8.3 Defrost Systems and Evaporator Maintenance

The Physics of Frost Accumulation on Coils

In industrial cooling applications where the evaporator coil temperature operates below the freezing point of water ($32^\circ\text{F}$ or $0^\circ\text{C}$), moisture in the air will inevitably condense and freeze on the coil surface. This ice buildup, known as frost, creates two severe operational problems:

1. Thermal Insulation Effect: Frost has a very low thermal conductivity (approximately $0.1$ to $0.3 \text{ W/(m\cdot K)}$) compared to aluminum ($205 \text{ W/(m\cdot K)}$) or carbon steel ($50 \text{ W/(m\cdot K)}$). The frost layer acts as an insulating blanket on the tubes, reducing the overall heat transfer coefficient ($U$-value) and restricting heat transfer from the room air to the ammonia.
2. Airflow Restriction: As frost builds up, it bridges the spaces between the fins (fin spacing). This restricts the open area of the coil, increasing static pressure drop across the unit and reducing the volumetric airflow rate (CFM) delivered by the fans.

The combination of these factors leads to longer compressor runtimes, high energy costs, and an inability to maintain room temperatures.

Overview of Defrost Methodologies

To restore operating efficiency, the frost layer must be regularly removed. The four primary defrost methods in industrial plants are:

• Air Defrost (Off-Cycle): Used only in rooms maintained above $36^\circ\text{F}$ (typically $38^\circ\text{F}$ to $40^\circ\text{F}$). The liquid refrigerant feed is shut off, but the evaporator fans continue to run. The warm room air melts the frost. This is energy efficient but slow.
• Water Defrost: Water is sprayed over the top of the coil via distributor pipes while the fans are off and the coil is isolated. Water defrost is fast and common in low-temperature freezers, but it consumes large amounts of water and requires a heated, insulated drain pan to prevent water from re-freezing in the drain line.
• Electric Defrost: Electric resistance heating elements are inserted into the fin pack of the coil. This method is rare in industrial ammonia plants due to high electrical operating costs and the potential for electrical spark hazards in an engine room.
• Hot Gas Defrost: The standard method for industrial ammonia systems. Warm, high-pressure discharge gas from the compressors is routed directly into the evaporator coil. The hot gas condenses inside the tubes, releasing its latent heat of condensation to melt the frost from the inside out.

Hot Gas Defrost: Operational Principles

Hot gas defrost is highly efficient because it utilizes waste heat from the compression process and heats the coil from the inside. However, because it introduces high-pressure, warm gas into a low-pressure, cold part of the system, it carries severe physical hazards.

During hot gas defrost, the evaporator temporarily acts as a condenser. The hot gas enters the coil and condenses into liquid ammonia. This condensed liquid must be drained from the coil. This is accomplished by venting it through a defrost relief valve or a pressure regulating valve set to maintain $70$ to $80 \text{ psig}$ inside the coil. This pressure corresponds to a saturation temperature of approximately $48^\circ\text{F}$ to $54^\circ\text{F}$—warm enough to melt ice quickly without causing excessive thermal expansion stresses on the coil structure.

Step-by-Step Valve Sequencing for Hot Gas Defrost

To ensure safety, hot gas defrost must follow a strict, automated sequence of valve operations. Operators must never manually override or accelerate this sequence.

[Step 1: Pump-Out] ----> Close liquid feed solenoid. Keep fans running to boil off liquid.
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                             v
[Step 2: Fan Stop]  ----> Stop evaporator fans.
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[Step 3: Isolation] ----> Close suction stop valve (coil is isolated).
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[Step 4: Hot Gas]   ----> Open hot gas supply solenoid slowly (coil rises to 70-80 psig).
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[Step 5: Defrost]   ----> High-pressure gas condenses, melting frost. Liquid vents to LPR.
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                             v
[Step 6: Bleed Down]---> Close hot gas solenoid. Open bleed valve to slowly lower pressure.
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[Step 7: Equalize]  ----> Suction pressure equalized.
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[Step 8: Fan Delay] ----> Open suction & liquid valves. Run coil for 1-3 mins BEFORE fans start.

1. Pump-Out (Pump-Down) Phase: The liquid supply solenoid valve is closed while the evaporator fans continue to run. This allows the remaining liquid refrigerant in the coil to boil off and return to the suction accumulator or LPR. This phase typically lasts $10$ to $20 \text{ minutes}$ and is critical to prevent hot gas from pushing a slug of cold liquid into the suction line.
2. Fan Stop Phase: Once the liquid is evacuated, the evaporator fans are turned off to prevent blowing heat into the cold room and to speed up the defrost process.
3. Suction Isolation Phase: The main suction stop valve is closed, isolating the evaporator from the suction header.
4. Hot Gas Injection Phase: The hot gas supply solenoid valve is opened. To prevent pressure shocks, this valve is often opened slowly or utilizes a small bypass solenoid first. The pressure in the coil rises to $70\text{--}80 \text{ psig}$, and the hot gas condenses, melting the frost. The condensate drains through a relief regulator back to the LPR or suction accumulator.
5. Defrost Termination & Bleed-Down Phase: The hot gas solenoid is closed. The coil is now full of high-pressure vapor and liquid. A small bleed-down valve (or equalizer valve) is opened to slowly depressurize the coil down to the suction header pressure. Opening the main suction valve immediately would cause a violent pressure shock.
6. Re-entry to Refrigeration: The suction stop valve is opened fully, and then the liquid feed solenoid is opened to resume cooling.
7. Fan Delay (Drip/Freeze Time): The evaporator coil pulls down to operating temperature for $1$ to $3 \text{ minutes}$ before the fans restart. This re-freezes any remaining water droplets on the coil fins, preventing "water blow-over" (which throws water mist into the room, creating ice on walls, ceilings, and product).

The Mechanics of Hydraulic Shock and Liquid Slugging

The single greatest safety hazard in an industrial ammonia facility is hydraulic shock (also called liquid hammer). Hydraulic shock occurs when a slug of incompressible liquid refrigerant is accelerated to high velocity by a pressure differential and slams into a stationary valve, elbow, or pipe cap. This impact creates localized pressure spikes exceeding $1,000 \text{ psi}$, which can easily shatter cast iron valves and rupture steel piping, releasing toxic ammonia.

During defrost, hydraulic shock is typically caused by two errors:

• Inadequate Pump-Out: If liquid refrigerant is left in the coil before hot gas injection, the hot gas will act as a piston, pushing the cold liquid slug forward at high velocity.
• Rapid Suction Valve Opening: If the suction stop valve is opened too quickly before the coil pressure has been equalized to the suction header pressure during bleed-down, the high-pressure gas (at $80 \text{ psig}$) will rush toward the low-pressure suction line (at $10 \text{ psig}$), dragging a wave of condensed liquid with it.

Hydrostatic Pressure Hazards of Trapped Liquid

Liquid ammonia is incompressible and has a high coefficient of thermal expansion. If liquid refrigerant is trapped inside a section of piping between two closed valves (such as a coil isolated for maintenance or during an incorrect defrost sequence) and is allowed to warm up, the pressure will rise rapidly.

For every $1^\circ\text{F}$ rise in temperature, the hydrostatic pressure of trapped liquid ammonia increases by $100$ to $150 \text{ psi}$:

$\Delta P_{\text{hydrostatic}} \approx 125 \text{ psi per } 1^\circ\text{F}$

If a coil is trapped at $30^\circ\text{F}$ and warms to a room temperature of $50^\circ\text{F}$ (a $20^\circ\text{F}$ differential), the pressure will attempt to rise by $2,500 \text{ psi}$, far exceeding the burst rating of the piping. Safe operation requires that relief valves or hydrostatic expansion bypass valves be installed on any pipe run where liquid trapping is possible.

Evaporator Fan Checks and Rotation Direction

Evaporator fans must deliver design airflow to maintain heat transfer efficiency. During routine maintenance, operators must check fan motors and blade assemblies:

• Rotation Direction: Evaporator fans must rotate in the correct direction to draw air across the coil and discharge it into the room. If a fan motor is wired in reverse, it will still move air, but the airflow volume will be reduced by $50%$ to $70%$, causing high coil pressures, ice accumulation, and loss of room cooling.
• Checking Rotation: To check rotation, view the fan blade from the motor side (looking through the motor toward the blade) and compare the spin direction to the arrow stamped on the motor housing or fan shroud.
• Correcting Rotation: Industrial fan motors are typically three-phase. If a fan is spinning in reverse, the operator must shut down and lock out the power (LOTO), then swap any two of the three power leads at the motor starter terminal box to reverse the motor's magnetic field rotation.