- 237 - Chapter – 18 Refrigerant Piping Systems Normally, the refrigerant flows in the refrigerant piping system are single phase flow of either vapor or liquid. It is a requirement to design the refrigerant piping system for single phase flow, unless it is purposely designed for two-phase flow such as the return flow of a liquid recirculation system. Refrigerant Piping for Single Phase Flow of Gas or Liquid: Pressure drops in refrigerant piping system must be optimized, particularly the pressure drop in suction line of the refrigeration system; it must be carefully exam and considered, because it impacts greatly the selection of the compressor which adversely reducing the capacity of the compressor and also increases the power consumption for the system, especially for low temperature application. Pressure drop allowance for the suction line is the balance of economic considerations of the size of suction line, power consumption, compressor size, suction line insulation and installation costs. As a general guide, suction line size should generally be selected for a pressure drop of 1 to 3 psi per 100 feet of pipe for evaporative temperature above 20˚F; 0.2 to 2 psi per 100 feet of pipe for evaporative temperature between 20˚F to -60˚F. Increase discharge line pressure drop would increase the compression ratio for the compressor and reduces the volumetric efficiency of the compressor. This reduces capacity of the compressor, also increase power consumption of the system. Discharge line is generally sized to have 2 to 4 psi per 100 feet of pipe run. Pressure drop can be critical as well in liquid lines since it is the most common cause of generating flash gas; flashing of liquid in the liquid line might cause loss of system capacity. Piping line from condenser to receiver is generally sized to have maximum liquid velocity of 300 fpm for a free draining installation and a liquid velocity not to exceed 150 fpm for trapped liquid line. For refrigerant piping from receiver to evaporator or intercooler, the maximum liquid velocity should be limited to 300 fpm and the line pressure drop is suggested to be less than 2 psi per 100 feet pipe run. When the liquid flow is against a riser column or when the pressure drop may generate flash gas, liquid subcooling is suggested to eliminate the flashing problem. Liquid pipe line should be insulated if subcooling method is used. The liquid line between the expansion or throttling device and the evaporator should be short; common practice is to use same size pipe as the expansion outlet or one size larger than normal liquid line. Two-phase flow calculation should be considered if the throttling device and the evaporator are not close, because this line is carrying both
Transcript
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Chapter – 18 Refrigerant Piping Systems Normally, the refrigerant flows in the refrigerant piping system are single phase flow of either vapor or liquid. It is a requirement to design the refrigerant piping system for single phase flow, unless it is purposely designed for two-phase flow such as the return flow of a liquid recirculation system. Refrigerant Piping for Single Phase Flow of Gas or Liquid: Pressure drops in refrigerant piping system must be optimized, particularly the pressure drop in suction line of the refrigeration system; it must be carefully exam and considered, because it impacts greatly the selection of the compressor which adversely reducing the capacity of the compressor and also increases the power consumption for the system, especially for low temperature application. Pressure drop allowance for the suction line is the balance of economic considerations of the size of suction line, power consumption, compressor size, suction line insulation and installation costs. As a general guide, suction line size should generally be selected for a pressure drop of 1 to 3 psi per 100 feet of pipe for evaporative temperature above 20˚F; 0.2 to 2 psi per 100 feet of pipe for evaporative temperature between 20˚F to -60˚F. Increase discharge line pressure drop would increase the compression ratio for the compressor and reduces the volumetric efficiency of the compressor. This reduces capacity of the compressor, also increase power consumption of the system. Discharge line is generally sized to have 2 to 4 psi per 100 feet of pipe run. Pressure drop can be critical as well in liquid lines since it is the most common cause of generating flash gas; flashing of liquid in the liquid line might cause loss of system capacity. Piping line from condenser to receiver is generally sized to have maximum liquid velocity of 300 fpm for a free draining installation and a liquid velocity not to exceed 150 fpm for trapped liquid line. For refrigerant piping from receiver to evaporator or intercooler, the maximum liquid velocity should be limited to 300 fpm and the line pressure drop is suggested to be less than 2 psi per 100 feet pipe run. When the liquid flow is against a riser column or when the pressure drop may generate flash gas, liquid subcooling is suggested to eliminate the flashing problem. Liquid pipe line should be insulated if subcooling method is used. The liquid line between the expansion or throttling device and the evaporator should be short; common practice is to use same size pipe as the expansion outlet or one size larger than normal liquid line. Two-phase flow calculation should be considered if the throttling device and the evaporator are not close, because this line is carrying both
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liquid and gas. The minimum size of liquid or gas connection to auxiliary devices such as liquid float valve, surge drum, etc. should not be smaller than the connection of the float valve. Table 18-1 lists the dimensions and the physical data for the copper or seamless steel tubing; Table 18-2 lists same information for the carbon steel piping. These tables also provide the tube or the pipe volume information for the estimation of refrigerant charge for the piping system. Table 18-3 is for the quick selection of suction line sizes at various suction temperatures, pressure drops and at 105˚F condensing temperature for R-22. Interpolation is allowed to determine the line capacity, however, the interpolation should be based between saturated suction temperatures at a fixed pressure drop. No interpolation is allowed between pressure drop columns. For other condensing temperature, use refrigerant flow rate and the pressure drop charts for the estimate. Use P-H diagram method to calculate the refrigerant flow if more accurate flow rate is desirable. Table 18-4 is the quick selection chart to determine the capacities of discharge and liquid lines at fixed pressure drop or velocity as shown in the table for R-22. This chart is based on 105˚F CT and 40˚F ET. Use refrigerant flow rate and the pressure drop charts for CT and ET temperatures other than 105˚F and 40˚F. Table 18-5 is the approximate K-Factors for valves and fittings for halocarbon refrigeration application. Table 18-6 lists the equivalent lengths for the valves and fittings for halocarbon refrigerant. Figure 18-1 is the quick reference chart for the approximate flow rate per TR for various ET and CT for R-22; the flow rate is Lbs. of R-22 per minute per TR. Again, use P-H diagram method to calculate the refrigerant flow if more accurate flow rate is desirable. Figure 18-2 is the chart to determine the psi pressure drop per 100 ft. for vapor flow of R-22 for carbon steel piping. (Figure 18-5 is for copper tubing). Figure 18-3 is to determine the velocity for the R-22 vapor flow for steel piping. (Figure 18-6 is for copper tubing). Figure 18-4 is to determine the velocity of liquid flow of R-22 for steel piping. (Figure 18-7 is for copper tubing). Figure 18-8 is the chart to convert the vertical R-22 liquid column to pressure drop between receiver to evaporator. Table 18-7 shows the maximum TR capacities for various pipe sizes under various conditions for R-717.
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Table 18-8 is the equivalent pipe lengths for the valves and fittings for R-717. Table 18-9 is the equivalent pipe lengths for welded ells and return bends for R-717 piping. Table 18-10 is the maximum refrigerant flow capacities in Lbs. per Minute for piping from condenser to receiver for R-717. Figure 18-9 is the quick reference chart for the approximate flow rate per TR for various ET and CT for R-717; the flow rate is Lbs. per minute per TR. Use P-H diagram method to calculate the refrigerant flow if more accurate flow rate is desirable. Figure 18-10 is the chart to determine the psi pressure drop per 100 ft. for vapor flow of R-717 refrigerant; this chart is for carbon steel pipe and it is for discharge gas line and for suction piping for temperature between 40˚F to -40˚F. Figure 18-11 is the same as Figure 18-10 except it is for suction piping for R-717 and for temperature between -40˚F to -100˚F. Figure 18-12 is to determine the pressure drop psi per 100 ft. for R-717 liquid flow. Figure 18-13 is the chart to estimate the vertical R-717 liquid static column to pressure drop between receiver and evaporator. General Guides for Pipe Size Selection and Estimation: 1.0 Design and develop the refrigerant flow diagram for the refrigeration system. 2.0 Sketch a proper P-H diagram for the refrigeration system. 3.0 Show all the design operating conditions on the P-H diagram including all the
pressures, temperatures, enthalpy points, CT, ET, TR, intermediate temperature if any.
4.0 Indicate all the design pressure drops and superheated allowance. 5.0 Calculate the refrigerant flows for each section. 6.0 Estimate the piping run between each component and each piping section;
estimate the number of elbows, tees, valves, etc. convert all the valves and fittings into equivalent length to obtain the total length of the pipe run for each section.
7.0 Select and adjust the pipe size to match the pressure drop allowed. 8.0 Pressure drop calculated must be less than the pressure drop allowed in the
design, particularly the suction piping pressure drop. If it is higher than the designed, either to change the pipe to a larger size pipe to reduce the pressure drop or changing the compressor selection to accept the larger pressure drop.
9.0 Check vertical static column to see if liquid subcooling is needed for the system. 10.0 Check to avoid liquid traps in the piping line to protect the compressor. Refrigerant Piping System for Two-Phase Gas & Liquid Flow: In refrigerant piping system, only three areas involve with two-phase flow.
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(A) One is inside the DX evaporator; the DX evaporator is designed for this
application and it is not part of the piping system. (B) The second area is the piping between the outlet of the expansion device and the
inlet of the evaporator, this section of the piping is very close; the usual rule of thumb is to use the same size of the outlet connection of the expansion device.
(C) The third area is the return piping of a liquid recirculation system.
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The pressure drop is considerably higher than a single phase vapor flow if the flow is a two-phase flow of gas and liquid combination. Figure 18-14 is the pressure drop multiplier factor for the two-phase flow for R-22 based on vapor pressure drop Lbs/Min flow of same pipe size. Figure 18-15 is the pressure drop multiplier factor for the two-phase flow for R-717 based on vapor pressure drop Lbs/Min flow of same pipe size. Both Figure 18-14 and Figure 18-15 are the simplified version of the pressure drop multiplier curves. It is suggested to use industrial acceptable software if more accurate estimate is required.
