Manufacturers use capillary tubes for upper cooling by changing the diameter and length of the capillary tube. But I don't know the good diameter and length of the capillary tube for an air conditioner with 500w cooling capacity!
Generally the diameter of the capillary tube varies from 0.6 mm-2.2mm, while length of the capilary tube is around 0.3-4 m. The size of the capilary tube depedns on type of rhe refrgerant used, suction pressure, delivery pressure, power required by the compressor, and capacity of compressor.
The data is being assumed
A R-12 system operates between 1.83 bar and 9.65 bar. The enthalpy at the begining of compression (h1) is 181.94 k J/kg, while at the end of compression (h2) is 209.3 k J/kg,Total enthalpy of refrigerant before evaporator= 74.77 kJ/kg; Obtain the length of capillary tube of inner diameter 1.8 mm for 0.5kW compressor.
Compressor work per unit area = w= h2-h1= 209.3-181.94= 27.4 kJ/kg;
Compressor capacity for 0.5kW= mqcx3600=(0.5/27.4) 181.94-74.77)3600=
7040 kJ/kg
From log graph, (variation of compressor capacity with the length of capillary tube for different refrigerants), length of the capillary tube can approximately be estimaed by knowing the diamter of the capillary tube. After conducting experiment, exact size of the capillary tube can be determined.
Ref:
Refrigeration and Air Conditioning By Manohar Prasad; New Age International Pub. Limited, New Delhi, 2010
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Good morning Mr. Majidreza Salehi,
there is not "the answer" to the question you posed. The length and the diameter of the capillary tube affect the mass flow rate through it. The mass flow rate depends not only on geometry but also on the thermodynamic prop of the refrigerant and the operating conditions: pressure and subcooling at the capillary tube inlet (pressure at the evaporator is important if the flow inside the capillary tube is not choked).
Once you design a heat pump or a refrigerator with a capillary tube, the inlet/outlet conditions at each component (compressor, condenser, evaporator and capillary tube) will depend on the sizing of each component itself and on the actual buondary conditions (temperature, flow rates of the fluids at the evaporator and condenser). So the balancing of the system will affect the inlet conditions to each components and the real conditions could be far from the nominal ones.
For this reason, usually, the procedure used inside the R&D labs for thermodynamic design, have some preliminary calculations of the diameter/length based on correlations available in literature at the nominal conditions of the system. Then simulations and tests in further conditions are required to check that, in the whole range of application of the system, the refrigerant at the exit of the compressor is feed by superheated refrigerant to prevent risks for its durability. Also the refrigerant charge has to be appropriately choosen depending on the refrigerant, the operating conditions and the volume of the system, otherwhise the capillary tube will not be correctly feed by a liquid at the inlet, with unstable working conditions for the system.
Taking into account for all these considerations, and also :
-for the fact that the equations for the sizing of the capillary tube have a poor accuracy ;
-for the fact that the diameter of such tubes is small (less than 1.8 mm usually) and of a size not so reliable,
the labs use a try-and-error approach following some procedures reccommended by standard. The ASHRAE handbook (part Refrigeration) reports such procedure.
The use of appropriate softwares could help to reduce the experiments in the labs.
Please account that based on researches from our labs the couple diameter/lenght of capillary tube, with the refrigerant charge and the size of the compressor will affect notably the performance of a heat pump or a refrigerator.
Practically, the capillary tube selection is complex process involving trials with internal diameter (ID) sizes in market. The size of ID and length of the old or previously used one can give a good starting. Based on it, one can easily play with dimensions and coil shapes and get a better element. One can design the capillary tube in theory despite the fact that numerous factors are involved (such as refrigerant properties, temperature at locations, required pressure drop, COP etc.). Smaller diameter and shorter length is better for system performance.
A new e-Tool, CapS, is available at website www.Refriglab.com for sizing CAP tubes with quite flexible options of contacting with suction pipe (separate/winding/tube-in-tube), employing single/twin/more, copper/stainless steel/plastic tubes, and 39 common and new refrigerants (R134a/R290/R404a/R410a/HFOs...).
With CapS/e-Tool at www.refriglab.com, CAP tube contacting factor=0 for detached/separate, =1 for common winding on, =2 for tube-in-tube arrangement with suction line pipe. Contacting Factor varies from 0 to 1.5 for winding on suction line to stand for different contacting degree.
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