5.1 Industrial Compressor Types

5.1 Industrial Compressor Types

In industrial ammonia refrigeration, the compressor serves as the mechanical heart of the vapor-compression system. Its operation is critical to maintaining continuous refrigeration by performing two essential thermodynamic functions:

1. Low-Pressure Maintenance: The compressor continuously draws refrigerant vapor out of the low-pressure side of the system (evaporators, accumulators, and surge drums). This maintains a low suction pressure, allowing the liquid ammonia in the evaporator to boil (evaporate) at the low temperatures required to absorb heat from the product or process.
2. Pressure and Temperature Elevation: The compressor compresses the low-pressure, low-temperature suction vapor, raising its pressure and temperature. By elevating the pressure, the saturation temperature of the ammonia is raised above the temperature of the condensing medium (cooling water in an evaporative condenser or ambient air in an air-cooled condenser). This temperature differential is what allows the refrigerant to reject its absorbed heat and condense back into a liquid, completing the cycle.

Operating personnel must understand the mechanical construction, capacity control methods, and thermodynamic limitations of the different compressor styles commonly found in industrial facilities.

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Classification: Positive Displacement vs. Dynamic Compressors

Industrial compressors are classified into two major categories based on their design and compression method: positive displacement and dynamic (kinetic).

Positive Displacement Compressors

Positive displacement compressors increase gas pressure by trapping a fixed volume of vapor in a compression chamber and then physically reducing the volume of that chamber. As the chamber volume decreases, the molecules of the gas are forced closer together, causing an increase in pressure and temperature. Once the compression process is complete, the gas is discharged into the high-pressure side of the system.

• Reciprocating Compressors: Use pistons moving back and forth (reciprocating) inside cylinders. They rely on suction and discharge valves that open and close based on pressure differentials.
• Rotary Screw Compressors (Twin and Single): Use intermeshing helical rotors. Twin-screw compressors use a male and a female rotor in a casing, while single-screw compressors use a main rotor meshed with two star-shaped gate rotors.
• Scroll Compressors: Use two spiral-shaped scrolls: one fixed and one orbiting. The orbiting scroll moves eccentrically, trapping and squeezing gas pockets toward the center.

Dynamic (Kinetic) Compressors

Dynamic compressors do not rely on trapping a closed volume of gas. Instead, they increase pressure by transferring kinetic energy to the gas using a high-speed rotating impeller. The gas enters the center of the impeller, is accelerated outward to extreme velocities by centrifugal force, and then passes through a stationary, expanding chamber called a diffuser. The diffuser slows the gas down, converting the kinetic energy of velocity into potential energy in the form of static pressure.

• Centrifugal Compressors: The primary dynamic compressor type in industrial processes. They are high-speed, high-volume machines.

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Compressor Type Comparison in Ammonia Systems

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Reciprocating Compressors: Operation and Capacity Control

Reciprocating compressors are positive displacement machines that operate similarly to an automobile engine. They utilize pistons driven by a crankshaft through connecting rods to compress refrigerant vapor within cylinders.

Compression Cycle Phases

The reciprocating compression cycle is divided into four distinct piston strokes:

1. Suction Stroke: As the piston moves downward from Top Dead Center (TDC) to Bottom Dead Center (BDC), the volume inside the cylinder increases, creating a low-pressure area. When the cylinder pressure drops below the suction line pressure, the pressure differential forces the suction valve reeds or plates open, allowing vapor to fill the cylinder.
2. Compression Stroke: When the piston reaches BDC and begins its upward stroke, the suction valve closes due to the rising pressure. As the piston continues upward, the volume of the trapped vapor is reduced, steadily increasing its pressure and temperature.
3. Discharge Phase: As the piston approaches TDC, the pressure within the cylinder exceeds the pressure in the discharge manifold. This pressure differential forces the discharge valve open, and the piston pushes the high-pressure gas out of the cylinder into the discharge piping.
4. Expansion Phase: When the piston reaches TDC, it cannot touch the valve plate; a small clearance volume remains. This volume contains high-pressure gas. As the piston begins its downward stroke, this gas expands, and the suction valve cannot open until the cylinder pressure drops below the suction manifold pressure.

Capacity Control: Cylinder Unloaders

To match system capacity with fluctuating refrigeration loads, reciprocating compressors use cylinder unloaders.

• Mechanism: Unloaders typically consist of hydraulic or pneumatic actuators that control unloader pins or lifter rings situated directly under the suction valve reeds.
• Operation: When capacity reduction is needed, a solenoid valve vents oil pressure (or air pressure) from the unloader actuator. This action allows springs to push the unloader pins upward, physically holding the suction valve reeds open off their seats.
• Thermodynamic Effect: During the upward compression stroke of the piston, the open suction valve prevents compression. The piston simply pushes the gas back into the suction manifold without doing work on it. This cylinder is now "unloaded."
• Staged Control: Reciprocating compressors are unloaded in discrete steps. For example, a 6-cylinder compressor can operate at 100% capacity (all 6 cylinders loading), 67% (4 cylinders loading), 33% (2 cylinders loading), or completely unloaded (0% capacity for starting). This step control maintains high part-load efficiency, but does not provide the infinite capacity adjustment found in screw compressors.

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Rotary Screw Compressors: Twin vs. Single Screw

Rotary screw compressors are the dominant type in modern industrial ammonia refrigeration. They are positive displacement machines known for high reliability, long operating life, and the ability to handle large volumes of refrigerant.

Twin-Screw Design

Twin-screw compressors utilize two intermeshing helical rotors housed in a close-tolerance casing.

• Rotor Lobes: The male rotor is typically driven by the electric motor and features 4 helical lobes. The female rotor meshes with the male rotor, driven by it, and features 6 helical flutes (though other designs like 5/6 are also used).
• Compression Mechanism: Low-pressure vapor enters the suction port at one end of the rotors, filling the space between the lobes and flutes. As the rotors turn, the meshing of the rotors seals the gas pocket, isolating it from the suction inlet. The pocket of gas is moved axially along the length of the screw. Because of the helical geometry, the size of the pocket decreases as it moves toward the discharge end, compressing the vapor. When the pocket reaches the end of the rotors, it is exposed to the discharge port, and the gas is forced out.

Single-Screw Design

Single-screw compressors feature a single main cylindrical rotor with helical grooves and two star-shaped gate rotors.

• Operation: Compression occurs in the grooves of the main rotor as the gate rotor teeth engage them.
• Balanced Radial and Axial Loads: Because the gate rotors are positioned 180 degrees apart on opposite sides of the main rotor, the radial and axial forces of compression cancel each other out. This balanced loading design minimizes forces on the main shaft bearings, significantly extending bearing life.

Capacity Control: The Slide Valve

Screw compressors provide infinitely variable capacity control (typically from 10% to 100%) through the use of a movable slide valve.

• Mechanical Operation: The capacity slide valve is a shaped metal block positioned axially directly beneath the rotors. It is moved back and forth by a hydraulic piston supplied with compressor oil.
• Bypass Action: When the system load decreases, the slide valve is moved toward the discharge end. This uncovers a bypass slot in the rotor casing, allowing a portion of the suction gas to escape back to the suction inlet before it can be trapped and compressed. By shortening the active length of the rotors, the compressor compresses less gas, reducing capacity and motor power consumption.

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Volume Ratio (Vi) in Screw Compressors

Unlike reciprocating compressors, which use pressure-activated valves that open dynamically, a screw compressor has fixed mechanical ports. The point at which the suction pocket is sealed and the point at which it is exposed to the discharge manifold are physically fixed by the geometry of the rotors and the casing. This design introduces the Volume Ratio (Vi).

Definition and Formula

The Volume Ratio ($Vi$) is the ratio of the volume of the suction pocket at the moment the suction port closes ($V_i$) to the volume of that same pocket just as the discharge port is uncovered ($V_d$):

$Vi = \frac{V_i}{V_d}$

The internal pressure ratio ($Pi$) developed by the compressor is directly tied to the geometric $Vi$ by the equation:

$Pi = Vi^k$

where $k$ is the isentropic exponent of the refrigerant. For ammonia ($NH_3$), $k \approx 1.3$.

Matching Vi to System Pressures

To achieve peak thermodynamic efficiency, the compressor's internal pressure ratio ($Pi$) must exactly match the system's operating pressure ratio ($Pc / Pe$, where $Pc$ is condensing pressure and $Pe$ is evaporating pressure, both in absolute units, psia). If these ratios do not match, the compressor suffers from efficiency losses known as under-compression or over-compression.

1. Under-Compression

• Condition: Occurs when the system operating pressure ratio is higher than the compressor's internal pressure ratio ($Pc / Pe > Pi$).
• Effect: The gas inside the rotor pocket is not compressed all the way up to condensing pressure by the time the pocket reaches the discharge port. When the port opens, high-pressure gas from the discharge manifold flows backward into the pocket, instantly raising the pocket pressure to match the manifold.
• Consequences: The motor must do extra work to push this backflow gas out of the compressor. This causes thermodynamic losses, high power consumption, severe turbulence, gas pulsation, increased vibration, and a loud rumbling noise.

2. Over-Compression

• Condition: Occurs when the system operating pressure ratio is lower than the compressor's internal pressure ratio ($Pc / Pe < Pi$).
• Effect: The gas inside the rotor pocket is compressed to a pressure higher than the discharge manifold pressure before the port opens.
• Consequences: When the discharge port finally opens, the over-compressed gas expands down into the discharge manifold. The work done to compress the gas above the condensing pressure is completely wasted. This causes increased motor power draw, elevated discharge gas temperatures, and accelerated oil degradation.

Variable Vi Slide Valves

To prevent under- and over-compression losses, modern screw compressors incorporate a variable Vi slide valve (often running in tandem with the capacity slide valve). This valve dynamically adjusts the physical location of the discharge port opening. By moving the Vi slide valve, the compressor changes $V_d$ to match the changing system condensing and evaporating pressures, maintaining maximum efficiency across a wide range of operating conditions.

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Worked Example: Calculating Volume Ratio and Energy Loss

Consider an ammonia refrigeration system operating under two different seasonal conditions. The compressor has a fixed volume ratio ($Vi$) of $3.6$. The isentropic exponent ($k$) for ammonia is $1.3$.

Compressor Internal Pressure Ratio (Pi)

$Pi = Vi^k = 3.6^{1.3} \approx 5.23$

This means the compressor is geometrically designed to increase the suction gas pressure by a factor of $5.23$.

Case 1: High Summer Condensing Pressures (Under-Compression)

• Evaporating Temperature: $-20^\circ\text{F}$ (Saturated pressure $Pe = 18.3 \text{ psia}$)

• Condensing Temperature: $95^\circ\text{F}$ (Saturated pressure $Pc = 195.8 \text{ psia}$)

• System Pressure Ratio:

  $R_p = \frac{Pc}{Pe} = \frac{195.8}{18.3} \approx 10.7$

Because the system pressure ratio ($10.7$) is much higher than the compressor's internal pressure ratio ($5.23$), the gas will only be compressed to $18.3 \text{ psia} \times 5.23 = 95.7 \text{ psia}$ inside the rotors. Upon port opening, $195.8 \text{ psia}$ discharge gas backflows into the compressor. Severe under-compression occurs, wasting energy and causing high vibration. To eliminate this, a variable Vi compressor would adjust its discharge port to increase its geometric Vi to:

$$Vi_{required} = (10.7)^{1/1.3} = 10.7^{0.769} \approx 6.1$$

Case 2: Low Winter Condensing Pressures (Over-Compression)

• Evaporating Temperature: $-20^\circ\text{F}$ (Saturated pressure $Pe = 18.3 \text{ psia}$)

• Condensing Temperature: $55^\circ\text{F}$ (Saturated pressure $Pc = 99.4 \text{ psia}$)

• System Pressure Ratio:

  $R_p = \frac{Pc}{Pe} = \frac{99.4}{18.3} \approx 5.43$

In this case, the system pressure ratio ($5.43$) is very close to the compressor's design ratio ($5.23$). If the condensing temperature dropped further to $45^\circ\text{F}$ ($Pc = 80.5 \text{ psia}$), the system pressure ratio would be:

$$R_p = \frac{Pc}{Pe} = \frac{80.5}{18.3} \approx 4.40$$

Since the system pressure ratio ($4.40$) is now lower than the compressor's internal pressure ratio ($5.23$), the gas inside the rotors will be over-compressed to $95.7 \text{ psia}$ before being released into the $80.5 \text{ psia}$ manifold, wasting work. A variable Vi slide valve would decrease the geometric Vi to:

$$Vi_{required} = (4.40)^{0.769} \approx 3.1$$

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Troubleshooting Compressor Operations

1. Under-Compression Symptoms and Corrective Actions

• Symptoms: Increased noise level (a distinctive low-frequency rumbling), higher discharge line vibration, and reduced volumetric efficiency. If monitored, the compressor power draw will show an increase relative to the mass flow rate of the gas.
• Causes: Fixed-Vi compressor operating at a pressure ratio higher than its design, or a variable-Vi slider mechanism stuck in a low Vi position (due to oil debris or a faulty slide valve actuator solenoid).
• Corrective Actions: Verify the operation of the Vi slide valve actuator. Clean or replace control solenoids, inspect the slider feedback potentiometer (if equipped), and ensure the oil pressure feed to the hydraulic slide cylinder is within specification (typically $30\text{--}50\text{ psi}$ above discharge pressure).

2. Over-Compression Symptoms and Corrective Actions

• Symptoms: High discharge gas temperatures, elevated motor current draw (kW), and thermal expansion noise from the discharge piping.
• Causes: Fixed-Vi compressor operating at a pressure ratio lower than its design (e.g., during winter when condensing pressure drops), or variable-Vi slider stuck in a high Vi position.
• Corrective Actions: Implement condensing pressure control (e.g., floating head pressure controls) to optimize the system, or troubleshoot the variable Vi actuator. Ensure the slide valve calibration is accurate.

3. Reciprocating Compressor Valve Failure (Liquid Slugging)

• Symptoms: High-pitched metallic clicking or knocking in the cylinder head, sudden drop in discharge pressure accompanied by a rise in suction pressure (indicating gas bypassing back to the suction side), and rapid frosting of the compressor suction manifold.
• Causes: Liquid ammonia entering the compressor suction port (liquid slugging). Unlike vapor, liquid ammonia is incompressible. When the piston moves upward, the liquid cannot pass through the discharge valves fast enough, leading to extreme hydraulic pressures that break the suction/discharge valve reeds, valve plates, or even bend the connecting rods.
• Corrective Actions: Shut down the compressor immediately. Inspect and rebuild the valve plate assembly. Adjust the suction accumulator or surge drum level controls to prevent liquid carryover. Check the superheat setting at the evaporator expansion valves (ensuring at least $10^\circ\text{F}$ of superheat at the compressor inlet is maintained).

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Scroll and Centrifugal Compressors in Ammonia Systems

While reciprocating and screw compressors are the industry standards, other types have distinct roles or limitations in ammonia applications.

Scroll Compressors

Scroll compressors are positive displacement machines consisting of a stationary scroll and an orbiting scroll.

• Operation: The orbiting scroll moves in a circular path, trapping gas pockets at the outer edge and carrying them inward toward the center, reducing their volume.
• Ammonia Applications: Scroll compressors have no suction or discharge valves, leading to high volumetric efficiency and quiet operation. They are highly tolerant of liquid slugging because the scrolls can separate radially under hydraulic force. However, due to limited displacement sizes, they are rarely used in large industrial plants, finding use primarily in small package water chillers and low-capacity cascade refrigeration systems.

Centrifugal Compressors

Centrifugal compressors are dynamic, kinetic machines. They do not trap gas but accelerate it through a high-speed impeller and convert that velocity to pressure in a diffuser.

• Ammonia Molecular Weight Limitation: Ammonia ($NH_3$) has a molecular weight of $17.03 \text{ g/mol}$. In dynamic compression, the pressure rise (polytropic head) generated is inversely proportional to the molecular weight of the gas. Because ammonia is extremely light compared to halocarbon refrigerants, a centrifugal impeller must rotate at extremely high speeds (often exceeding $20,000 \text{ RPM}$) or use multiple impellers in series (multi-staging) to achieve the pressure lift required for standard industrial refrigeration. This mechanical complexity makes centrifugal compressors economically impractical for most refrigeration duties.
• The Hazard of Surge: Centrifugal compressors must maintain a minimum flow rate of refrigerant to function. If the system load drops and flow falls below this limit, the compressor experiences a condition called surge. The compressor can no longer generate enough head to overcome the discharge pressure, causing a sudden, violent reversal of gas flow from the discharge line back through the impeller. Surge causes rapid, severe pressure pulsations, extreme noise, high temperatures, and catastrophic mechanical damage to bearings, impellers, and seals.