ACHRNEWS

Refrigeration piping I: Here are the piping basics

April 28, 2000
Refrigeration is the process of moving heat from one location to another by the use of refrigerant in a closed cycle. A refrigeration cycle consists of oil management, gas and liquid separation, subcooling, superheating, piping of refrigerant in a liquid and gas, and two-phase flow in the condenser drain line.

Applications include air conditioning, commercial refrigeration, and industrial refrigeration.

Desired characteristics of a refrigeration system may include:

  • Year-round operation, regardless of outdoor ambient conditions;
  • Possibly wide load variations during short periods without serious disruption of the required temperature levels;
  • Frost control for continuous-performance applications;
  • Oil management for different refrigerants under varying loads and temperatures; and
  • System efficiency, maintainability, and operating simplicity.

A successful refrigeration system depends on a good piping design and an understanding of the required accessories.

First you solder a joint

Probably the first skill that any refrigeration technician learns is to make a soldered joint.

Running pipe is so common a task that often its critical importance in the proper performance of a system is overlooked. But in a system with improper piping, it is common for a service person to add gallons of oil that seemingly disappear without a trace.

It is, of course, lying in the bottom of tubing in the system, usually in the evaporator or suction line. This oil must be removed when it returns to the compressor after operating condition change. The excess oil must be removed or the compressor will fail.

Refrigeration piping involves extremely complex relationships in the flow of refrigerant and oil. Fluid flow is the study of the flow of any fluid, whether it is a gas or a liquid, and the interrelationship of velocity, pressure, friction, density, and the work required to cause the flow.

The design of a refrigeration piping system is a continuous series of compromises. It is desirable to have maximum capacity at minimum cost and proper oil return. Since oil must pass through the compressor cylinders to provide lubrication, a small amount of oil is always circulating with the refrigerant.

Oil and refrigerant vapor, however, do not mix readily, and the oil can be properly circulated through the system only if the mass velocity of the refrigerant vapor is great enough to sweep the oil along. To ensure oil circulation, adequate velocities of refrigerant must be maintained in the suction, discharge lines, and evaporator.

The design of a refrigerant piping system should:

  • Ensure proper refrigerant feed to evaporators;
  • Provide practical refrigerant line sizes without excessive pressure drop;
  • Prevent excessive amounts of lubricating oil from being trapped in any part of the system;
  • Protect the compressor at all times from loss of lubricating oil;
  • Prevent liquid refrigerant or oil slugs from entering the compressor during operating and idle time; and
  • Maintain a clean and dry system.


Refrigerant line velocities

Economics, pressure drop, noise, and oil entrapment establish feasible design velocities in refrigerant lines. These are:

Suction line - 700 to 4,000 fpm

Discharge line - 500 to 3,500 fpm

Condenser drain line - 100 fpm or less

Liquid line - 125 to 450 fpm

Higher gas velocities are sometimes found in relatively short suction lines on comfort air conditioning or other applications where the operating time is only 2,000 to 4,000 hrs per year and where the low initial cost of the system may be more significant than low operating cost.

Industrial or commercial refrigeration applications, where equipment runs almost continuously, should be designed with low refrigerant velocities for the most efficient compressor performance and low equipment operating cost.

The liquid line from the condenser to the receivers should be sized for 100 fpm or less to ensure positive gravity flow without incurring a backup of liquid flow. Liquid lines from the receivers to the evaporator should be sized to maintain velocities below 300 fpm, thus minimizing or preventing liquid hammer when solenoids or other electrically operated valves are used.

Line sizing

In sizing refrigerant lines, cost considerations favor keeping the line size as small as possible. However, suction and discharge line pressure drops cause loss of compressor capacity and increased power usage.

Excessive liquid line pressure drops can cause the liquid refrigerant to flash, resulting in faulty expansion valve operation. Refrig-eration systems are designed so that friction pressure losses do not exceed a pressure differential equivalent to a corresponding change in the saturation boiling temperature.

The primary measure for determining pressure drop is a change in saturation temperature. Pressure drop in a refrigerant line causes a reduction in system efficiency. Correct sizing must be based on minimizing cost and maximizing efficiency.

Pressure drop calculations are determined as normal pressure loss associated with a change in saturation temperature of the refrigerant. Typically, the refrigeration system will be sized for pressure losses of 2°F differential or less for each segment of the discharge, suction, and liquid lines. An HFC refrigerant liquid line is sized for pressure losses of 1° differential or less.