Cascaded Refrigeration Fundamentals

1. Multi-Stage Refrigeration Principles

Single-stage vapor-compression refrigeration becomes impractical when the required temperature lift between evaporator and condenser exceeds a certain threshold. The fundamental limitation is the compression ratio: as the ratio of condenser pressure to evaporator pressure increases, compressor volumetric efficiency drops, discharge temperatures rise to levels that degrade lubricating oil, and overall coefficient of performance (COP) deteriorates rapidly. Industry practice limits single-stage compression ratios to approximately 6:1 for reciprocating compressors and 4:1 for screw compressors.

For midstream gas processing applications that require cooling to temperatures below −40°F, single-stage propane refrigeration cannot provide adequate performance. The evaporator pressure drops below atmospheric (creating air-ingress risk), the compression ratio exceeds practical limits, and the COP falls below economically viable levels. Multi-stage or cascaded refrigeration overcomes these limitations by dividing the total temperature lift into smaller increments, each handled by an independent or coupled refrigeration stage.

Why Single-Stage Fails at Large Temperature Lifts

Optimal Interstage Temperature Selection

For a two-stage system operating between an overall cold temperature T_cold and hot temperature T_hot, the optimal intermediate temperature that equalizes the COP (and approximately the compression ratio) across both stages is found using the geometric mean of the absolute temperatures:

Where T_R denotes absolute temperature in degrees Rankine (°R = °F + 459.67). This geometric-mean selection ensures that the compression ratio is equal across both stages, which minimizes the total compressor power for a given overall temperature lift. For a three-stage system, the two intermediate temperatures are found by dividing the absolute temperature range into three equal geometric intervals:

COP Improvement with Multiple Stages

The overall COP of a multi-stage system is defined as the ratio of useful cooling duty to total compressor power input. The improvement over single-stage operation arises from two effects: reduced compression ratio per stage (improving volumetric and isentropic efficiency) and reduced flash gas losses at each expansion step. Typical COP improvements are summarized below:

The Carnot COP sets the theoretical upper bound for any refrigeration system and depends only on the absolute temperatures of the heat source and sink:

Real systems achieve 30–50% of Carnot COP depending on compressor efficiency, heat exchanger approach temperatures, and pressure drops. Multi-stage cascading improves the fraction of Carnot efficiency achieved by reducing irreversibilities in each compression and expansion step.

2. Cascade System Configurations

A cascade refrigeration system uses two or more independent vapor-compression cycles thermally coupled through a cascade heat exchanger. The condenser of the lower-temperature (colder) cycle rejects heat to the evaporator of the higher-temperature (warmer) cycle. Each cycle uses a different refrigerant optimized for its operating temperature range. This is distinct from a multi-stage system that uses a single refrigerant with multiple compression stages and economizers, though the terms are often used interchangeably in industry.

Propane / Ethylene Cascade

The most common cascade arrangement in midstream gas processing uses propane in the warm stage and ethylene in the cold stage. Propane condenses against ambient air or cooling water at 100–130°F and evaporates at −20°F to −40°F. Ethylene condenses against the propane evaporator in the cascade heat exchanger at approximately −30°F to −50°F and evaporates at the required process temperature, typically −100°F to −150°F for deep NGL recovery. This configuration provides process cooling to approximately −150°F with compression ratios of 3:1 to 5:1 per stage.

Propane / Ethane Cascade

Where ethane is available from the gas processing plant itself, a propane/ethane cascade can be more economical than propane/ethylene. Ethane has similar thermodynamic properties to ethylene but is typically available on-site as a product stream, eliminating the need to purchase and store a separate refrigerant. The ethane stage can reach evaporator temperatures of −120°F to −140°F. However, ethane has a lower critical temperature (90°F) than ethylene (49°F), requiring careful design of the cascade condenser approach temperature.

Three-Stage: Propane / Ethylene / Methane

For cryogenic applications requiring temperatures below −150°F, such as LNG production or deep ethane recovery, a third stage using methane (or mixed refrigerant) is added. The methane stage condenses against the ethylene evaporator at approximately −140°F to −160°F and evaporates at −250°F to −260°F, approaching methane liquefaction temperatures. This three-stage cascade is the basis of the classical cascade LNG process.

Refrigerant Selection by Temperature Range

Cascade Condenser / Evaporator Design

The cascade heat exchanger is the critical link between stages. In this exchanger, the cold-stage refrigerant condenses on one side while the warm-stage refrigerant evaporates on the other. The approach temperature (temperature difference between the condensing cold-stage refrigerant and the evaporating warm-stage refrigerant) is typically 5–8°F for kettle-type cascade exchangers and 3–5°F for plate-fin exchangers. A smaller approach temperature improves overall system COP but requires more heat exchanger surface area and capital cost. The economic optimum is determined by balancing incremental exchanger cost against compressor power savings.

3. Compressor Considerations

Compressor selection for cascade refrigeration systems depends on the refrigerant, required capacity, compression ratio, and operating temperature range. Each stage may use a different compressor type based on these factors.

Reciprocating Compressors with Intercoolers

Reciprocating compressors are widely used in small to medium cascade systems (up to approximately 2,000 HP per stage). For multi-stage compression within a single refrigerant loop, intercooling between stages reduces discharge temperature and improves volumetric efficiency. In a two-stage reciprocating compressor, gas is compressed in the first-stage cylinders, cooled in an intercooler (typically to within 15–20°F of suction temperature), and then compressed in the second-stage cylinders. This arrangement keeps discharge temperatures below the 275°F limit and improves overall compression efficiency by 8–15% compared to single-stage compression over the same pressure ratio.

Screw Compressors

Rotary screw compressors are commonly used for individual stages in cascade systems, particularly for the propane stage where capacities of 500–5,000 HP are typical. Screw compressors offer the advantages of continuous compression (no pulsation), oil injection for cooling and sealing, and the ability to handle moderate liquid carryover. Single-stage screw compressors can achieve compression ratios up to 4.5:1 efficiently. For the ethylene or ethane stage, screw compressors are also used but require special materials and sealing systems rated for the lower temperatures (−150°F and below).

Centrifugal Compressors for Large Systems

For large-capacity installations such as baseload LNG plants (above 5,000 HP per stage), centrifugal compressors offer superior efficiency and reliability. Multi-stage centrifugal compressors with two or three impellers per casing can achieve the required pressure ratios within a single machine. Gas turbine or electric motor drivers are typical. The propane stage in large LNG facilities commonly uses multi-stage centrifugal compressors with capacities of 20,000–100,000 HP. Frame size selection is based on inlet volumetric flow (ACFM) and the required head per stage.

Inter-Stage Cooling and Desuperheating

Desuperheating the compressor discharge gas is essential in cascade systems to protect the cascade condenser and improve heat transfer. Desuperheating is accomplished by liquid injection (spraying subcooled liquid refrigerant into the discharge line), external desuperheaters (shell-and-tube exchangers using cooling water), or economizers that flash intermediate-pressure liquid to cool the discharge gas while providing additional refrigeration capacity. Liquid injection is the simplest method but reduces system COP slightly because the injected liquid must be re-compressed. External desuperheaters add capital cost but do not penalize the thermodynamic cycle.

Compressor Sizing by Stage

4. Heat Exchanger Design

Heat exchangers in cascade refrigeration systems serve three primary functions: process cooling (chiller/evaporator), heat rejection to ambient (condenser), and inter-stage thermal coupling (cascade condenser/evaporator). The design and selection of these exchangers has a significant impact on system efficiency, capital cost, and operability.

Cascade Condenser Design

The cascade condenser is where one refrigerant condenses while another evaporates. This simultaneous phase change on both sides produces high heat transfer coefficients and compact exchanger designs. The overall heat transfer coefficient U for condensing/evaporating service is typically 100–200 Btu/hr·ft²·°F for shell-and-tube exchangers and 200–400 Btu/hr·ft²·°F for plate-fin (brazed aluminum) exchangers.

The heat duty of the cascade condenser equals the condenser duty of the cold stage, which is the sum of the cold-stage evaporator duty plus the cold-stage compressor work:

Approach Temperatures

The approach temperature in the cascade condenser is the temperature difference between the condensing cold-stage refrigerant and the evaporating warm-stage refrigerant. Typical values by exchanger type are:

LMTD and Sizing

Because both sides of the cascade condenser involve phase change (isothermal or near-isothermal processes), the log-mean temperature difference (LMTD) simplifies to approximately the approach temperature itself. The required surface area is calculated from:

Where Q is the cascade condenser duty (Btu/hr), U is the overall heat transfer coefficient, and ΔT_approach is the approach temperature. Fouling factors for clean refrigerant service are small (0.0005–0.001 hr·ft²·°F/Btu) but must be included per TEMA standards.

Kettle-Type vs. Thermosiphon

For the cascade condenser application, kettle-type reboiler/evaporators are the most common choice in midstream gas processing. The cold-stage refrigerant condenses inside the tubes while the warm-stage refrigerant evaporates in the shell-side pool. The kettle configuration provides a disengagement space above the tube bundle for liquid-vapor separation, eliminating the need for a separate knockout drum. Thermosiphon arrangements use natural circulation driven by the density difference between the two-phase mixture in the riser and the liquid in the downcomer. Thermosiphon exchangers require more plot space and piping but can handle higher heat fluxes without dry-out.

Process Chiller (Evaporator) Design

The process chiller cools the actual process stream (gas, NGL, or liquid hydrocarbon) against the evaporating refrigerant. Shell-and-tube chillers with refrigerant evaporating on the shell side and process fluid on the tube side are standard. For cryogenic service below −150°F, brazed aluminum plate-fin exchangers are preferred due to their high surface-area-to-volume ratio and multi-stream capability. Approach temperatures for process chillers are typically 8–15°F, depending on the trade-off between exchanger cost and compressor power.

5. Applications and Economics

NGL Recovery (Cryogenic)

Cascade refrigeration is essential for cryogenic NGL recovery processes that target high ethane recovery (90%+). The gas subcooled process and similar cryogenic schemes require cooling the inlet gas to −100°F to −160°F, well beyond the capability of single-stage propane refrigeration. A two-stage propane/ethylene cascade provides the external refrigeration, while turboexpanders provide additional cooling through isentropic expansion. The cascade system supplies the refrigeration to cool the inlet gas, condense heavier hydrocarbons, and maintain the cold separator at the required temperature. Capital cost for the cascade refrigeration system typically represents 25–35% of the total NGL plant cost.

LNG Production

Baseload LNG plants have historically used three-stage cascade refrigeration (propane, ethylene, methane) or mixed-refrigerant processes. The classical cascade process provides precise temperature control at each level and simple operability, though it requires more equipment than mixed-refrigerant alternatives. Peak-shaving LNG facilities, which are smaller in capacity, often use a simplified two-stage cascade or nitrogen expander cycle. The choice between cascade and mixed-refrigerant processes depends on plant capacity, feed gas composition, ambient conditions, and operator preference.

Petrochemical Cooling

Ethylene plants, propylene units, and other petrochemical facilities use cascade refrigeration to provide process cooling at multiple temperature levels simultaneously. The cascade system can deliver cooling at −40°F, −100°F, and −150°F from a single integrated system by taking side-draws from the appropriate evaporator pressure level. This multi-level cooling capability makes cascade systems particularly well suited to complex petrochemical processes with diverse cooling requirements.

Capital vs. Operating Cost Tradeoffs

Efficiency Benchmarks

Industry benchmarks for cascade refrigeration system efficiency are typically expressed as specific power consumption (HP per MMBTU/hr of cooling) or as a fraction of Carnot COP:

Key factors that drive efficiency improvements include tight cascade condenser approach temperatures (3–5°F), high compressor isentropic efficiency (78–85%), proper interstage temperature selection using the geometric-mean method, and minimizing pressure drops through suction piping and heat exchangers. A well-designed two-stage cascade system can achieve 15–25% power savings over single-stage compression for the same cooling duty and temperature span. For three-stage systems spanning more than 300°F of temperature lift, the power savings can exceed 30–40%, making the additional capital investment economically justified for continuous operation.

References

1. GPSA, Chapter 14 — Refrigeration
2. ASHRAE Handbook — Fundamentals, Thermodynamics and Refrigeration Cycles
3. API RP 686 — Recommended Practice for Machinery Installation and Installation Design
4. TEMA Standards — Tubular Exchanger Manufacturers Association