Document wgmnrrwYEn12bMO8QEvQ4518D

it-- 5 i \ i i 734 CHAPTER 45 quentiy suitable for defrost relief from'evaporator to suction pressure, as large capacity relief from a pressure vessel to the low side,- or as a liquid -refrigerant pump relief from pump discharge to the accumulator-to prevent excessive pump pres sures when some evaporators are valved closed. PART III: CAPILLARY TUBES Allrefrigerating units require a pressure-reducing device' to meter the Sow of refrigerant to the low side in'accordance with the demands placed on the system: With the advent of the hermetic'compressor and the halocarbon refrigerants,the capil lary tube became practicable and rapidly achieved popularity, especially with the grnaller unitary hermetic equipment, such 'as household refrigerators and freezers,. dehumidifiera, and ronm Air conditioners More recently, it bus been extended to larger units 6uch as unitary air conditioners in sizes lip to 10 tons capacity. - The capillary operates on the principle that liquid passes through it much more readily than gas. It consists of a small diameter line, which, when used for the purpose of controlling the flow, of refrigerant in a system, connects the outlet of the rnnripnqpr to the inlet of the evaporator. It is sometimes soldered to the outer surface of the suction line for heat ex change-purposes. > A high-side liquid receiver is not normally used withla capillary, and a corresponding reduction in refrigerant charge could-result. In a few instances, such as with household-re frigerators and freezers, it is customary to employ a.small low. side accumulator. The pressure equalizing characteristic of a capillary makes possible the use of a low starting torque motor compressor.- Inherently, a capillary,does not operate as efficiently over a wide range of conditions as does a thermo static expansion valve, but due to counterbalancing factors in most applications, its performance is generally very good. The simplicity of the capillary gives it the advantage of greatly reduced cost.- ' Theory Aa has been indicated, it is the characteristic of the capil lary to pass liquid much more readily than gas that makes it a practical metering device. Let it be assumed that a con- Rg. 32 .....Pressure end Temperature Distribution Along Typical Capillary Tube - - 1965 Guide And Data .Book* denser-to-evaporator capillary has been sized to permit the desired flow of refrigerant with a liquid'seal at its inlet If a system unbalance occurs so that some gas (uncondensed refrigerant) enters the capillary,- this gas will tend to consider ably reduce the mass flow of refrigerant with little, or no change in the system pressures. If the opposite type of un= balance occurs, liquid refrigerant will back up in the conden ser. This condition will tend to cause subcooling and inrrPA^ the mass flow of refrigerant. If properly razed for the applica tion, tiie capillary thus tends to automatically compensate for load and system variations and will give acceptable - per formance oyer a wide range of operating conditions. A common flow condition involves subcooled liquid at the entrance to the capillary. Bolstad1 demonstrates the flow be havior from temperature and pressure measurements along the tube: - .. "With subcooled liquid altering the capillary tube, the pressure distribution along the tube is similar to that shown in Fig. 32. At the entrance to the tube, section 0-1, there is slight pressure drop which was usually unreadable on the gages* From point 1 to point 2. the pressure drop is linear. In the portion of the tube 0-1-2, the refrigerant is entirely in the liquid state, and at point 2, the first bubble of vapor forma. From point 2 to the end of the tube, the pressure drop is not linear, the pressure dtop per unit length increasing aa the end of the tube ia approached. For this portion of the time, both the saturated Liquid and saturated vapor phases are present, the percent and volume of vapor increasing in the direction of flow. In most of the runs there was observed a.significant pressure drop from tire end of the tube into the evaporator apace. With a saturation temperature corresponding to the pres sure scale superimposed along the vertical axis, it is: possible, to plot the observed temperatures in a more meaningful way than if a uniform temperature scale were used. The temperature is con stant for the first portion of the tube 0-1-2. At point 2t the pres sure has dropped to the saturation pressure corresponding to this temperature. Further pressure drop beyond .point 2 is accom panied by a corresponding drop in temperature, the temperature bong tiie saturation temperature corresponding to the pressure. As a consequence, the pressure and temperature tnM coincide from point 2.to the end of the tube." The rate of. refrigerant flow through a capillary always increases with an increase in inlet pressure. It also increases with a decrease in external outlet pressure'down to a cer tain critical value, below which the'flow does not change. Fig. 32 illustrates a case where the outlet-pressure inside the capillary has reached the critical value, which happens to be higher than the external pressure.'Point 2 in'Fig. 32, where the first gas bubble appears, is called the bubble point. The preceding portion of capillary is called the liquid length; and that following is called the two-phase length. '. System Design Factors The high side must be carefully designad for use with a capillary. If liquid backs up in the condenser, enough re frigerant may be removed from the evaporator to cause an undercharged condition. This can be improved by decreasing the high-side volume so that less liquid refrigerant will be required to increase the discharge pressure to achieve bal anced conditions. The total volume of the high side usually represents a compromise, since it' may be necessary to also provide sufficient refrigerant storage volume to protect against excessive discharge pressures during high load con ditions. The high-side volume should be sufficient to contain the entire refrigerant charge to prevent hydrostatic rupture failure in case of capillary stoppage. Another consideration in high-side design, where cyclic operation is involved, is that of unloading during the off period. When unit operation ceases, the capillary continues to Refrigerant Control Devices 735 pfoa refrigerant from the high ride to the low ride until press ures are equalized. Care must be taken to provide good drain-' age of the liquid into the capillary during this unloading interval. If liquid is trapped in the high side, it will evaporate there during tire off cycle. It will then pass to the low ride as a warm gas, where it condenses and adds latent heat to the evaporator, liquid trapping may also increase the time re^ quired for the pressures to equalize after the compressor stops operating. If this interval is too long, the compressor may not be sufficiently unloaded at cut-in to permit easy starting. The important design parameters in low-side design are total refrigerant charge and charge tolerance: For many reasons the total refrigerant charge should be held to a minimum consistent with satisfactory performance. However, a high tolerance may be necessary to minimize the effect of a varying distribution of refrigerant in the system during operation. A good low-ride design will achieve both objectives. The amount of refrigerant in the evaporator is at its maxi mum during the off cycle and at ite minimum value during the running cycle. The suction piping should be arranged to reduce to a minimum the adverse effects of the variable charge distribution. A suitable liquid accumulator is sometimes necessary. When a suction line heat exchanger is used, the excess capil lary may be coiled and located at either end of the exchanger. Although more efficient heat exchange may be obtained if the excess capillary, is coiled at the evaporator, experience has shown that system stability is enhanced if a portion of the capillary is located at the condenser. Care should, of course, be exercised in forming the bends and coils to avoid local restrictions. In general, one should not locate all of the capillary at the evaporator and precede it with a conventional large bore liquid fine in heat-exchange with the suction line. With this particular arrangement, an overcharge into the suction line-' places the low ride and high side in heat exchange relationship in the heat exchanger and will result in serious instability. The increased cooling of the refrigerant entering the capillary, due to the liquid in the suction line, increases the capillary ca pacity and reduces the liquid stored in the condenser. Capacity Balance Characteristic b Too oucb eaptBarr ren'rionc*--ttquid lufirfgeraaf bodi ep in eondwrMr and comes evaporator to bo undercharged. Compressor ditdtargo pressure may be aboorpuffy high. Suction pressure betow normal. Bottom of condenser cubcoafed. Fig. 33 ...: Effect of Capillary Tube Selection . on Refrigerant Distribution*-1* . _ The desired suction pressure may be obtained by regulating the heat input to the low side, usually by means of electric heaters; the desired discharge pressure may be obtained by means of a suitable controlled water-cooled condenser. A liquid indicator is located at the entrance to the capillary'. The usual test procedure is to hold the high-ride pressure constant and, with gas bubbling through the right glass, slowly increase the suction pressure until a liquid seal forms atthe capillary entrance. Repeating this procedure at various discharge pres sures.will readily determine the capacity balance characteris tic curve similar to that shown in Fig. 34. This equipment may The selection of a capillary tube will depend on the appli cation and the anticipated range of operating conditions. One approach to the problem involves the concept of capacity balance. A refrigerating system may be said to be operating at the condition of capacity balance when the resistance of the capillary is sufficient to maintain a liquid seal at its en trance without excess liquid accumulating in the High ride of the .system. (See Fig. 33.) Only one such capacity bidance point exists for any given compressor discharge pressure. A curve through the capacity balance points for a range of com pressor discharge pressures is called the capacity balance characteristic of the system. Such a curve is shown in.Flg. 34. Ambient temperatures are drawn on the chart for a typical air-cooled system. A given set of compressor discharge suction pressures are associated with fixed condenser and evaporator pressure, .drops; therefore they establish the capillary inlet and outlet pressures. The .capacity, balance characteristic curve for any combi nation of a compressor, and capillary may be determined experimentally .by means of the arrangement shown in Fig. :* possible to independently vary the suction and discharge pressures until capacity balance is obtained. Capillary System*