The main components in this primary loop are the pump, electrical heater, test section, condenser and pressure control vessel.
The working fluid is circulated with a pump through the electric pre-heater, the test section and the condenser. As depicted in Figure 4-1, the refrigerant flows through a filter/dryer (Carly DCY164) before it enters a closed gear pump (Micropump type GN-N23). The pump circuit also features a bypass and is coupled to a 3 phase variable speed motor to control the mass flux. The flow rate is then measured by a Titan Instruments 945 turbine flowmeter that can measure flows ranging from 0.2 to 4.5 L/min with an accuracy within ±3%.
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The flow enters the pre-heater which is a 1.27 cm outside diameter electrically heated steel tube of overall length of 2.95 m that is bent into a serpentine configuration. The somewhat long section ensures that sufficient heat can be transferred to the working fluid at low to moderate heat fluxes, well below the critical heat flux. The electrical current flows in the tube wall, and the tube resistance facilitates Joule heating of the tube which is transferred to the fluid. The entire preheater is wrapped in sufficient insulation that heat losses to the ambient are negligible. The pre-heater is connected to the main flow loop with dielectric fittings see Figure 4-2 (SWAGELOK SS-8-DE-6). The range of power required and the power supply specification are listed in Appendix A. An Agilent Technologies System power supply (N5742A DC) was selected and connected to the tube ends and used to control the outlet thermodynamic state of the working fluid which is then routed to the test section.
Figure 4-2. New pre heater design
It is important to keep the thermodynamic quality of the working fluid within the preheater low enough that the risk of local dryout and “hot spots” forming on the steel tube surface is mitigated. The constant heat flux of the electric power coupled with a vapour barrier may cause high temperatures to occur on the heater surface and cause degradation of the fluid, as the maximum heat flux of 18 W/cm2 is specified by 3M [54] for HFC 7000. For the experiments considered here, the fluid exit was slightly subcooled such that the fluid was in a liquid state throughout the test section.
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With the fluid in a liquid state, the quantity of heat transferred from the pre heater to the working fluid can be calculated from the sensible energy change across the heater section, using the measured flow rate and working fluid inlet and outlet temperature difference, as logged by two calibrated type T thermocouples. The heat gained is given by the expression:
(62)
An electrical connection is provided to measure the voltage and supplied to the electrical pre heater in order to perform an energy balance. It can also be used to estimate the exit enthalpy, and thus quality, for cases when the exit is a two phase flow. The electrical heat supplied is simply calculated as;
(63)
A 0-10 bar electronic pressure transmitter (Bourdon Sedeme type E7-13) measures the fluid system pressure (Pin) at the exit of pre heater. For single phase flow, the pressure and associated temperature measurement at the exit is sufficient to determine the fluid thermodynamic state. If two phase flow is desired at the exit, the thermodynamic state can be determined with knowledge of the heat supplied to the working fluid.
As shown in Figure 4-5, the working fluid is routed from the pre-heater to the test section where it is forced upward through a 400 mm long developing length of 8 mm ID glass tubing before entering the main heated test section. Glass was used in order to visualize the inlet flow regime. For the cases studied here where the flow is slightly subcooled, the transparent developing length was used to ensure no bubbles were entering the test section. During testing, the section was wrapped in insulation to reduce heat losses to the ambient.
On exiting the test section, which will be discussed later, the working fluid continues flowing upward through another 400 mm length of glass tube, identical to the lower developing length section. At the top of the test section the refrigerant exits the glass tube through a machined fitting with an exit port machined with a 135o angle to
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reduce the elbow pressure effect that would occur with an abrupt right angle fitting see Figure 4-3.
As there is an elevation change from the test section inlet to the test section level, another pressure transmitter (Omega PX4201) was installed to record the pressure (Pexit) exiting the test section. This pressure reading allows a correction of the quality to be re-calculated as the two-phase fluid exits the test-section.
The working fluid is condensed in a compact plate heat exchanger supplied with cold water from a temperature controlled chiller unit (Thermo Scientific Accel 250 LC). All the piping and equipment are contained within a framework constructed of modular aluminium profile bar (Bosch Rexroth) which allows for flexibility and modification. Most of the joints in the primary and cooling loops are Swagelok fittings except for some threaded joints which were sealed with epoxy adhesive. The main sapphire tube test section pressure sealing were fabricated in-house and tested satisfactorily to 2 bar, where the normal rig operating pressures range between 1.2-1.6 bar.
For the purpose of redesigning the system, the rig is required to be drained often. A drainage tap at the bottom of the rig is thus used for this purpose, see Figure 4-1. The refrigerant is collected in a clean bottle for reuse. Fresh refrigerant was used for the final tests.
To fill the rig with refrigerant, the rig is first vacuumed up to ~ -100 kPa through the refrigerant reservoir to remove air from the system. A flexible pipe, connected to the drain is immersed in refrigerant bottle, the valve opened slowly and the refrigerant allowed to flow into the rig up to the test section.
Preliminary tests found that air was still inside the system, and the following degassing procedures was carried out; the reservoir is connected to a tank was charged with a vacuum; the rig is operated and the vent valve slowly opened and the gas is collected at the reservoir vessel; the vacuum tank valve is slowly opened to allow the collected gas to carry over to the vacuum tank; the valve is closed and the
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system run again allowing gas to reach the top of the system; the gas is vented off to the vacuum tank. It takes several charges of vacuum and recovery to get the rig to an acceptable working state. Once achieved, one charge every few days is sufficient to keep the rig gas free.