A supermarket refrigeration rack system uses R-404A and has a common hot gas defrost line. The main hot gas header is 2 inches (50.8 mm) and feeds several medium-temperature cases. If the total defrost load requires a flow rate equivalent to 10 kW (2.84 tons) of refrigeration, and considering a typical pressure drop of less than 35 kPa (5 psi) per 30 metres (100 ft) of equivalent length, what would be an appropriate nominal size for a branch hot gas line feeding a single case requiring 1.5 kW (0.43 tons) defrost?

Question: A supermarket refrigeration rack system uses R-404A and has a common hot gas defrost line. The main hot gas header is 2 inches (50.8 mm) and feeds several medium-temperature cases. If the total defrost load requires a flow rate equivalent to 10 kW (2.84 tons) of refrigeration, and considering a typical pressure drop of less than 35 kPa (5 psi) per 30 metres (100 ft) of equivalent length, what would be an appropriate nominal size for a branch hot gas line feeding a single case requiring 1.5 kW (0.43 tons) defrost?

Answer options: ✅ 1/2 inch (12.7 mm)

• 3/4 inch (19.05 mm)
• 1 inch (25.4 mm)
• 1 1/4 inch (31.75 mm)

Correct answer: 1/2 inch (12.7 mm)

Explanation: For a 1.5 kW (0.43 tons) R-404A hot gas defrost branch line, a 1/2 inch (12.7 mm) nominal pipe size is generally appropriate. This size balances sufficient flow velocity for heat transfer during defrost, minimizing pressure drop, and ensuring effective oil return from the coil to the hot gas header during the defrost cycle, without oversizing and losing velocity. Calculation relies on standard industry piping charts for hot gas defrost lines.