Ammonia refrigeration compressors are mainly divided by structural form and motion mode into three major categories: reciprocating, screw, and centrifugal. There are also subdivided types such as scroll and diaphragm, but their application in ammonia refrigeration engineering is relatively limited. There is no single “best” answer in equipment selection; only by comprehensively weighing the refrigeration capacity required by the system, operating conditions, energy efficiency requirements, and investment and maintenance budgets can the most suitable model for a specific project be determined.
With regard to reciprocating ammonia compressors, a crank-connecting rod mechanism drives the piston in reciprocating linear motion within the cylinder. The suction stroke draws in low-pressure ammonia vapor, and the compression stroke raises its pressure before discharging it to the discharge line. This type has mature technology and extensive manufacturing experience, and maintains a large number of operating references in the field of ammonia refrigeration. Its advantages include a wide applicable temperature range, stable operation at evaporation temperatures from −65°C to +5°C, relatively high indicated efficiency, good adaptability to variable operating conditions, a wide single-unit cooling capacity range, and relatively low initial purchase cost. However, its inherent limitations cannot be ignored: the unit is bulky; there are numerous wearing parts such as valves, piston rings, and crankshaft bearings; periodic inspection and replacement involve a heavy workload, resulting in significantly higher long-term maintenance costs. In terms of capacity control, reciprocating compressors mostly use cylinder unloading to achieve stepped control, making it difficult to satisfy wide-range stepless regulation requirements. The discharge flow exhibits pulsation, and a buffer vessel is usually required to suppress pipeline vibration. Overall, this type is suitable for refrigeration systems in the medium-to-high capacity range and for applications requiring staged cooling capacity control.
The screw ammonia compressor uses a pair of intermeshing male and female helical rotors as its core components. As the rotors rotate at high speed within the cylinder, the interlobe volume changes periodically, thereby completing the suction, compression, and discharge of ammonia vapor. Oil-injected screw compressors are widely used in ammonia refrigeration systems. Their prominent advantages include a compact structure and a footprint far smaller than that of a reciprocating compressor of the same cooling capacity; moreover, the moving parts are only the rotors and bearings, with very few wearing parts, significantly improving operational reliability. Because the rotors have good dynamic balancing characteristics, the unit has low vibration and continuous, stable discharge. In addition, oil injection cooling effectively suppresses the rise in discharge temperature. Screw compressors are insensitive to wet compression and permit liquid carryover operation without damage. In terms of capacity control, a slide valve mechanism enables continuous stepless regulation within the range of 10% to 100%. However, this type also has shortcomings: its power consumption per unit cooling capacity is slightly higher than that of a high-efficiency reciprocating compressor; the oil injection system requires auxiliary devices such as an oil separator, oil cooler, oil pump, and filter, making the overall unit system relatively complex; and the machining accuracy required for the rotor profile is extremely high, requiring large investment in manufacturing equipment. Screw compressors are currently the dominant type in large industrial ammonia refrigeration systems and are particularly suitable for process cooling applications requiring long-term continuous operation.
The working principle of the centrifugal ammonia compressor is entirely different from the previous two types. It relies on a high-speed rotating impeller to transfer mechanical energy to the ammonia vapor, giving it relatively high kinetic energy and pressure energy. The gas then decelerates in the diffuser and volute casing, further converting kinetic energy into pressure energy, thereby achieving pressure rise. As a dynamic-type compressor, its single-stage pressure ratio is relatively low, and multiple impellers arranged in series are usually required to obtain the required pressure differential. The most notable feature of centrifugal compressors is their extremely large volumetric flow rate; the cooling capacity of a single unit can reach tens of thousands of kilowatts. In addition, there is no direct contact friction between the gas flow and rotating components, resulting in smooth operation and low vibration and noise levels. Since there are no wearing parts such as valves or piston rings, the inspection cycle is very long, and overhaul intervals can be as long as several years. However, its initial investment is far higher than that of reciprocating and screw types. Because extremely high rotational speeds are required, it often relies on a speed-increasing gearbox or high-speed motor drive; the technical thresholds for precision manufacturing and dynamic balancing are high; and the performance curve has a surge boundary in the low-flow region, resulting in a relatively narrow operating control range and higher requirements for the professional technical competence of operation and maintenance personnel. Therefore, centrifugal ammonia compressors are mainly used in ultra-large industrial installations with extremely large cooling demands, such as petrochemical bases, large air separation units, and district cooling stations.
In the selection decision-making process, multiple dimensions must be considered, including applicable scale, operating efficiency, initial investment, maintenance cost, control characteristics, and tolerance to wet compression/liquid carryover. Reciprocating compressors offer a cost-performance advantage in the small-to-medium and medium-to-large cooling capacity ranges, but their maintenance workload is relatively heavy. Screw compressors cover large to extra-large cooling capacity demands, offer moderate initial investment and stepless capacity control, and have become the mainstream choice in modern industrial refrigeration. Centrifugal compressors are dedicated to extra-large to ultra-large projects, offering very high efficiency but also the most obvious initial investment and control limitations. Actual selection should be comprehensively justified in combination with the project’s specific cooling load, annual operating energy efficiency ratio, on-site installation space, cooling water source conditions, payback period, and other factors. After preliminary screening, it is recommended to entrust a professional design institute or compressor manufacturer to conduct detailed operating condition calculations and techno-economic analysis, so as to ensure that the selected model achieves optimal overall benefits throughout the project life cycle.
