Handbook of air conditioning and refrigeration / Shan K

11.14 CHAPTER ELEVEN
Size of Copper Tube, Refrigeration Load, and Pressure Drop
Sizing Procedure
During the sizing of refrigerant piping, the system refrigeration load Qrl, in tons of refrigeration, is
a known value. Because the refrigerant velocity v � m˙ r /(�Ai) , the refrigerant mass flow rate
2 m˙ r � Qrl /�h and the inner surface area of the copper tubing Ai � �Di /4. Here � represents the
density of the refrigerant, and �h is the enthalpy difference of the refrigerant leaving and entering
the evaporator. For a specific size of copper tubing, the relationship between the outside diameter D
and inside diameter Di is a fixed value. From the Darcey-Weisbach equation, the pressure drop in
the refrigerant piping can be expressed as
D � K� 8
� C pipe�
� 2 g c
2 fLQrl 2 fLQrl �(�h) 2 �p �1/5
�(�h) 2 �p �1/5
where g c � dimensional constant, 32.2 lb m�ft/lb f�s 2
L � length of piping, ft (m)
Q rl � refrigeration load, Btu/min (W)
�h � enthalpy difference of refrigerant, Btu/lb (kJ/kg)
�p � pressure loss, lb f/ft 2 (kPa)
(11.1)
In Eq. (11.1), constant C pipe � K [8/(� 2 g c)] 1/5 . The friction factor of refrigerant f can be calculated
by the Colebrook equation and is covered in Chap. 18. If the absolute roughness of the copper tube
is taken as 0.000005 ft (0.0015 mm), f has only a minor influence on the outside diameter of the
copper tubing D. Constant K takes into account the difference between D and D i for a specific size
of copper tubing.
The size or outside diameter of the copper tubing D is
● 1 Directly proportional to the length of the refrigeration pipe L to the �5 power
● 2 Directly proportional to refrigeration load Qrl to the �5 power
● 1 Inversely proportional to the density of the suction vapor or hot gas � to the �5 power, which is affected
by suction and condensing temperature, respectively
● Inversely proportional to the enthalpy difference �h (which is closely related to the suction tem-
2 perature Tsuc and condensing temperature Tcon) to the �5 power
● 1 Inversely proportional to the maximum allowable pressure drop �p to the �5 power
Refrigerant piping design and size should be determined as follows:
1. Make an optimum refrigerant piping layout, and measure the length of the piping.
2. Find the correction factor of the refrigeration load from Table 11.1, based on the actual suction
and condensing temperatures.
3. Estimate the equivalent length of refrigerant piping including the pipe fittings and accessories
L eq, ft (m). It is usually 1.5 to 5 times the measured straight length of the piping, depending on
the number of fittings and accessories.
4. Based on the corrected refrigeration load and L eq, determine the tentative diameter of the copper
tubing from refrigeration load versus equivalent length Q rl-L eq charts (see Figs. 11.10, 11.11,
and 11.12). All suction, discharge, and liquid line Q rl-L eq are plotted based on the data given in
ASHRAE Handbook 1998, Refrigeration.