21.1 Hot-Gas Defrost Sequence

Frost on an evaporator is not cosmetic. Ice is a thermal insulator and a mechanical blockage. On a finned air unit, frost fills the gaps between fins, so air that should wash the entire coil face is forced through a shrinking free area. Face velocity through the remaining openings can rise while total mass airflow falls. Fan static pressure climbs. Motor amps may increase even as delivered cfm drops. On the refrigerant side, that same ice layer sits on the tube and fin metal that is supposed to conduct heat from room air into boiling ammonia. Overall U falls. Effective surface A falls. In Q = UAΔT, if UA collapses, the only way the coil can still pull heat is for ΔT — the temperature difference (TD) between entering room air and refrigerant saturation — to grow.

Operators see this as a room that will not hold setpoint, a suction pressure pulled lower than design, or a coil that looks “too cold” for the work it is doing. Capacity in tons is down even though compressors are loaded. That is the industrial refrigeration reason to defrost: restore UA and restore the air path, not to make the unit look clean. Leave frost in place and you also raise compressor lift (colder suction for the same room), raise kW/ton, and eventually ice-bridge the coil until it is a solid block.

Why industrial NH3 uses hot-gas defrost

Industrial ammonia plants almost always hot-gas defrost freezer air units, and they use the same method on many cooler units. The heat source is already in the engine room: compressor discharge vapor taken after the oil separator onto a hot-gas header. That vapor is high pressure and high enthalpy. When you admit it into an isolated evaporator, the coil stops being an evaporator and becomes a temporary condenser. Discharge gas gives up superheat and then condenses, dumping a large latent-heat load into the frost. Melt water runs to the drain pan. You are using heat the plant already paid to create at the compressors, instead of installing hundreds of kilowatts of electric elements inside a blast cell.

That is why hot gas is the CIRO-level default on large NH3 air units. Water spray and electric heat appear mainly as exceptions on small commercial equipment or on rooms that stay above freezing — covered in the termination and alternatives section. Hot-gas defrost is also a process, not a single valve. The coil contains liquid during refrigeration. The suction header is at low pressure. Discharge is at high pressure. If you simply open a hot-gas solenoid into a live, flooded coil, you can shove liquid into the suction, hammer the piping, and overpressure the low side. The sequence exists to make the coil a safe, warm condenser for a few minutes and then return it to refrigeration without a slug.

Supervisor-level sequence

Know this order and the reason for each step. Controllers differ (PLC, dedicated defrost controller, dual-position gas-powered valves), but the physics does not.

1. Stop liquid feed. De-energize the liquid-line solenoid — and any overfeed feed valve to that coil. You must stop adding liquid before you add heat. If the coil is still being fed, hot gas will push a wet mixture toward the suction or the drain regulator, pump-out will never finish, and the coil will not empty.
2. Pump down / isolate. Leave the suction path open long enough for remaining liquid to boil off under room load. Many programs keep fans running for a short pump-out so the coil still has air-side heat. Wet suction may dip as inventory leaves. Then close the suction stop (commonly a dual-position gas-powered suction stop) so the coil is isolated from the suction header. Isolation is what lets coil pressure rise during defrost instead of blowing hot gas straight into the low side.
3. Stop fans for the melt. Fans left on during hot gas blow warm, wet air into the room, throw water off the fins, glaze product, and steal defrost heat into the space instead of into the ice.
4. Admit hot gas. Many plants use a two-step or slow-opening hot-gas solenoid (soft hot gas) so pressure in the emptied coil rises over tens of seconds instead of jumping from suction pressure to near discharge in one gulp. Then the main hot-gas valve opens full. Gas often enters a drain-pan circuit first so the pan is hot before melt water arrives, then flows into the coil.
5. Melt frost and drain condensate. Coil pressure rises to the setting of the defrost drain pressure-relief regulator. Frost melts. Condensed ammonia and leftover vapor leave through that regulator into a defrost return or wet suction downstream of the closed suction stop. Melt water leaves through the pan and heated drain — not through the refrigeration piping.
6. Drip time. Close hot gas and wait. Water still clings to fins. If you refrigerate immediately, that water becomes glaze ice, which is denser and harder to remove on the next cycle.
7. Equalize (soft gas / bleed). Bleed coil pressure down toward suction through a small equalizing path before the main suction stop opens. This is the other plant use of “soft gas.” It prevents liquid hammer and compressor or separator slugging.
8. Open suction, restart liquid, delay fans. Open the main suction stop when the ΔP is small. Open liquid feed. Keep fans off until remaining droplets freeze on the coil (fan delay) so you do not fog the room. Then return to refrigeration.

Overfeed, flooded, and DX coils

CIRO plants are mostly liquid overfeed air units. Those coils hold a large liquid inventory (often a 3:1 to 4:1 recirculation ratio during refrigeration), so pump-out time is not optional. Direct-expansion coils hold less liquid and pump down more completely after the solenoid closes, but they still need isolation, fan stop, controlled hot-gas admission, drip, and equalization. Flooded shell-and-tube or plate units are a different vessel problem; this chapter is about air-unit defrost, which is what Content Area 8 is built around.

Header capacity, oil, and shock

If several large freezer coils defrost at once, two things fail together: the rooms lose capacity, and the hot-gas header cannot supply enough mass flow, so coil pressure never reaches a useful saturation temperature and ice remains. Stagger defrosts. Watch discharge pressure. Plants that float head pressure for efficiency often need a defrost floor on head pressure so there is enough driving ΔP into the coil.

Take hot gas after the oil separator. Oil that rides along still fouls distributors and pans; a wet, oily coil defrosts poorly. Pitch and trap the hot-gas main so condensed liquid in the header cannot slug into the coil when the solenoid opens. Opening a hot-gas valve into a liquid-logged header is a classic setup for hydraulic shock — a liquid slug accelerated through steel pipe. Documented ammonia releases have started from defrost-related shock. Treat valve speed, pump-out, and one-coil-at-a-time logic as safety controls, not conveniences.