Low Pressure Receiver Systems

System Overview

Low-pressure receivers (LPR) serve as the central component in liquid overfeed refrigeration systems, operating at evaporator pressure to separate vapor from liquid refrigerant returning from evaporators. The LPR maintains a constant supply of saturated liquid refrigerant for continuous circulation while accumulating flash gas generated during the expansion process.

Primary Functions

The low-pressure receiver performs multiple critical functions:

Vapor-Liquid Separation - Removes flash gas and entrained vapor from liquid refrigerant returning from evaporators, ensuring only liquid feeds the recirculation pump. Separation efficiency directly impacts pump performance and system capacity.

Refrigerant Storage - Maintains adequate liquid inventory to supply all connected evaporators under varying load conditions. Storage capacity must accommodate system charge migration during load changes, defrost cycles, and pump-down operations.

Surge Damping - Absorbs sudden changes in liquid return flow caused by rapid load variations, defrost termination, or multiple evaporators cycling simultaneously. Inadequate surge capacity results in liquid level instability and potential pump cavitation.

Pressure Equalization - Establishes a common suction pressure reference point for all connected evaporators operating at the same temperature. The receiver pressure determines the system’s saturation temperature.

Receiver Design and Construction

Vessel Configuration

Low-pressure receivers are fabricated as horizontal or vertical cylindrical pressure vessels complying with ASME Section VIII Division 1 requirements.

Horizontal Receivers - Preferred for most installations due to superior vapor-liquid separation characteristics and larger liquid surface area for flash gas release. Horizontal orientation provides stable liquid level control and better accommodates internal components.

Vertical Receivers - Used when floor space is limited or when tall vessels improve gravity drainage from elevated evaporators. Vertical configuration requires larger diameter for equivalent liquid volume and may require additional separation enhancement.

Dimensional Sizing

Receiver sizing depends on multiple factors requiring careful analysis:

Internal Components

Inlet Baffle - Deflects high-velocity liquid return flow to prevent turbulence and re-entrainment. Baffle design dissipates kinetic energy and directs liquid downward into the liquid pool.

Demister Pad - Wire mesh pad installed in vapor space removes entrained liquid droplets from vapor flowing to the compressor suction. Typical efficiency exceeds 99% for droplets larger than 10 microns at design vapor velocity.

Liquid Outlet - Located at the lowest point in horizontal receivers to ensure pump draws only liquid. Outlet pipe extends into vessel with perforated screen to prevent vortexing and vapor ingestion.

Level Instrumentation Connections - Multiple tapped connections at varying elevations accommodate different level control technologies. Connections must be sized for specific instruments and located to avoid turbulent zones.

Material Selection

Receiver shells are constructed from carbon steel for ammonia systems and carbon steel or stainless steel for halocarbon refrigerants. Material thickness is determined by design pressure, vessel diameter, and code requirements.

Receiver Sizing Methodology

Volume Calculations

Total receiver volume equals the sum of working liquid volume, surge capacity, vapor space, and unusable dead volume.

Working Liquid Volume - Calculated based on the total refrigerant circulation rate and desired residence time, typically 1-3 minutes at design load:

V_working = (CR × t) / (ρ_liquid × 60)

Where:

• V_working = Working liquid volume (ft³)
• CR = Total circulation rate (lb/min)
• t = Residence time (minutes)
• ρ_liquid = Liquid density (lb/ft³)

Surge Capacity - Additional volume to absorb transient liquid return without overflow or pump starvation. Minimum surge equals the largest single defrost termination load plus simultaneous load variations on operating evaporators.

Vapor Space - Minimum 30% of total volume to accommodate vapor-liquid interface fluctuations and provide adequate separation zone above the liquid surface.

Practical Sizing Example

For an ammonia system with 10,000 lb/hr total circulation rate at 20°F suction temperature:

• Circulation rate: 10,000 lb/hr ÷ 60 = 167 lb/min
• Residence time: 2 minutes (typical)
• Liquid density at 20°F: 38.5 lb/ft³
• Working volume: (167 × 2) ÷ 38.5 = 8.7 ft³
• Surge capacity (50%): 4.3 ft³
• Vapor space requirement: 40% of liquid volume
• Total liquid volume: 8.7 + 4.3 = 13 ft³
• Required vapor space: 5.2 ft³
• Total vessel volume: 18.2 ft³ minimum

Select standard receiver with 20 ft³ capacity providing design margin.

Level Control Methods

Float-Operated Control

Mechanical float switches or modulating float valves maintain liquid level within operating range without requiring external power.

Ball Float Switches - SPDT or DPDT switches activate at preset levels to control makeup solenoid valves or provide high/low level alarms. Typical differential between on/off points is 2-6 inches.

Modulating Float Valves - Direct-acting valves throttle makeup liquid flow proportionally to liquid level, providing stable control without electrical components. Limited to smaller systems due to valve capacity constraints.

Electronic Level Controls

Capacitance Probes - Measure dielectric constant changes between refrigerant liquid and vapor. Single probe provides continuous level indication with programmable switch points. Accuracy ±0.5% of probe length.

Displacer-Type Controls - Buoyant displacer element responds to liquid level changes, actuating pneumatic or electronic transmitters. Provides proportional output for modulating control strategies.

Ultrasonic Sensors - Non-contact measurement using ultrasonic pulses reflected from liquid surface. Eliminates refrigerant exposure but requires vapor space free of excessive turbulence or foam.

Differential Pressure Transmitters - Measure hydrostatic head between top and bottom vessel connections. Output signal proportional to liquid level independent of density for most applications.

Operating Procedures

System Startup

Initial Charging - Liquid refrigerant is charged into the receiver until the desired operating level is established. Total system charge includes receiver inventory plus liquid in evaporators, piping, and auxiliary components.

1. Evacuate receiver and connected piping to 500 microns or less
2. Introduce initial refrigerant charge as liquid into receiver
3. Start recirculation pump and verify liquid flow to evaporators
4. Add refrigerant incrementally until normal operating level is achieved
5. Monitor suction pressure and verify proper evaporator superheat

Pump-Down Sequence - Before starting the recirculation pump, ensure adequate liquid level prevents pump cavitation and vapor lock.

Normal Operation

The receiver operates at relatively constant pressure corresponding to the design evaporator temperature. Liquid level fluctuates within the control band as load varies, with the level control system modulating makeup liquid flow from the high-pressure side.

Makeup Control - Solenoid valve or modulating expansion valve admits high-pressure liquid refrigerant from the condenser or high-pressure receiver. Flashing during expansion generates vapor that must be vented to prevent pressure buildup.

Level Management - Maintain level between 40-60% of horizontal receiver diameter or 30-50% of vertical receiver height during normal operation. Excessive level reduces vapor space and separation effectiveness while low level risks pump cavitation.

Pump-Down Procedures

Routine Pump-Down - Required for maintenance, equipment isolation, or system troubleshooting.

1. Close liquid supply valve to isolated section
2. Continue operating recirculation pump to transfer refrigerant to receiver
3. Monitor suction pressure rise as refrigerant migrates from evaporators
4. Stop pump when evaporator superheat exceeds 50°F or pressure stabilizes
5. Close isolation valves and verify no pressure equalization

Emergency Pump-Down - Rapid refrigerant transfer from affected evaporators to receiver during refrigerant leak or equipment failure. Receiver must have adequate capacity to accept full evaporator charge without overfilling.

Safety Considerations

Pressure Relief Protection

Every low-pressure receiver requires pressure relief protection per ASHRAE 15 and applicable codes. Relief devices protect against overpressure from external heat exposure, control system failure, or blocked discharge scenarios.

Relief Valve Sizing - Calculated based on fire exposure scenario per ASME Section VIII or CGA S-1.3 for ammonia systems. Required relieving capacity depends on vessel surface area exposed to fire.

Rupture Disc Backup - Many jurisdictions require dual protection with pressure relief valve and rupture disc in series for ammonia systems. Rupture disc provides backup protection if relief valve fails to reseat.

High/Low Level Alarms

Critical safety alarms prevent hazardous operating conditions:

High Level Alarm - Activates before liquid reaches demister pad or vapor outlet, typically at 75-80% of vessel diameter. Prevents liquid carryover to compressor suction causing liquid slugging and potential mechanical damage.

Low Level Alarm - Warns of insufficient liquid inventory before pump cavitation occurs, typically at 20-25% of vessel diameter. Triggers before automatic pump shutdown protection.

Overfill Prevention

Automatic Shutdowns - High-high level switches stop recirculation pump and close liquid makeup valve to prevent continued filling. Reset requires manual intervention after investigating root cause.

Receiver Sight Glass - Full-length sight glasses on horizontal receivers or gauge glass assemblies on vertical vessels provide visual verification of liquid level. Essential during charging and troubleshooting operations.

Performance Optimization

Separation Efficiency

Effective vapor-liquid separation depends on proper vapor velocity and adequate disengagement height above the liquid surface.

Entrainment Prevention - Maximum vapor velocity through the receiver vapor space must remain below the entrainment velocity for the specific refrigerant and operating pressure. Conservative design limits vapor velocity to 50-70% of calculated entrainment velocity.

Vapor Quality - Properly designed receivers deliver vapor with less than 1% liquid content by mass to the compressor suction. Demister pads reduce carryover to near-zero levels when vapor velocity remains within design limits.

Refrigerant Charge Optimization

Minimum receiver liquid level during lightest load conditions determines the base refrigerant charge requirement. Excess charge unnecessarily increases system cost and environmental impact without performance benefit.

Charge Inventory Analysis - Total system charge equals:

• Receiver operating charge (working level)
• Evaporator charge (tubes and headers)
• Piping charge (supply and return lines)
• Condenser charge (normal operating level)
• Compressor charge (oil separator and crankcase)

Energy Efficiency

Lower receiver pressure reduces system lift and decreases compressor power consumption. However, excessively low suction pressure may limit capacity and reduce evaporator heat transfer coefficients.

Optimal Suction Pressure - Determined by balancing compressor efficiency against evaporator performance for the specific application. Industrial refrigeration systems typically operate at the warmest acceptable suction temperature meeting load requirements.

Troubleshooting Common Issues

Maintenance Requirements

Annual Inspections - Verify pressure relief valve operation, inspect internal components through manholes or handholes, test level control accuracy, and document refrigerant charge inventory.

Periodic Cleaning - Remove oil accumulation that reduces effective volume and interferes with level control operation. Frequency depends on compressor oil carryover rates and system cleanliness.

Instrument Calibration - Verify level control accuracy and alarm setpoints every 6-12 months. Recalibrate transmitters per manufacturer specifications.

Low-pressure receiver design and operation significantly impacts liquid overfeed system reliability, efficiency, and safety. Proper sizing, level control selection, and operational procedures ensure optimal performance across varying load conditions.