Chiller Sizing Fundamentals

1. Refrigeration Principles for NGL Recovery

Mechanical refrigeration is used extensively in midstream gas processing to cool natural gas and NGL streams below temperatures achievable by ambient cooling alone. The primary purpose of refrigeration in NGL recovery is to condense heavier hydrocarbons (C₃+) from the gas stream by lowering the temperature below the hydrocarbon dewpoint. In plants that do not use turboexpander technology, mechanical refrigeration — typically using propane as the working fluid — provides the primary cooling for NGL extraction. Even in turboexpander-based plants, upstream refrigeration is often used to precool the inlet gas and to provide supplemental cooling for specific process streams.

The fundamental thermodynamic cycle underlying all mechanical refrigeration in gas processing is the vapor-compression cycle. A liquid refrigerant at high pressure is throttled (flashed) through an expansion valve to a lower pressure, producing a cold two-phase mixture. This cold refrigerant absorbs heat from the process stream in the chiller (evaporator), boiling the refrigerant at constant pressure and temperature. The low-pressure refrigerant vapor is then compressed back to the condensing pressure by the refrigerant compressor, condensed against ambient cooling (air or water), and returned to the expansion valve to repeat the cycle.

Coefficient of Performance

The thermodynamic efficiency of a refrigeration cycle is expressed as the coefficient of performance (COP), defined as the ratio of useful refrigeration (heat absorbed from the process) to the work input (compressor power):

Where Q_evap is the evaporator duty (BTU/hr) and W_comp is the compressor shaft power (BTU/hr equivalent). For a propane refrigeration system operating between a −30°F evaporating temperature and a 120°F condensing temperature, the ideal Carnot COP is approximately 2.9, and the actual COP with real equipment efficiencies is typically 2.0–2.5. This means that for every BTU of cooling provided, the refrigerant compressor consumes 0.4–0.5 BTU of work input.

Single-Stage vs. Multi-Stage Refrigeration

The required evaporating temperature determines whether a single-stage or multi-stage refrigeration system is needed. Single-stage propane systems are economical for process temperatures down to approximately −20°F to −30°F. Below this range, the compression ratio across a single stage becomes excessive (greater than approximately 5:1), leading to high discharge temperatures, reduced volumetric efficiency, and poor COP.

Multi-stage (typically two-stage or three-stage) propane systems extend the practical cooling range to −40°F. Each stage operates at a different evaporating pressure, with interstage economizers that flash the liquid refrigerant to produce intermediate-temperature cooling and reduce the total compressor power. A three-stage propane system with evaporating levels at +20°F, −10°F, and −40°F is a common configuration in NGL recovery plants.

Economizer Operation

Economizers (also called flash drums or flash tanks) are integral to multi-stage refrigeration systems. At each intermediate pressure level, a portion of the high-pressure liquid refrigerant is flashed to produce vapor that is routed directly to the corresponding interstage suction of the compressor. This flash vapor provides cooling to subcool the remaining liquid before it passes to the next lower pressure stage. The economizer reduces the mass flow of refrigerant that must be compressed through the lowest-pressure stage, thereby reducing total compressor power by 10–20% compared to a system without economizers at the same evaporating temperature.

2. Refrigerant Selection

The choice of refrigerant determines the achievable chiller temperature, the system operating pressures, the required compressor size, and the overall efficiency of the refrigeration cycle. In midstream gas processing, propane is the dominant refrigerant, but mixed refrigerants, ethane, and ethylene are also used for specific applications requiring lower temperatures.

Propane (R-290)

Propane is the standard refrigerant for NGL recovery plants because it is readily available on-site (as a product or byproduct of the gas processing), has favorable thermodynamic properties, and operates at pressures compatible with standard industrial equipment. Propane evaporates at −43.7°F at atmospheric pressure, providing a practical minimum evaporating temperature of approximately −40°F (with slight positive pressure to prevent air infiltration). Condensing against ambient air or water at 100–120°F produces condensing pressures of 175–250 psig, which are well within the range of standard reciprocating or screw compressors.

Key advantages of propane refrigeration include its compatibility with hydrocarbon process streams (no contamination concerns if a tube leak occurs), its availability at gas processing facilities, well-established design and operating procedures, and the large installed base of propane refrigeration equipment in the midstream industry.

Mixed Refrigerants (MR)

Mixed refrigerant systems use a blend of hydrocarbons (typically methane, ethane, propane, and butane) or a hydrocarbon-nitrogen mixture as the working fluid. Unlike a pure-component refrigerant that boils at a constant temperature, a mixed refrigerant evaporates over a temperature range (glide) that can be tailored to match the cooling curve of the process stream. This temperature glide reduces the thermodynamic irreversibility of the heat exchange, improving the overall cycle efficiency compared to a pure-component system operating between the same temperature extremes.

Mixed refrigerant technology is used in two primary applications in gas processing: single mixed refrigerant (SMR) processes for LNG liquefaction and mixed refrigerant NGL recovery processes that replace the traditional propane + turboexpander combination. In NGL service, a properly optimized MR system can achieve cooling to −60°F to −100°F in a single refrigeration loop, eliminating the need for separate propane and ethane/ethylene circuits.

Refrigerant Property Comparison

Cascade Refrigeration

When the required process temperature is below the practical range of a single refrigerant (−40°F for propane), a cascade refrigeration system is used. In a cascade system, two or more refrigeration cycles are thermally connected in series: the condenser of the lower-temperature cycle is cooled by the evaporator of the higher-temperature cycle. The most common cascade arrangement in gas processing is propane (high-temperature loop, −40°F) cascaded with ethane or ethylene (low-temperature loop, down to −120°F to −150°F).

The cascade heat exchanger, where the propane evaporator cools the ethane/ethylene condenser, is a critical design element. The approach temperature across this exchanger (typically 5–10°F) directly affects the operating pressures of both loops and the total compressor power. A closer approach reduces compressor power but requires a larger (more expensive) cascade exchanger.

Refrigerant Selection Criteria

• Target temperature — The required process outlet temperature determines the minimum evaporating temperature and therefore the refrigerant or refrigerant combination
• Available compression — Higher-pressure refrigerants (ethane, ethylene) require compressors rated for higher discharge pressures and may need special metallurgy
• Process compatibility — Hydrocarbon refrigerants are compatible with hydrocarbon process streams; a tube leak introduces the same components already present in the process
• Site conditions — Ambient temperature affects condensing pressure; hot-climate installations require higher condensing pressures or larger air-cooled condensers
• Turndown requirements — Multi-stage propane systems offer good turndown; mixed refrigerant systems may require composition adjustment at reduced throughput

3. Cooling Curve Analysis

The cooling curve is the fundamental tool for chiller design. It plots the temperature of the process stream against the cumulative heat removed (duty) as the stream cools from the inlet temperature to the target outlet temperature. For multi-component hydrocarbon streams, the cooling curve is not a simple straight line because phase change (condensation of heavier components) occurs over a range of temperatures, producing regions of varying heat capacity and heat transfer coefficient.

Construction of Cooling Curves

A rigorous cooling curve for a natural gas or NGL stream requires vapor-liquid equilibrium (VLE) calculations at multiple temperature increments along the cooling path. At each temperature point, a flash calculation determines the vapor and liquid compositions, the enthalpy of each phase, and the total enthalpy of the two-phase mixture. The cooling curve is then constructed by plotting temperature vs. cumulative enthalpy change from the inlet condition.

The shape of the cooling curve reveals important design information. A steep section indicates gas-phase cooling with relatively low heat capacity (sensible heat removal only). A flatter section indicates simultaneous condensation and cooling, where latent heat of condensation supplements the sensible heat and increases the effective heat capacity. The point where the curve begins to flatten is the dewpoint of the mixture, and the point where it steepens again near the outlet indicates that condensation is largely complete and the remaining cooling is primarily sensible heat removal from the liquid phase.

Composite Cooling Curves

In chiller design, both the hot composite curve (process stream being cooled) and the cold composite curve (refrigerant being heated/evaporated) are plotted on the same temperature-duty diagram. For a pure-component refrigerant such as propane, the cold curve is a horizontal line at the evaporating temperature (constant temperature during boiling). For a mixed refrigerant, the cold curve has a slope (temperature glide) that can be matched to the process stream cooling curve.

The vertical distance between the hot and cold composite curves at any point along the duty axis represents the local temperature driving force for heat transfer. The minimum vertical distance between the curves is the minimum approach temperature (ΔT_min), also called the pinch point. This pinch point constrains the chiller design: a smaller approach temperature requires more heat transfer area but reduces the thermodynamic irreversibility and potentially allows a warmer (higher-pressure) evaporating temperature, reducing compressor power.

Zone Analysis

Because the heat transfer coefficient and temperature driving force vary along the length of the chiller, the cooling curve is divided into zones for accurate sizing. Each zone represents a section of the chiller where the process conditions (phase fraction, fluid properties, flow regime) and the corresponding heat transfer coefficient are approximately uniform. Typical zones include:

• Gas cooling zone — Process stream is entirely vapor; heat transfer coefficient is governed by gas-phase convection (typically 30–80 BTU/hr·ft²·°F for hydrocarbon gases at moderate pressures)
• Condensation zone — Heavier hydrocarbons are condensing from the vapor; heat transfer coefficient increases significantly due to film condensation (typically 80–200 BTU/hr·ft²·°F)
• Two-phase cooling zone — Mixture of vapor and liquid is being cooled; heat transfer coefficient depends on liquid fraction and flow regime
• Liquid cooling zone — Process stream is entirely liquid; heat transfer coefficient is governed by liquid-phase convection (typically 100–250 BTU/hr·ft²·°F)

Each zone has its own LMTD and overall heat transfer coefficient (U), and the required area for each zone is calculated independently. The total chiller area is the sum of the zone areas.

4. LMTD and Chiller Sizing

The log mean temperature difference (LMTD) is the fundamental driving force parameter used to size heat exchangers, including chillers. It provides the effective average temperature difference between the hot (process) and cold (refrigerant) streams, accounting for the logarithmic temperature profile along the exchanger length. For a single-pass counterflow or parallel-flow arrangement, the LMTD is calculated as:

Where ΔT₁ is the temperature difference at one end of the exchanger and ΔT₂ is the temperature difference at the other end. For a chiller with a pure-component refrigerant (constant evaporating temperature), the calculation is straightforward: ΔT₁ is the process inlet temperature minus the refrigerant evaporating temperature, and ΔT₂ is the process outlet temperature minus the refrigerant evaporating temperature.

LMTD Correction Factor

For multi-pass shell-and-tube exchangers (which most gas processing chillers are), the LMTD must be corrected with a configuration factor F_t to account for the deviation from true counterflow. The corrected LMTD is:

The correction factor F_t depends on the number of shell passes, the number of tube passes, and the temperature effectiveness of the exchanger. For a 1-shell/2-tube-pass configuration (the most common for chillers), F_t typically ranges from 0.80 to 0.95. Values below 0.75 indicate that the selected shell-and-tube configuration is thermodynamically inefficient, and additional shell passes or a different exchanger type should be considered.

For chillers using a pure-component refrigerant that evaporates at constant temperature, the cold-side temperature is constant along the entire exchanger length. In this special case, the F_t factor is 1.0 regardless of the number of passes, because the cold-side temperature profile is flat and there is no temperature cross. This is one reason propane chillers are thermodynamically efficient despite using simple shell-and-tube construction.

Overall Heat Transfer Coefficient

The overall heat transfer coefficient (U) combines the resistances of the tube-side fluid film, the tube wall, fouling on both sides, and the shell-side fluid film into a single parameter:

Where h_i and h_o are the tube-side and shell-side film coefficients, R_fi and R_fo are the fouling resistances, t_w is the tube wall thickness, k_w is the wall thermal conductivity, and r_o/r_i is the ratio of outside to inside tube radius.

Typical U Values for Gas Processing Chillers

Sizing Calculation

The required heat transfer area is determined from the fundamental heat exchanger equation:

Where A is the required area (ft²), Q is the total heat duty (BTU/hr), U is the overall heat transfer coefficient, and LMTD is the log mean temperature difference. When the cooling curve is divided into multiple zones, this calculation is performed for each zone independently:

Design Margin

Standard design practice adds margin to the calculated area to account for uncertainties in fluid properties, fouling rates, and operating variations. Typical design margins for gas processing chillers are:

• Clean service, well-defined composition: 10–15% excess area
• Variable composition or inlet conditions: 15–25% excess area
• Fouling service (waxy crude, high-paraffin condensate): 25–40% excess area
• Process with uncertain thermodynamic data: 20–30% excess area

Excessive overdesign should be avoided because an oversized chiller operates with a reduced temperature driving force, which can lead to laminar flow on the process side, reduced heat transfer coefficients, and potentially inadequate tube velocity to prevent fouling deposits.

5. Mechanical Design Considerations

Gas processing chillers are shell-and-tube heat exchangers designed per TEMA (Tubular Exchanger Manufacturers Association) standards and constructed per ASME Section VIII. The most common configuration places the boiling refrigerant on the shell side and the process stream on the tube side, designated as a TEMA AES or BEM type depending on the head and shell configuration.

Kettle-Type vs. Flooded Chillers

Two shell-side configurations are used for the refrigerant in gas processing chillers:

• Kettle-type (TEMA AKT) — The tube bundle is submerged in a pool of boiling refrigerant in an oversized shell. Vapor disengages above the liquid surface and exits through the top nozzle. The shell diameter is typically 1.5–2.0 times the bundle diameter to provide adequate disengagement space. Kettle chillers offer stable operation, uniform shell-side temperature, and tolerance for liquid level variations, but require more refrigerant charge and a larger shell
• Flooded (TEMA AES/BEM) — The tube bundle occupies most of the shell cross-section, and the refrigerant liquid level is maintained just above the top tube row by a level control valve. Flooded chillers use less refrigerant charge and smaller shells than kettle types, but require more careful level control to prevent tube starvation (dry-out) at the top of the bundle

For most NGL recovery applications, kettle-type chillers are preferred because they provide more forgiving operation, accommodate load variations without risk of tube dry-out, and simplify the refrigerant level control scheme.

Tube Material and Layout

Chiller tubes in hydrocarbon service are typically carbon steel (SA-179 or SA-214) for temperatures above −20°F. For cryogenic service below −20°F, low-temperature carbon steel (SA-334 Gr 1) or austenitic stainless steel (SA-249 TP304) is required to meet ASME impact test requirements. Tube diameters of 3/4-inch or 1-inch OD are standard, with wall thicknesses of 14 BWG (0.083 inch) or 12 BWG (0.109 inch).

Tube layout is typically triangular pitch (30-degree) for chillers where the shell side does not require mechanical cleaning (boiling refrigerant is inherently clean). The tube pitch is usually 1.0 inch for 3/4-inch tubes (1.33 pitch ratio) or 1.25 inches for 1-inch tubes. Tube lengths range from 16 to 24 feet for standard installations.

Cryogenic Design Requirements

Vibration and Tube Support

Shell-side boiling can induce flow-induced vibration (FIV) in the tube bundle, particularly at the outlet end of the bundle where the vapor velocity is highest. Baffle spacing and tube support plates must be designed to limit the unsupported tube span to below the critical span for acoustic resonance and fluidelastic instability. TEMA guidelines recommend maximum unsupported spans based on tube OD and material, but additional analysis per HTRI or ASME PTC 30 methods is advisable for large chillers with high shell-side vapor generation rates.

Refrigerant Piping and Safety

The refrigerant piping system connecting the chiller to the compressor and condenser must be sized for the volumetric flow of low-pressure refrigerant vapor, which is the largest-diameter piping in the refrigeration circuit. Excessive pressure drop in the suction piping raises the effective evaporating temperature and reduces the cooling capacity. Standard practice limits suction line pressure drop to 1–2 psi equivalent, which corresponds to 1–2°F of evaporating temperature penalty.

Safety considerations for propane refrigerant systems include relief valve sizing per API 521 for fire exposure, leak detection systems, refrigerant inventory management, and compliance with OSHA PSM (Process Safety Management) requirements for facilities with more than 10,000 lb of propane refrigerant. Emergency depressuring provisions must account for the large liquid inventory in kettle-type chillers and the flammable nature of hydrocarbon refrigerants.

Performance Monitoring

Key performance indicators for chiller operation include:

• Process outlet temperature — Direct measure of chiller cooling capacity; deviation from design indicates fouling, reduced refrigerant flow, or loss of refrigerant charge
• Approach temperature — Difference between process outlet and refrigerant evaporating temperature; increasing approach indicates fouling or reduced U value
• Refrigerant liquid level — Proper level ensures full tube submergence in kettle types; low level causes tube dry-out and reduced performance
• Shell-side pressure drop — Increasing pressure drop may indicate liquid carry-over, frosting, or internal fouling
• Compressor suction pressure — Lower suction pressure indicates higher chiller load or excessive suction line pressure drop

References

1. GPSA, Chapter 14 — Refrigeration
2. GPSA, Chapter 9 — Heat Exchangers
3. TEMA Standards of the Tubular Exchanger Manufacturers Association, 10th Edition
4. ASME Boiler and Pressure Vessel Code, Section VIII, Division 1
5. API Standard 521 — Pressure-Relieving and Depressuring Systems