CHOOSING A SIZE PIPES
The possibility to determine the differential pressure in the refrigerant lines may be crucial, but is the solution, how much pressure drop (or fall in temperature of saturation)to indicate. While the optimization process discussed in C. 9.5 would be ideal, as a rule, designers have resorted to some conventions, at least give a reasonable size pipes. Various sections of the pipeline decide individually:
- Suction in the compressor. The General fall in the saturation temperature is generally chosen to be 0.5 to 2C (from 0.9 to 3.6F). Exception of the vertical risers as for halocarbons direct expansion and liquid ammonia overfeed coils. For Halocarbon direct expansion systems speed of the refrigerant vapor must be high enough to pass the oil back to the compressor. Liquid ammonia-overfeed steam coils speed riser must be high enough to blow liquid so that it cannot fill the riser.
- Extract from the compressor, the condenser. The General fall in the saturation temperature is usually chosen from 1.0 to 3.0C (1.8 to 5.4F).
This decline saturation temperature in the discharge pipe, somewhat fewer penalties for compressor power than lowering the temperature of the suction side.
- High-pressure liquid. Pressure drop in this section, can accurately penalties on overall system performance, as the pressure drop does not occur in the pipe will be held in the expansion device or level control valve. Expansion device provides the final reduction of the load on the intermediate pressure (two-stage compression) or low pressure (in a single-stage compression). Concern about the pressure drop in this line, there is more to take care that the pressure drops pressure saturation corresponding to the existing temperature of the refrigerant. The pressure was reduced to the point, the liquid will flash into steam, exacerbate the pressure gradient, and may limit the flow through the expansion device. Refrigerant speed that is selected for liquid lines in the range from 1 to 2.5 m/s (3 to 8 m/s).
- Liquid/vapor return from evaporators for low-pressure receiver.
Line of evaporators back of low pressure in the liquid receiver recirculation system carries a mixture of liquid and vapor. Calculations of pressure drop in the flow of a liquid/vapor mixtures, perhaps, are complex. To avoid cumbersome calculatons, but still make adjustments in the presence of liquid, some designers choose the size of the string, the first by determining the appropriate size, if a pipe is carried only in pairs, then step up to the next pipe size to allow for joint fluid flow.
Hot-gas defrost lines. To make an informed choice pipe size, the required flow rate of hot gas as a function of the evaporator size must be known. Approximate hot gas consumption, that it is twice the refrigerant mass flow is used in the cooling services. With this assumption, the recommended dimensions of ammonia hot gas branches, proposed Hansen9 used as a base speed of 15 m/s (3000 ft) with 21C (70F) of hot gas. This speed would be appropriate for hot gas industry lines serving one evaporator cluster thawing evaporators at the same time. Hot gas pipelines can be engineered to carry half the total for all connected evaporators on the assumption that no more than half of evaporators will be defrosted at one time.
Recent efforts to plants as low temperature condensation as the possible impact of the desired size of the hot-gas line. The ultimate criterion is the saturation temperature at which the gas defrost can condensate to the evaporator be thawed, so the fall of the saturation temperature at the hot-gas line appears as the most appropriate basis for selecting the size of the pipe. As the temperature of the condensing plant drops, defrost gas becomes less dense, and when the temperature of the condensing plant drops from 35C (95F) to 15C (59F), for example, the fall of saturation temperature for some of the most common refrigerants doubles.
THE OPTIMAL SIZE OF THE PIPE
The calculation of the differential pressure of the refrigerant flowing in the pipe is only one step in the process of deciding the size of the pipe. Ultimately, the decision size pair in the pipe, economic, trading off the extra cost of the big pipe to an energy conservation compressor during the lifetime of the equipment. For this situation, price trends, as shown in Fig. 9.3 where all costs, given current costs.
It may seem at first that for a given refrigerant flow and condition for the optimal diameter of a long pipe will be more than a short one. Richards showed, however, that, setting a zero of the derivative of the total cost, length cancels. The summary form of the equation representing the costs shown in Fig. 9.3:
Length L cancels, which shows that the optimal diameter of independent length.
In principle, optimization of calculation, subject to such restrictions as the minimal diameter to achieve a certain speed or maximum diameter to satisfy the limits in space can be performed on each project. Such an effort is not practical, and the best that can be hoped for is the periodic inspection optimally accommodate shifts in the cost of materials and energy.
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