Comprehensive Study Guide to Ammonia Refrigeration Systems

Physical and Chemical Properties of Ammonia (R-717)

Ammonia, scientifically designated as $R-717$, is a refrigerant composed of one nitrogen atom and three hydrogen atoms ($NH_3$). In industrial settings, the refrigerant tank used for ammonia is color-coded silver for identification. It is classified as an inorganic compound, although it is also described as an organic-based refrigerant. Physically, ammonia is lighter than air and is utilized in refrigeration primarily because it is very inexpensive and offers a high refrigeration effect. From a safety and regulatory standpoint, it falls under the Group 2, B2(L) classification, which indicates it is toxic and slightly flammable. Its boiling point is $-28\,^{\circ}\text{F}$ at atmospheric pressure. Ammonia is a colorless substance that is highly water-soluble and possesses a strong, pungent odor. This odor makes the substance easy to detect in the event of a leak. However, due to its toxicity and odor, it is explicitly not used for human comfort cooling. Ammonia works effectively with multiple types of metering devices, including Thermostatic Expansion Valves (TEV), Automatic Expansion Valves (AEV), and both low-side and high-side floats.

Industrial Applications and Material Compatibility

Ammonia systems are deemed suitable for use in frozen food plants, cold storage warehouses, skating rinks, and other similar industrial or high-demand refrigeration scenarios. A critical constraint in system design is that ammonia cannot be used with copper, as it reacts destructively with the metal. However, it is compatible with aluminum. In these systems, a reciprocating compressor is generally the standard. Piping requirements are specific to the line's function: piping on the liquid line is typically Schedule 80, while piping on the suction line is Schedule 60. Ammonia plants generally utilize vertical shell and tube type condensers, and it is required that these condenser tubes be composed of steel. These systems are normally located in designated Class T Machinery Rooms.

Specialty Compounds and Lubrication Management

There are specific variations of ammonia and specialized materials used within these systems. Anhydroid refers specifically to dry ammonia. Conversely, Anhydrous Calcium Sulfate is prohibited for use as a drier in ammonia systems. Lubrication is managed by force-fed oil pumps, where spring pressure is used to determine and adjust the oil pressure within the pump. To maintain the purity of the refrigerant and the health of the system, several components are used. A Scale Trap is preferably located on the suction line close to the compressor to catch any scale or debris. An Oil Separator or Regenerator is placed as close to the condenser inlet as possible; it employs a plate baffle to direct the flow of the refrigerant. Additionally, an Oil Still is used to recover refrigerant from the lubricating oil within the ammonia system.

Compressor Mechanics and Capacity Control

Centrifugal compressors in ammonia or large-scale systems can run at speeds of up to $30,000\,\text{rpm}$. These compressors are most suitable for handling large quantities of refrigerant vapor. Operationally, the high-velocity gas leaving the impeller is transformed into static pressure by a stationary diffuser. Stationary diffuser vanes are necessary for this conversion of velocity energy to pressure. The capacity of a centrifugal compressor is governed by its condenser pressure. The shaft seal used is an oil seal with pressure applied from the outside. The preferred methods for capacity control include suction line throttling dampers or peroration guide vanes. The lowest operating capacity a centrifugal compressor can reach is $10\%$ of its full capacity. In a vertical, single-acting ammonia compressor, the suction valves are usually located in the piston, with a striking clearance of $-1/64\,\text{inch}$.

Safety Systems and Leak Detection Protocols

Safety is paramount in ammonia systems due to toxicity. A Pressure Relief Valve (PRV) may discharge ammonia into a tank containing water. The discharge pipe must distribute the ammonia into the bottom of the tank, no lower than $33\,\text{feet}$ below the maximum liquid level. The ratio for this containment is $1\,\text{gal}$ of water for every pound of ammonia released in one hour from the largest connected pressure relief device. The rated discharge capacity of these relief valves is measured in pounds of air per minute. Alternatively, a PRV may discharge into the atmosphere outside the building. In such cases, the discharge must be at least $15\,\text{feet}$ above the ground or accessible roof level and at least $25\,\text{feet}$ away from any window, ventilation opening, or building exit.

For leak detection, sulfur sticks (or tapers) are used; one end is set on fire and passed around the suspected leak, creating white smoke upon contact with ammonia. Litmus paper can also detect small leaks; when wetted and passed near a leak, it turns red or pink. The Occupational Exposure Limit (OEL) for ammonia is $25\,\text{ppm}$. Machinery rooms must have a leak detector alarm set not to exceed $300\,\text{ppm}$. If a leak reaches $300\,\text{ppm}$, a full-face air-purifying respirator (or equivalent) is required. Above $300\,\text{ppm}$, a Self-Contained Breathing Apparatus (SCBA) is mandatory. Emergency shutdown procedures must be located as close to the compressor as possible, and machinery room exit doors must open outward.

Auxiliary Components and System Seals

Specific hardware is required for system integrity. Solenoid valves must be installed upright to prevent moisture damage. Gaskets on flanged joints, specifically on the high side of the system, should be of asbestos composition. Companion flanges are typically used on piping connections for TEVs and pressure regulators. A check valve is employed to prevent refrigerant migration to the lowest pressure evaporator during the off cycle. Additionally, a Hot Gas Bypass Regulator is used to balance system capacity to load demand by bypassing discharge gas into the low side; this component requires a solenoid valve to function.

Evaporator and Crankcase Regulation

Pressure regulation is managed through the Evaporator Pressure Regulator (EPR) and the Crankcase Pressure Regulator (CPR). The EPR is an upstream regulator located on the warmest evaporator outlet in a multi-evaporator system, allowing for three or four different temperatures simultaneously. The final adjustment of the EPR is determined by the desired return air temperature of the fixture. The CPR is a downstream pressure-reducing valve installed on the suction side of the compressor. Its purpose is to prevent the motor from overloading when the crankcase pressure rises above the design working pressure, particularly during startup or after a defrost cycle in freezer systems.

Refrigeration System Classifications

Systems are classified based on how they handle refrigerant flow. TEV Systems, known as "dry systems," ensure exiting refrigerant is liquid-free. While they are simple, cost-effective, and have fewer oil logging problems, they are less efficient because all refrigerant must be boiled off before entering the compressor, requiring a larger evaporator coil. Flooded Systems are more efficient because they use a wetted evaporator surface ($25\%$ less surface area than a TEV system for the same heat removal). They use a surge drum and liquid level control (low-side float), causing a boiling action that recirculates $2$ to $3$ pounds of refrigerant for every pound evaporated. Liquid Recirculation Systems (or "overfeed systems") use pumps to force refrigerant through the system with a recirculation ratio as high as $4:1$. Wetted vapor returns to a low-pressure receiver where liquid and gas separate, maintaining a constant liquid level in the accumulator. These systems offer high efficiency, simplified oil return, lower coil temperatures, and efficient hot gas defrosting, though they require a larger refrigerant charge and piping.