Enthalpy tunnel

Details Enthalpy tunnel Enthalpy tunnel

2000-06-06

2003-04-08

Picard, Leo (Department: 2125)

Thermal measuring and testing

Heat flux measurement

C374S010000, C374S030000, C700S276000, C702S182000

Reexamination Certificate

active

06543932

ABSTRACT:

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 tunn

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