Designers are called on to make repeated pressure-drop calculations, but for a limited number of refrigerants and for specified pipe sizes. The situation is ready-made for the construction of charts to duplicate the calculations using the equation. Typical of these charts is a set developed by D.D. Wile for ASHRAE4,5 for R-12, R-22, R-502 and ammonia. Three charts are presented for each refrigerant: liquid, vapor at suction conditions, and vapor at compressor discharge conditions. A sample page for low-pressure R-22 vapor is shown in Fig. 9.2.
The numbers on the right side of the vertical 20° F evaporator line show a drop in saturation temperature of 2.1°F which corresponds to the pressure drop of 2.3 psi.
The conditions in Example 9.3 are identical to those in Example 9.2 where a pressure drop of 2.2 psi was calculated, so the two approaches agree approximately, but the chart in Fig. 9.2 is based on 20°F superheat which increases the specific volume over that for saturated vapor that was assumed in Example 9.2. Reference 4 states that for 20°F of superheat, the pressure drop will be 5.2% higher than for saturated vapor, so reducing 2.3 psi by 5.2% results in good agreement between Examples 9.2 and 9.3. It is no surprise that the chart in Fig. 9.2 and the pressure-drop equation Eq. 9.2 agree well, because the charts in Reference 4 were based on this standard equation.
Equations 9.1 and 9.2 apply to incompressible flow, so certainly are valid for liquid, but are sufficiently accurate for gases as well, so long as the change in pressure are very small with respect to the pressure level. At some of the high velocity regions of the charts, the compressibility of gas should have been taken into consideration6. This situation is not serious, because in the normal design of refrigeration systems, practical choices do not normally drift into the extremely high-velocity regions.