Refrigeration Rack: Suction Riser, Machinery Room, and Relief Discharge
A supermarket rack using R-448A serves a -20 F frozen-food case and a 35 F dairy case. The frozen-case suction line is 1-1/8 in copper with an equivalent length of 80 ft, including a 14-ft vertical riser. The rack holds 1,800 lb of refrigerant. The machine room is 20 ft by 15 ft by 10 ft and has a 1,200 cfm exhaust fan but no refrigerant detector. The receiver relief valve is 3/4 in and is piped 30 ft horizontally to a wall termination 8 ft above grade with a rain cap. The contractor proposes to add a second 1-1/8 in suction line for the dairy case and to reduce the riser to 7/8 in to save cost. The owner reports oil-logging symptoms at the frozen-case compressor after low-load periods.
1. Evaluate the proposed 7/8-in riser and the oil-return design. What should be checked?
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Reducing the riser to 7/8 in raises velocity and can improve oil return, but it also increases pressure drop and may reduce capacity at full load. The correct check is the minimum load: the line must maintain enough velocity to carry oil up the riser, while the full-load pressure drop must stay within the manufacturer's/ASHRAE line-sizing limits. A single 1-1/8 in riser may be oversized at low load, causing the reported oil logging; a double riser or an oil separator/trap may be the proper fix. The design should include a trap at the base of the riser, horizontal suction sloped toward the compressor, and an oil-return strategy. Do not simply reduce the riser without checking both full-load and minimum-load velocities and the dairy-case line.
2. Evaluate the machine room ventilation and detector requirements.
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Room volume = 20 x 15 x 10 = 3,000 ft^3. 1,800 lb / 3,000 ft^3 = 0.6 lb/ft^3, far above the RCL for R-448A, an A1 refrigerant. The room must be treated as a machinery room. 1,200 cfm x 60 / 3,000 ft^3 = 24 air changes per hour. Verify the adopted ASHRAE 15/UMC emergency ventilation rate; if 30 ACH is required, 24 ACH is insufficient. A refrigerant detector is required, with alarm and interlock to ventilation and alarm, and it should be located based on R-448A vapor density (heavier than air, so low-level sensing). The contractor needs to add detection and correct the ventilation.
3. Evaluate the relief valve discharge piping.
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The relief discharge is questionable. It terminates only 8 ft above grade; refrigerant relief outlets are typically required to be at least 15 ft above grade and away from windows, doors, and air intakes. A rain cap can obstruct flow and trap moisture; relief piping must not be trapped or restricted. The 30-ft horizontal run may create back pressure; the pipe must be sized for the valve capacity and same size as the outlet. It must be supported and terminate outdoors with an approved fitting. The design should be recalculated for relief capacity and termination height/location.