P2 establish the basic operating conditions of vapour compression systems UEERA0036

Comprehensive vocabulary flashcards covering evaporator classifications, temperature difference calculations, refrigerant flow controls, service gauge operation, and leak detection methods.

Liquid cooling

A classification where the evaporator cools a liquid or secondary refrigerant which then cools the product.

Direct or dry expansion (DX)

An evaporator system in which liquid is introduced through a restrictor at one end of a tube and evaporates as it flows; it is generally composed of $25\%$ liquid and $75\%$ vapour.

Flooded evaporator

An evaporator system designed to maintain a set level of liquid refrigerant, generally comprising $75\%$ liquid and $2\%$ vapour.

Evaporator

A heat exchanger that absorbs heat from a product space by maintaining a lower temperature than the medium to be cooled, causing refrigerant to change from liquid to vapour through latent heat.

Bare tube evaporators

Evaporators normally used for cooling liquids, but sometimes utilized for air cooling in freezer applications.

Plate surface evaporators

Evaporators used for air and contact cooling in domestic refrigerators as well as commercial and industrial plants.

Finned evaporators

Evaporators mainly used for air cooling, utilizing either natural or forced air convection.

Shell and tube evaporators

Evaporators where the refrigerant is in a tank-like vessel and the liquid to be cooled is circulated through tubing inside said vessel.

Natural or gravity convection

A circulation method where the movement of the air or liquid is caused by the increased density of the product when it is cooled.

Forced convection (forced draft)

A method where movement of air or liquid is induced over evaporator surfaces by a fan or pump.

Non-frosting evaporators

Evaporators whose operating temperature is high enough to prevent ice formation, often featuring $300$ to $400$ fins per meter.

Baudelot cooler

A liquid cooling application consisting of a bare tube, natural convection, non-frosting, flooded evaporator.

Temprite (IBC)

Also known as an Instantaneous Beverage Cooler; a shell and coil, forced convection, flooded evaporator that requires defrosting.

Relative Humidity (RH%)

The amount of water vapour contained in the air compared with the amount of water vapour the air could hold at a given temperature, stated as a percentage.

Evaporator temperature difference (Td)

The difference between the refrigerant saturation temperature in the evaporator and the temperature of the 'air on' (return air) to the evaporator.

Evap Td Formula

$$Evap\,Td = \text{Evap 'air on' temp} - \text{Sat Evap. Temp}$$

Condenser temperature difference (Td)

The difference between the refrigerant saturation temperature in the condenser and the temperature of the air on to the condenser.

Cond Td Formula

$$Cond\,Td = \text{Sat. Cond. temp} - \text{Cond 'air on' temp}$$

Direct refrigeration

The cooling effect produced when an evaporator cools a product or space directly.

Indirect cooling

The cooling effect produced when an evaporator cools a fluid (secondary refrigerant) which is then pumped to the product or space.

Secondary refrigerants

Fluids such as water, chloride solutions (calcium or sodium chloride), and glycol solutions used to transfer heat.

Flow Control (RMD)

The component used to reduce high pressure from the high side to low pressure in the low side while governing the flow of liquid refrigerant into the evaporator.

Hand Expansion Valve

A manually operated valve typically found in industrial applications where liquid recirculation requires monitoring by an operator.

Low Side Float

A flow control found on the low pressure side that operates based on the level of liquid refrigerant in the evaporator, typically used in a Temprite.

High Side Float

A flow control in the high pressure side used in critical charge systems like air conditioning chillers where the evaporator level is controlled by the high side liquid level.

Automatic Expansion Valve (AXV)

A valve that maintains constant evaporator pressure responding to spring and evaporator pressure; it cannot adjust to varying loads.

Thermostatic Expansion Valve (TXV)

The most common flow control that regulates liquid refrigerant flow proportionately to the rate of vaporisation.

Thermal-electric Expansion Valve (TEXV)

A valve controlled by changes in input voltage varied by a sensor, used in residential air conditioning and supermarket cabinets.

Electronic Expansion Valve

A regulator that adjusts electrical output to the valve based on sensor signals, often used on multiple evaporator supermarket cabinets.

Two Way Flow Control Valve

Also known as Chatleff or Accurator valves, commonly used in reverse cycle residential air conditioning.

Capillary Tube

The simplest refrigerant control using fixed length and bore to create pressure drop; it requires a critical refrigerant charge and negates the need for a liquid receiver.

Refrigerant Distributors

Components used to connect the metering device to the evaporator and ensure even refrigerant flow to each circuit.

Bourdon Tube

A curved copper tube in service gauges that straightens as pressure is introduced, moving the gauge needle.

Backseated position

The state of a service valve where the service gauge access port is closed off and the valve is fully open for normal system operation.

Frontseated position

The state of a service valve where the line to the compressor or component is closed off.

Schrader valves

Access valves that open when the gauge line depressor is fitted; they must reach into the gauge line to allow refrigerant flow.

Bullet piercing valves

Temporary access valves used on small systems with no access valves; they are prone to leaking and must be replaced with Schrader valves.

Halide lamps

Leak detectors that burn butane and detect refrigerants containing chlorine, such as $R22$, by changing flame color.

Electronic leak detectors

Devices using a heated ceramic sensor to ionise halogen-bearing refrigerants, converting the resulting electrical current into an audible noise.

Fluorescent dyes

Substances injected into a system that use oil as a transport agent and become visible under a UV lamp to identify leaks.