RETA CIRO key terms

Heat energy absorbed or released when a substance changes phase at constant temperature, such as refrigerant boiling in an evaporator or condensing in a condenser. It does the cooling work without changing the refrigerant's temperature.

Heat added to or removed from a substance that changes its temperature but not its phase. In refrigeration it is the heat that warms or cools liquid, vapor, air, or product between phase changes.

The number of degrees a refrigerant vapor is above its saturation temperature at the same pressure. Measured at the evaporator outlet, it confirms the liquid has fully boiled and protects the compressor from liquid carryover.

The number of degrees a liquid refrigerant is below its saturation temperature at the same pressure. Subcooling at the condenser outlet confirms a solid liquid column reaches the expansion valve and limits flash gas.

The temperature at which a refrigerant boils or condenses at a given pressure, with liquid and vapor present together. Raising the pressure raises this temperature; lowering the pressure lowers it.

Vapor formed instantly when liquid refrigerant drops in pressure through an expansion valve or across a restriction. It cools the remaining liquid but provides no useful refrigeration in the evaporator itself.

A rate of heat removal equal to 12,000 Btu per hour, roughly the heat needed to freeze one ton of ice in 24 hours. Refrigeration equipment capacity is commonly stated in tons.

Pressure read on a gauge that ignores atmospheric pressure, shown as psig. It equals absolute pressure minus atmospheric pressure, so a gauge at sea level reads zero when open to air.

Pressure measured from a perfect vacuum, shown as psia. It equals gauge pressure plus atmospheric pressure and must be used when reading refrigerant property tables and pressure-enthalpy charts.

A system that pumps more liquid refrigerant through evaporators than can boil off, then returns the surplus liquid to a low-pressure receiver for pumping again. Common in large ammonia plants.

A vessel on the low side that separates liquid droplets from suction gas, stores liquid for recirculation pumps, and provides surge volume. Also called an accumulator or surge drum.

The ratio of liquid refrigerant pumped to an evaporator to the amount that actually evaporates, often written as 3:1. Higher rates improve coil wetting and heat transfer at the cost of pump energy.

A system that meters liquid refrigerant into an evaporator at a rate that fully boils it before the outlet, so only vapor returns to the compressor. It uses a thermostatic or electronic expansion valve.

An evaporator kept full of liquid refrigerant by a liquid level control, with vapor drawn off the top. It wets the entire coil surface and needs a separator to prevent liquid carryover.

A defrost method that routes hot compressor discharge vapor through an evaporator to melt frost from the coil. Melted frost drains away and the coil is then returned to normal cooling.

A circulation loop driven by density difference instead of a pump: warmer, lighter fluid rises while cooler, denser fluid falls. Used for oil cooling, condenser subcooling, and some receiver applications.

A system that compresses vapor in two steps: a booster raises it to an intermediate pressure, and a high-stage compressor takes it to condensing pressure. Used for very low temperatures.

A heat exchanger or vessel that cools hot gas discharged from the booster compressor before it enters the high-stage compressor, reducing discharge temperature and improving overall efficiency.

A written procedure for isolating energy sources, applying locks and tags, and verifying zero energy before repair or maintenance. It protects workers from unexpected startup or release of stored refrigerant pressure.

A regulatory program for facilities handling highly hazardous chemicals such as anhydrous ammonia. It covers process hazard analysis, operating procedures, training, mechanical integrity, management of change, and incident investigation.

Air, nitrogen, or another gas that will not condense at normal condenser conditions. It blankets condenser surfaces, raises head pressure and power use, and must be removed by a purge unit.

A device that collects and vents noncondensable gases from the high side of a refrigeration system, usually losing a little refrigerant in the process, to keep condensing pressure near design.

A safety device that opens automatically at a set pressure to vent refrigerant and protect vessels and piping from overpressure, then recloses as the pressure falls back.

A small relief valve installed on liquid piping that can be blocked in by valves. It relieves pressure caused by liquid expansion as the trapped refrigerant warms, preventing a ruptured pipe or coil.

Fixed sensors that measure ammonia concentration in machinery rooms and trigger alarms, ventilation, and sometimes automatic shutdown. They warn occupants before concentrations reach dangerous or flammable levels.

A planned sequence that stops compressors and other equipment, isolates refrigerant, and starts ventilation when a leak, fire, or other hazard occurs. Operators must know and practice the procedure.

An area with limited entry and exit that is not designed for continuous occupancy, such as a vessel, pit, or vault. Entry may require a permit, atmospheric testing, ventilation, and an attendant.

The electrical pressure or potential difference between two points that pushes current through a circuit. It is measured in volts with a voltmeter connected across the source or component.

The rate at which electric charge flows through a conductor, measured in amperes. In a series circuit it is the same through every component; in a parallel circuit it divides among the branches.

Opposition to current flow in a circuit, measured in ohms. Conductors have low resistance, while insulators and heating elements have high resistance that converts electrical energy into heat.

The relationship stating that voltage equals current multiplied by resistance, written E = I x R. Knowing any two of the three values lets you calculate the third for a circuit or component.

A circuit with only one path for current, so the same current flows through every component and the individual voltages add up. A break anywhere in the path stops all current.

A circuit with two or more paths for current, so the voltage is the same across each branch and the branch currents add together. One branch can fail without stopping the others.

A control schematic drawn with two vertical power rails and horizontal rungs between them, read left to right. Each rung shows the switches and contacts that energize a coil or load.

One horizontal circuit path in a ladder diagram, running from the left rail to the right rail through contacts to a load. The load energizes only when the contacts in that rung are closed.

A switch symbol that is open when its device is de-energized or at rest and closes when the device is actuated. Used for start push buttons and for switches that make on a rise.

The output symbol on a ladder rung, drawn as a circle or parentheses, representing the actuator of a relay, contactor, solenoid, or starter. It energizes when its rung is complete.

A normally open auxiliary contact wired in parallel with a start push button so the coil stays energized after the button is released. It forms the holding circuit of a start-stop control.

A protective device that senses excess motor current, usually through heaters or current sensors, and opens the control circuit to stop the motor before the windings are damaged.

The combination of a contactor and overload protection that switches a motor on and off and protects it from overload. Its coil is controlled by contacts arranged in the ladder logic.