TOTAL DE HORAS A LA SEMANA: H/T: 2

The instrumentation used in the experimental facility was selected and applied for the measuring of all the variables necessary for determining the performance of the heat exchangers. The quantities measured, in both water and refrigeration system tests, basically included temperatures, pressures, and flow rates. There are 14 sensors that were employed in the study, of those 7 were used for the water test, 9 for the refrigeration test, some of which were used for both tests. All instruments have analogue outputs which are logged onto a PC via the data acquisition system. A description of the measuring instruments is presented in this section, organized separately for the water test system and the refrigeration test system. For detailed technical data and wire connections, Appendix B refers.

3.5.1 Water Test System

Temperature T

Temperatures are measured at four points, i.e., at inlets and outlets of both hot and cold water streams, approximated 200 mm from the PHE ports. Measurements were taken by sensors located at pertinent points with LM35DZ semi-conductor sensors, which are integrated circuit sensors that give electrical

voltage output proportional to the temperature in Celsius. The sensors are each powered by a 9V battery and give a 0-1 V signal corresponding to 0-100 °C temperature spectrum, via a three wire system.

The LM35DZ sensor is a tiny semi-conductor with a plastic tip. To use this sensor for the measurement of water temperature, a probe needed to be made which encloses the sensor in a copper sheath and thus can be inserted into the water pipe. The temperature sensor probe is shown schematically in Figure 3.7.

Volumetric flow rate Q

Volumetric flow rates of water on both sides are measured with two RS 257-026 turbine flowmeters. These flow meters have a rotor which spins when liquid passes through the flowmeter. The speed of the rotor is directly proportional to the flow velocity. As the rotor spins, the stainless steel blade tips pass a magnetic field and an AC voltage is induced in a coil, which is converted into an output 4- 20 mA current signal proportional to the flow. The two flowmeters have the capacity of 0-100 l/min and were, for the present experimentation, calibrated in the range of 0-60 l/min.

Many piping configurations and fittings generate disturbances with unknown characteristics, for this reason, flow conditioning, i.e., minimum length of straight pipe runs before and after the meter, is required for certain types of flowmeters. This information is not available from the manufacturer’s data sheet, therefore as a general rule, a minimum length of 20 diameters of straight run piping (Feener, 1999) was applied.

Differential Pressure ∆∆∆∆P

Differential pressure is the pressure drop between the PHE inlet and outlet, on the cold water side in the test. The pressure drop is measured by a Rosemount 3051 CD differential pressure transmitter. The transmitter has a working range of 0-80 kPa and was calibrated in the same range using water. Figure 3.8 shows the layout of the measurement arrangement.

It is important to note that the measured pressure drop has many components, and is not the frictional pressure drop ∆P_corein the corrugated channel, which is of the interest of the test. Evaluations of other components and an extraction procedure are required to obtain ∆P_core, as will be given in Chapter 4.

3.5.2 Refrigerant Evaporator Test System

Temperature T

Temperatures are measured at four points: inlets and outlets of both refrigerant and water. Four resistance temperature detectors (RTD’s) are used for the measurements. All the RTD sensors use a VDC 24V power supply and give standard 4-20 mA current signals. On the water pipe line, two RTD’s are

mounted using pressure couplings. On the refrigerant pipe line, the two sensors are mounted in well insulated T-connections where sealed copper pipes are brazed into a Tee and the sensors are inserted into the pipes. All sensors were extended approximately to the pipe center lines. Schematics of the mounting arrangement are shown in Figures 3.9 and 3.10.

Refrigerant Absolute Pressure P

Static pressures at the refrigerant inlet and outlet are measured by two WIKA pressure transmitters. The two sensors are mounted in well insulated T- connections on the pipe line. Also on the compressor suction line a pressure gauge is fitted on the back pressure regulator, for monitoring of the surge drum pressure. Both the WIKA sensors and the pressure gauge give readings of the “gauge pressure”, which is the absolute pressure less the local atmosphere pressure. The WIKA sensors use a VDC 24V power supply and give 4-20 mA current signals.

Refrigerant Differential Pressure P∆

The pressure drop through the evaporator is measured with a differential pressure transmitter 3051 CD, from Rosemount Inc. Careful consideration is needed for this measurement. The inlet refrigerant is liquid and the outlet refrigerant is a liquid and vapour mixture. The transmitter can not simply be installed with vertical connection between the inlet and outlet pipe lines because the connection could be partially filled with liquid and vapour. The unknown liquid column in the pressure connection can exert static pressure and thus has an effect on the measured result. For heavy refrigerants such as R134a this could be of the order

Figure 3.9: Water line RTD mounting

Figure 3.10: Refrigerant line RTD mounting

of the measurement itself. To solve this problem, all sensor connection tubes must be horizontally oriented, and the evaporator outlet port and the sensor low- side connection must be at the same height. The arrangement in the current facility is shown schematically in Figure 3.11. This arrangement eliminates the influence of refrigerant fluids in the connecting tubes on the pressure measurement.

Water Volumetric Flowrate Q_w

Volumetric flow rate of water on the process liquid sides is measured with one RS 257-026 turbine flowmeter, which is one of the two flowmeters which were used for the water test. The flowmeter was positioned at the water pipe line exit. The same flow conditioning was applied for this flowmeter, namely a minimum length of 20 diameters of straight run piping provided on both sides.

Refrigerant Volumetric Flow rate Qr

The flow rate of the liquid refrigerant is measured with a Trimec MP 15S multipulse flowmeter, from Trimec Industries (Australia). The flow meter utilizes the oscillating-piston principle for measuring the flow rate. The passage of liquid causes a piston to oscillate smoothly in a circular motion within the round measuring chamber. Each piston cycle displaces a known volume of liquid

from the inlet to the outlet port, while small high energy magnets in the piston activate a reed switch and a solid state sensor (Hall effect sensor), which provides pulse outputs for remote flow monitoring. The flowmeter has a flow rate range of 0.2-10 l/min, the calibration was done in the range of 0-8.5 l/min for the current study.

This flowmeter is insensitive to viscosity changes and does not require flow conditioning. The manufacturer’s instruction states that the flowmeter does not create pulses in the flow, however, pulsation of the water stream was observed during the calibration (refer to Appendix C), more apparent at relatively smaller flow rates. The device uses a 5-24V VDC power supply and gives a variable frequency signal. The frequency signal is converted to a current signal via a converter (Model 1100P, IQ Instruments cc, South Africa), and then sent to the data acquisition system.