calculations so dictate. 
     After passing through the flow straightening element  61 , the air stream passes into the sampling tube  62 . The resultant air stream within the sampling tube  62  is a completely turbulent moving air volume with a flat parabolic velocity profile. Although not included in the present embodiment, it is contemplated that future designs of the sampling system will include a fine diffusion screen in the air stream to help scrub any wall effects within the sampling region. The local differential pressure gradients within the sampling tube  62  across any cross section of the sampling tube  62  are at or very close to zero. This type of air stream is necessary to ensure accurate and repeatable test results. 
     Referring again now to FIG. 7, the sampler tube sub-assembly  60  with the associated instrumentation may be seen. In the preferred embodiment of the present invention and in enthalpy tunnels in general, there are three instruments in the sensing section of the sampling assembly  50 : a dry bulb measuring device  65 ; a wet bulb measuring device  66 , and a differential pressure transmitter  67 . The dry bulb measuring device  65  utilizes a 100 Ω Plinum Bulb RTD, 4 wire configuration, with a high-accuracy transmitter. The RTD sensor has a sheath, but the tip is perforated so that the air stream can directly impinge the platinum element. The wet bulb RTD sensor is identical to the dry bulb, except that the tip of the probe is not perforated. This is eliminates the possibility that the water would short the platinum element. The dry bulb RTD and wet bulb RTD use identical transmitters. The source of the water for the wet bulb RTD is a wick that covers the entire length of the sensor element inside the sampler tube sub-assembly  60 . The other end of the wick is in the water cup  64  which is maintained full of water by a water reservoir system described in full below. 
     A draft-range (0-6 inches of water column differential pressure transmitter  67  provides information on the local static pressure inside the sampler tube sub-assembly  60  to provide a measurement for compensating for the atmospheric pressure utilized in the enthalpy calculations. The instrumentation is sealed in the sampler tube sub-assembly  60  by modified (enlarged center bore) laboratory beaker stoppers that interface with mated (same angle) conical holes through the wall of the sampling tube  62 . A barometric pressure transmitter  68 , is included as part of the sampling tunnel  50  instrumentation, providing an input for calculations. 
     A water reservoir is used to maintain the water cup  64  filled during testing. The water reservoir  80  is contained within the insulated enclosure housing the sampling tunnel  50 . The temperature of the water is maintained at virtually the same temperature as the conditioned air. This is important for good wet bulb temperature readings as a water temperature different from the air temperature would affect the wet bulb measurement. 
     Referring now to FIG. 8, the sample blower assembly may be seen, indicated generally at  70 . The sample blower assembly  70  comprises a blower motor and fan  71 , transitional/connective piping, and a motor operated discharge damper  72 . The blower motor and fan  71  provide a constant discharge flow rate from the sampling tunnel assembly  50 . The discharge damper  72  controls the velocity of the air through the sampling tunnel  50 . Both the blower motor and fan  71 , and the discharge damper  72  are controlled by a loop control system  100 , which will be described below. 
     The Air Speed and Mass Flow Rate Control System 
     Referring to FIG. 9, a schematic of the loop control system may be seen, indicated generally at  100 . The loop control system  100  is designed with a microprocessor-based, real-time data acquisition and control system. In the preferred embodiment, the data acquisition and control system is an Allen Bradley SLC 5/04 Programmable Logic Controller (PLC)  101 , although other PLC&#39;s may be used and the use of a particular controller is not critical to the invention. Programmable Logic Controllers are commercially available units well known in the art. PLC&#39;s can be programmed with closed-loop algorithms as in the preferred embodiment. They normally come equipped with instrumentation interfacing and provide for communication with Supervisory Control Systems which may be used to operate a test cell. The Supervisory Control System and the test cell are not aspects of the present invention, but are typical components used in enthalpy testing environments. In the preferred embodiment, the PLC is used to provide deterministic control of the actuators and drivers. The deterministic control algorithm used in the present invention is part of its inventive quality, and will be described in detail later in this specification. 
     Main Tunnel Air Mass Flow Rate Control 
     The sensors and actuators used on the enthalpy tunnel  10  provide input to and are controlled by the loop control system  100 . The mass flow rate of air through the main tunnel  40  is a parameter that must be closely controlled. The loop control system provides a positive deterministic control that continually monitors the mass flow rate using a single flow element, and adjusts the discharge rate of the main blower assembly  80  as necessary to maintain a constant mass flow rate. 
     Referring again to FIG. 9, the main blower assembly indicated generally at  80  comprises a variable frequency drive  91  which in turn drives a fan/motor unit, a discharge damper actuator  92  which controls a discharge damper, and a housing and support structure. The PLC  101  performs a real-time, approximate Standard CFM compensation used for closed-loop control of the mass flow rate through the main tunnel  40 , using inputs from the main flow sensor  30 , the calculated wet bulb and calculated dry bulb temperatures. A closed loop control program adjusts the frequency of the electrical supply to variable frequency drive  91  thereby controlling the speed and the discharge rate of the connected fan. Alternatively, the position of the discharge damper  92  may be varied, or a combination of controlling both the main blower  80  and the discharge damper can be used to maintain the desired mass flow rate of air through the main tunnel  40 . In the preferred embodiment, the discharge damper  92  is used when an air conditioning unit under test has its own discharge fan, and has a low output flow of air. 
     The mass flow rate of air is determined by measuring the humidity and temperature of the air, and its velocity through the main tunnel. Flow sensors  30  and  52  utilize an array of pitot sensors to generate a differential pressure signal proportional, via square-root extraction, to the velocity of air in the sensor. The sensor pressure ports are connected to flow transmitters which convert the pressure to a current loop (4-20 mA) signal proportional to the volumetric flow rate through the sensor. These current loop signals are wired to input modules on the PLC  101 . The temperature and humidity of the air volume are measured by the instrumentation in the sampling tunnel  50 , and input to the PLC  101 . The sampling tube  62  three different sensors. There are two different temperature readings and a differential pressure reading. The pressure differential sensor  67  is a draft-range (0-6 INH 2 O) pressure transmitter with the high pressure port open to atmosphere and the low pressure port connected to the clear sampler sensor section  60 . The pressure sensor  67  is equipped with a transmitter that generates a current loop (4-20 mA) signal proportional to the pressure and is also connected to an input module on the PLC  101 . This sensor provides a local correction, relative to the Barometric pressure, at the sensing location. There are also two 4 wire, 100 Ω Platinum Bulb RTD sensors. One reports the Dry Bulb (actual) temperature (item  65 ), and the tip of the probe is perforated to allow air to directly impinge the RTD element. The other reports the Wet Bulb temperature (item  66 ) and the tip is not perforated. Instead, it is covered by a sock. The other end of the sock is in a reservoir of water, the water cup  64 , described above. The cables (4 wires, each) are connected to temperature transmitters specifically calibrated to the sensor it is paired with. The transmitter generates a current loop (4-20 mA) signal proportional to the programmed temperature range of the transmitter. 
     Sampling Tunnel Air Velocity Control 
     The air velocity through the sampling tunnel assembly  50  is maintained at 1000 fpm +/−10 fpm by a closed loop control program in response to a signal generated by the sampling tunnel flow sensor  52 . The sampler flow sensor signal, generated by the sample flow element  52 , is divided by the area of the sensing section of the sampler tube assembly  50  to determine the velocity of the air stream. 
     Referring to FIGS. 7 and 10, the sampling tunnel  50  has a fixed-speed blower fan  71  with a control valve  72  functioning as a flow control damper as described above. In the sampling tunnel  50 , an actual flow rate ACFM of air as calculated by the controller  101  using the (differential pressure) indicated by the sampling tunnel flow sensor  52  is converted into a velocity. A closed loop control algorithm in the controller  101  compensates for changes in velocity, as indicated by the flow sensor  52 , by selectively opening or closing the control valve  72 . The velocity of air in the sampling tunnel  50  is maintained at a velocity of 1000 fpm (+/−10) for a valid standard wet bulb measurement. 
     The closed loop feedback control system  100  is an integral part of the overall enthalpy tunnel  10  design. By varying blower speeds and controlling back-pressure to achieve the desired flow rates, the controller  101  allows the enthalpy tunnel  10  to meet any flow conditions necessary to accurately and consistently determine the rating of air conditioning units that operate in the 500-20,000 SCFM range. The overall applicable range of the control system will cool flow rates from 500-20,000 SCFM, however, the preferred embodiment of the enthalpy tunnel  10  operates in the 2500-10,000 SCFM range. 
     Closed Loop Control Algorithm 
     As mentioned above, the PLC may be programmed by the user to perform a variety of calculations and provide output control signals. It is an aspect of the present invention that the enthalpy tunnel  10  is equipped with a loop control system and that the system incorporates a unique control algorithm. Internally, there are two major calculations performed by the loop control program in real time. Referring to FIG. 10, a flow diagram of the loop control system algorithm may be seen. In general, the main tunnel flow rate as measured in ACFM is modified to a standard flow rate (SCFM) which is equivalent to a mass flow rate, and the main blower assembly  80  is controlled to maintain a mass flow rate. 
     The PLC  101  receives input from the main flow sensor  30 , the sample tunnel flow sensor  52 , the dry bulb RTD  65 , the wet bulb RTD  66 , the differential pressure transmitter  67 , and the barometric pressure transmitter  68 . These are continuous analog inputs to the PLC  101 , and are represented on FIG. 10 as the PLC input Sequence  201 . The PLC then computes an absolute wet bulb temperature  202 , an absolute dry bulb temperature  203 , the sampling tunnel static pressure  204 , the main flow rate  205 , and the air stream velocity in the sampling tunnel  206 . The flow rate (ACFM) is multiplied by the cross-sectional area of the sampler to generate the air velocity at the sensor section. 
     The status of the Supervisory Operating System  208  is then determined. As discussed above, the Supervisory Operating System is external to the enthalpy tunnel  10  and the merely provides signals to start and stop a test run. If the Supervisory Operating System is sending a stop signal, the PLC will send a signal to its outputs indicated at  209 , to shut down or not start the enthalpy tunnel equipment. Communication to the enthalpy tunnel equipment is indicated as PLC Background Comm Processing, block  210 . If the Supervisory Operating System is operating, then the computed values are sent to the output portion of the algorithm. 
     There are two closed-loop control schemes in the output algorithm. Each uses the internal PID (Proportional/Integral/Derivative) instruction that is a part of the Allen Bradley software for the SLC 5/04 used in the preferred embodiment. PID controllers are well known in the art. 
     The first output algorithm loop  212  compares the Main Tunnel Standard Flow Rate (SCFM) computed at  205 , to a setpoint downloaded from the supervisory operating system. As the flow deviates from the setpoint, the speed of the main blower assembly  80  is adjusted to bring it back to the setpoint. The second output algorithm loop  213  compare the calculated sampler velocity  206  to a fixed setpoint. In the preferred embodiment, this is 1000 ft/min. As the calculated and velocity deviates from 1000, it will open or close the control valve  72 , accordingly, to bring the velocity back to 1000. Both signals are then transferred to output registers  214 , integral to the PLC  101 , and then to the PLC output  209 .