Usually the path of the liquid mains and branches is dictated by the geometric configuration of the plant. After the layout is established, the designer must position the shutoff valves and decide what pressure relief provisions must be made in pipe sections between valves that could be shut off. Pressures could rise to unacceptable magnitudes if trapped liquid warms up. After the piping and valve layout has been determined, the major tasks are to:
(1) compute the rate of liquid flow
(2) select the pipe diameters
(3) compute the system pressure drop
(4) select the pump
Computing the liquid flow rate. The flow rate vaporized, m . ev, is dictated by the total refrigeration rate, qtotal,
The latent heat of vaporization, hfg, is the difference between the enthalpy of saturated vapor, hg, and the enthalpy of saturated liquid, hf, at the temperature of the low-pressure receiver.
The flow rate of liquid delivered by the pump through the liquid distribution system,m . pump, is
where n is the circulation ratio as defined in Eq. 8.1.
Selecting the pipe diameters. Choosing the pipe diameters and selecting the pump becomes an iterative process, because the arbitrary choice of pipe diameters may result in a system pressure drop so high that a multistage pump is needed, or conversely the pressure drop is low enough that larger than necessary pipe is being used. Experienced design engineers can usually choose an appropriate size, perform a pressure-drop calculation, and if the pressure drop is unreasonable, can adjust the size. The target pressure drop for the system that results in a reasonably-sized pump is between 200 to 350 kPa (30 to 50 psi). The pressure drop in the pipe and fittings (elbows, tees, and open valves) is only one contributor to the total pressure drop, so perhaps less than 1/2 the total pressure difference available can be allocated to the pipe and fittings. The following equations can provide a rough guide for the liquid line size:
Thus, if the liquid flow rate is to be 3 kg/s (400 lb/min) and 100 kPa (14.5 psi) is reserved for the pipe and fittings, the trial pipe size would be 1.40 or 1–1/2 inch.
Computing the system pressure drop. Knowledge of the pressure drop to be experienced in the liquid distribution system is required for selection of the pump. The components and geometry that contribute to the pressure drop include:
• straight pipe
• elbows, tees, and reducing fittings
• open shutoff valves
• balancing valves
• overcoming the head if the evaporators are at a higher elevation than the liquid level in the low-pressure receiver
• liquid/vapor return line
• back-pressure regulators on any evaporators
Chapter 9 on piping provides instructions on how to compute the pressure drop in straight pipe and fittings. Manufacturers’ data are available to compute the pressure drop in open shutoff valves. The balancing valves, which many practitioners call expansion valves, should initially be partially closed in order to be able to open more if the liquid supply to the coil is inadequate. A reasonable estimate for the pressure drop through the balancing valves is between 35 and 70 kPa (5 to 10 psi).
Equations 8.4 and 8.5 may be used to compute the pressure difference associated with the rise in elevation of a line. In many closed liquid-pumping systems it is not necessary to compensate for the rise in elevation, because when the piping rises at one place it will drop somewhere else. The refrigeration system is different, however, when the liquid must be pumped up to an evaporator and this difference in pressure is not recovered in the liquid/vapor suction line. Some practicioners indulge in a philosopical debate as to whether the pump force provides all the impetus for the movement of refrigerant, or whether the compressor is also drawing vapor and thus assisting the circulation. It would appear, however, that the pump is responsible for the refrigerant from the liquid level in the low-pressure receiver and back to that point.
Sometimes one or more evaporators in a medium-temperature recirculation system are equipped with back-pressure regulators or evaporator-pressure regulators, as shown in Figure 8.24. This regulator prevents the pressure and temperature in the evaporator from falling so low that products would be damaged or that frost would form on the coil requiring defrost provisions. If the temperature of the low-pressure receiver and that of most of the evaporators is 2°C (35.6°F), but an evaporator-pressure regulator holds one evaporator at 5°C (41°F), the pressure in that evaporator will be, for ammonia, 53.3 kPa (7.7 psi) higher than the other evaporators. Unless the pump has been chosen to provide this excess pressure, the evaporator with the pressure regulator will not be supplied with refrigerant.
Selecting the pump. The two numbers needed to select the pump from a manufacturer’s catalog are the flow rate and the pressure rise. Section 8.9 and Fig. 8.18 emphasized that the design point should be at the high-pressure part of the curve, rather than to the far right. For a given rotative speed, for example, 1800 rpm, the impeller diameter controls the pressure difference that the pump can develop, and the width of the impeller passages regulates the flow rate. Both of these influences are embodied in the pump performance curves.