Cold room glossary
Cold room terms, in plain English.
Panel thickness, heat load, evaporators, superheat, refrigerants… The terms you meet when you draw, build and price a cold room, kept short and clear.
Popular terms Heat loadPanel thicknessBlast freezerEvaporator
All terms
175 termsAdiabatic coolingSystems and equipmentCooling the air entering a condenser or gas cooler beforehand with a wetted pad or a water mist. Water is turned on only in the hot hours; in return the condensing temperature is pulled down.Air curtainDoorsA measure that keeps inside and outside air from mixing while the door is open. It is either a strong air stream blown down from above the door or clear PVC strips covering the opening.Air flowFundamentalsThe volume of air the evaporator fans move per hour, in m³/h. It decides how evenly the cold spreads through the room and how fast the product cools.Air infiltrationHeat loadThe load brought by warm, humid air coming in each time the door opens. Door type, strip curtains and an anteroom reduce it.Air throwSystems and equipmentThe distance the air blown by an evaporator reaches. If it is shorter than the room, a second unit is needed on the opposite wall.Air-cooled and water-cooled condenserSystems and equipmentA condenser rejects heat either to outdoor air with fans or to water from a tower or a well. The first is air-cooled, the second water-cooled; capacity, climate and water availability decide which one is used.Aisle widthRacking and storageThe clear width between racks that a forklift needs to turn. It varies with the type of truck.AnteroomRooms and panelsThe buffer space in front of a cold room door. It cuts down the warm, humid air that comes in when the door opens.
Back-to-back rackingRacking and storageA layout where two rows of racking stand back to back, with an aisle on each side.Banana ripening roomProduct and storageA special room that brings green bananas to the same yellow across the whole batch in a few days. It works with an ethylene dose, high humidity and a system that can both cool and heat.Bill of materialsProject and quotationThe breakdown of panel, door, equipment and accessory quantities taken off from the project.Blast freezerRooms and panelsA room running at −35 / −40 °C where product is frozen in a short time. Most of the load comes from freezing the product.Blast freezingProduct and storageFreezing a product within a few hours under very cold, fast-moving air. The ice crystals stay small, so texture and flavour are preserved.BTU/hFundamentalsThe heat rate unit of American and British catalogues. 3,412 BTU/h is about 1 kW, and 12,000 BTU/h is 1 ton of refrigeration.
Capacity controlOperation and measurementThe ways of raising and lowering the output of the refrigeration plant to match the need of the moment. The method chosen affects electricity consumption, how much the room temperature swings and how often the compressor starts.Cascade systemSystems and equipmentAn arrangement of two separate refrigeration circuits with different refrigerants working in series: the upper circuit cools the condenser of the lower one. It is used for duties between −50 and −80 °C and in industrial CO₂ plants.Central systemSystems and equipmentAn arrangement where many rooms are served by a single compressor rack. Used in large cold stores.Check valveComponentsA valve that lets flow pass one way and closes by itself when the flow tries to turn back. When the system stops, it keeps gas and liquid from escaping to where they should not go.Chiller (water chilling unit)Systems and equipmentA unit that cools water or a glycol mixture circulating in a loop, not the air of a room. The chilled water is piped to a process line, a tank or an air handling unit.Chilling injuryProduct and storagePermanent tissue damage in fruit and vegetables from warm climates when they wait in too much cold, even though they do not freeze. Bananas cannot take less than 13 °C, cucumbers less than 10 °C.Cold chainProduct and storageA perishable product never leaving its own temperature limits at any stage from the producer to the final buyer. For fresh food that limit is usually 0–4 °C, for frozen food −18 °C at most, for medicines 2–8 °C.Cold roomRooms and panelsAn enclosed space built from insulated panels, with its own refrigeration system, that keeps product at a set temperature.Cold room doorDoorsA special door with an insulated leaf, gaskets all round and, in freezer rooms, a heated frame. Hinged, sliding, swing and high-speed types are chosen by traffic and room temperature.Cold room lightingRooms and panelsInterior lighting made with dust-tight and waterproof fittings that withstand cold and moisture. Today nearly all of it is LED, and every watt it burns is added to the heat load of the room.Cold room quotationProject and quotationThe bill of materials, priced and presented to the customer as a document.Cold storeRooms and panelsA large warehouse made up of several cold rooms, an anteroom and a dispatch area, worked with forklifts and racking.CompressorComponentsThe machine that drives the refrigeration cycle. It draws low-pressure vapour from the evaporator, compresses it and discharges it to the condenser at high pressure; most of the electricity the system uses goes here.Compressor oil (POE, mineral, PAG)RefrigerantsThe oil that protects the rubbing parts of the compressor, seals the clearances and carries heat away. Its type depends on the gas: mineral with R22, POE with HFC and HFO gases, PAG in car air conditioning.CondenserSystems and equipmentThe coil and fans that reject the heat removed from the room to the outside air.Condensing temperatureOperation and measurementThe temperature at which the refrigerant turns from vapour to liquid in the condenser. It is 10–15 K above ambient; the lower it is kept, the less electricity the compressor uses.Condensing unitSystems and equipmentThe refrigeration unit outside the room that houses the compressor and the condenser.ContactorComponentsAn electromagnetic switch that closes its main contacts on a small control current through its coil and so powers heavy loads such as compressors, fans and heaters.Controlled atmosphere storage (CA)Product and storageA cold room sealed gas-tight, with the oxygen in its air reduced and the carbon dioxide kept in check. Fruit such as apples, pears and kiwifruit waits here months longer than in an ordinary store.Cooling towerSystems and equipmentAn outdoor device that cools condenser water. It evaporates part of the water, the rest cools as a result and returns to the condenser; the temperature it can reach is limited by the wet-bulb temperature of the air.COP (coefficient of performance)FundamentalsThe ratio of the heat a system removes to the electricity it uses. At a COP of 2.5, each 1 kW of electricity takes 2.5 kW of heat out of the room.Crankcase heaterComponentsA low-power heater that keeps the oil warm while the compressor is stopped. It stops refrigerant from mixing into the oil and collecting there, so the oil does not foam at start-up and no liquid enters the compressor.
DefrostSystems and equipmentMelting the frost that builds up on the evaporator, at set intervals.Defrost heaterComponentsA rod-shaped electric heater fitted in the evaporator fin block and the drip tray. It switches on during defrost to melt the frost, and is the most common defrost method in freezer rooms.Dew pointFundamentalsThe temperature at which the water vapour in air starts to turn into droplets as the air is cooled. Any surface colder than this sweats, and ices up if it is below zero.Discharge temperatureOperation and measurementThe temperature of the compressed gas as it leaves the compressor. Being superheated, it is well above the condensing temperature; 60–110 °C is read on the pipe, and beyond the limit the oil breaks down and the compressor is damaged.Dock shelterDoorsThe flexible frame at a loading dock door that closes the gap between the vehicle body and the building.Drain line heaterComponentsA heating cable that keeps the defrost drain pipe warm in a sub-zero room. It is fitted so the water does not freeze and block the line before it gets outside.Drive-in rackingRacking and storageHigh-density storage where the forklift drives into the rack. For many pallets of the same product.
EC fanComponentsA brushless fan with its motor and drive electronics in one housing. It connects straight to the mains, and its speed is set to any value with a 0–10 V signal.EconomiserComponentsAn auxiliary circuit that cools the liquid once more ahead of the expansion valve and feeds the vapour produced into the intermediate port of the compressor. It adds capacity and efficiency at low temperature.Electronic expansion valve (EEV)ComponentsAn expansion valve driven by a stepper motor. A controller calculates superheat from temperature and pressure sensors and moves the valve to exactly the opening required.Energy efficiency (SEPR, ecodesign)Operation and measurementThe amount of cooling obtained for the electricity consumed. It is measured by COP for a single operating point and by SEPR for the weather of a whole year; the ecodesign regulation sets the minimum values.EnthalpyFundamentalsThe amount of heat a substance carries per kilogram, measured in kJ/kg. What counts in refrigeration is the difference between two points: it tells how much heat the refrigerant or the air has picked up or given off in between.EthyleneProduct and storageA colourless gas (C₂H₄) that fruit and vegetables give off by themselves; it triggers ripening and ageing. Even a very small amount, such as one part per million, is enough. It is dosed on purpose in ripening rooms and removed in other stores.EvacuationOperation and measurementEmptying the air and moisture inside the pipes with a vacuum pump before refrigerant is charged. The target is to get below 500 microns (0.67 mbar); a skipped vacuum comes back as acid, an iced-up valve and high pressure.Evaporating temperatureOperation and measurementThe temperature at which the refrigerant boils inside the evaporator. It is lower than room temperature by the TD; this point sets compressor capacity, efficiency and evaporator selection.Evaporative condenserComponentsA condenser whose tubes are wetted with water while air is blown across them. The evaporating water carries the heat away; condensing temperature falls markedly compared with an air-cooled condenser.EvaporatorSystems and equipmentThe fan unit inside the room that cools the air. How many there are and where they go depends on getting air to the whole room.Expansion valve (TXV)ComponentsThe valve that throttles liquid refrigerant at the evaporator inlet. It drops the pressure and feeds the evaporator as much refrigerant as the load requires.
F-gas regulationRefrigerantsThe rules that limit the sale, use and leakage of fluorinated greenhouse gases. They ban high-GWP gases in stages and require leak checks, record keeping and certified personnel.Fan motorComponentsThe motor that turns the propeller of an evaporator or condenser. The airflow through the unit, its noise and part of its electricity use depend on how this component is chosen.Fan speed controlComponentsAn arrangement that changes fan speed according to pressure or temperature. On a condenser it stops condensing pressure from falling too far in winter; the fans use less electricity and make less noise.Filter drierComponentsA filter fitted in the liquid line that traps moisture, acid and solid particles in the refrigerant. It keeps the expansion valve from blocking with ice and protects the compressor windings.Fin spacingComponentsThe gap between two fins of an evaporator or condenser, in millimetres. The colder the room, the wider the spacing; close fins for chill rooms, wide fins for freezer rooms.Floating head pressure controlOperation and measurementA control that does not lock the condensing temperature at one setting but follows the outdoor air and pulls it down. The compressor does less compression work and annual electricity consumption falls by 10–20%.Floor panelRooms and panelsThe floor insulation used in freezer rooms. Without it, heat comes in through the floor and the ground can freeze and heave the concrete.Free coolingSystems and equipmentUsing cold outdoor air as a free source of cooling. When the air is suitable the compressor drops out, the work is left to the fan and pump, and the electricity bill shrinks noticeably in the cold months.FrostingOperation and measurementMoisture building up as thin frost on evaporator fins and cold pipes that are below zero. A little is in the nature of the job; when it grows, capacity drops and a fault is looked for behind it.Frozen storageProduct and storageA room where product that arrives already frozen is held for months at −18 °C and colder. It does not do the freezing; it only protects what is frozen.
Glycol system (secondary loop)Systems and equipmentAn indirect cooling arrangement: the refrigerant circulates only in the chiller in the plant room, and pumped glycol–water carries the cooling to the coolers in the rooms. Refrigerant charge and the chance of a leak drop sharply.GWP (global warming potential)RefrigerantsA number that compares a gas’s effect on the climate with CO₂. It tells how many kg of CO₂ would warm the atmosphere as much over 100 years as 1 kg of leaked gas; 3,922 for R404A, 3 for R290.
HACCPProduct and storageA food safety system that identifies hazards in advance, measures at critical points and ties everything to records. Cold room temperature is the most common critical control point.Heat exchangerSystems and equipmentA device that passes heat from one fluid to another through a metal wall, without mixing them. The evaporator and the condenser are heat exchangers too.Heat loadHeat loadThe total heat (kW) the refrigeration system has to remove to hold the room at the required temperature.Heat of respirationHeat loadThe heat fruit and vegetables produce for as long as they stay alive in storage. It rises with temperature.Heat pumpSystems and equipmentA refrigeration machine used for heating. It collects heat from a cool source such as outdoor air, well water, the ground or waste heat, raises its temperature and delivers it where it is needed.Heat recoverySystems and equipmentUsing part or all of the heat a refrigeration system rejects at the condenser for hot water, space heating or underfloor heating. It lowers the fuel bill of the plant.Heat transfer coefficient (U-value)Heat loadThe number that tells how easily a building element lets heat through: watts per square metre per kelvin (W/m²K). A low value means good insulation; panels, doors and glazing are compared by it.Hermetic and semi-hermetic compressorComponentsA hermetic compressor has a welded shell and cannot be opened or repaired. A semi-hermetic compressor has a bolted housing; it can be opened and its parts replaced.HFC, HFO and HCFC refrigerantsRefrigerantsThe generations of fluorinated refrigerants. CFCs and HCFCs contain chlorine, thin the ozone layer and have been phased out; HFCs are chlorine-free but have a high GWP; HFOs are the latest generation, with a very low GWP.High-speed doorDoorsA PVC door that opens and closes in seconds. It reduces air infiltration where traffic is frequent.Hinged doorDoorsA cold room door with a leaf that swings on hinges. For pedestrian access and small rooms.Hot gas defrostOperation and measurementMelting the ice on an evaporator by passing hot vapour from the compressor through the inside of its tubes. It is quicker than heater defrost and uses less energy, but the pipework is busier.Humidification (ultrasonic humidifier)Product and storageRaising relative humidity by adding a water mist to the room air that is fine enough not to wet anything. Vegetables, fruit and flowers lose less water. The most widely used device is the ultrasonic humidifier, which vibrates water into mist.
Ice bankSystems and equipmentA tank that stores cooling as ice. Ice forms around its pipes over several hours; at the moment of need the melting ice supplies water at 0–1 °C, and a small unit copes with a large, short-lived load.Icing (ice block on the evaporator)Operation and measurementFrost that does not fully melt during defrost refreezes and covers the evaporator in a hard mass of ice. Air cannot pass and the room warms up; a defrost or drainage problem is usually behind it.Inner and outer dimensionsRooms and panelsThe room measured from panel face to panel face (inner) or from outside of panel to outside of panel (outer). Panels are ordered on outer dimensions. Usable space is discussed in inner dimensions.Internal loadsHeat loadThe heat given off inside the room by lighting, workers, forklifts and evaporator fans.Inverter (variable frequency drive)ComponentsAn electronic drive that changes the frequency of the current fed to a motor and so matches the speed of a compressor, fan or pump to the load. Capacity is adjusted steplessly and the electricity bill falls.IQF (individually quick frozen)Product and storageFreezing each piece of a small product separately, within minutes. The pieces do not stick together, so any amount can be taken from the pack.
Latent heatFundamentalsWhile a substance freezes, melts, evaporates or condenses, its temperature stays constant but it takes in or gives off heat. That heat is called latent heat.Leak testingOperation and measurementSearching the refrigeration circuit for escaping refrigerant with a detector, foam and pressure methods. Under the F-gas rules it is done regularly on systems whose charge reaches 5 tonnes CO₂ equivalent.Liquid receiverComponentsThe pressure vessel that collects liquid refrigerant leaving the condenser. It holds the surplus as load and season change, and takes the whole charge during service.Liquid sluggingOperation and measurementThe compressor swallowing liquid refrigerant or oil instead of gas. Liquid does not compress, so valves, connecting rods and scroll spirals break; it is among the most expensive compressor failures.Loading dockDoorsThe platform and pit arrangement that brings the vehicle bed to the same level as the store floor.Loading dock doorDoorsThe sectional door where the vehicle docks and loading takes place.
Nitrogen pressure testOperation and measurementPressurising the circuit with dry nitrogen and holding it before any refrigerant goes in. It proves both tightness and strength; evacuation and charging follow.Non-condensable gasesOperation and measurementAir or nitrogen left in the circuit that does not liquefy in the condenser. It takes up condenser surface and raises discharge pressure and power consumption; it is usually left over from poor evacuation.
ODP (ozone depletion potential)RefrigerantsA number showing how much a refrigerant thins the ozone layer, relative to R11. It is above zero for older gases that contain chlorine and zero for HFCs, HFOs and natural refrigerants.Oil returnOperation and measurementThe oil that the compressor throws out with the gas travelling round the pipework and coming back to the crankcase. If the suction line is not designed for it, oil stays in the evaporator and pipes and the compressor runs short of oil.Oil separatorComponentsA component in the discharge line that catches the compressor oil carried along with the vapour and returns it to the crankcase. It stops oil collecting in the pipework and the evaporator.
Pallet capacityRacking and storageThe number of pallets a room can hold. It is set by the rack layout, the number of levels and the ceiling height.Pallet rackingRacking and storageSteel racking that holds pallets on several levels. Every pallet is reached from the aisle.Panel thicknessRooms and panelsThe insulation thickness of the panel. It goes up as room temperature goes down: 80–100 mm for a 0 °C room, 120–150 mm for −20 °C, 150–200 mm for a blast freezer.Phase monitoring relayComponentsAn electronic relay that watches the three-phase supply. When a phase is lost, the sequence is reversed, the voltages become unbalanced or go out of limits, it opens the control circuit and the compressor stops.Plate freezerSystems and equipmentA machine that freezes product by contact, pressed between metal plates with refrigerant flowing inside them. It freezes regular-shaped blocks faster and with less energy than air blast freezing.Polyurethane (PUR) and PIR foamRooms and panelsThe closed-cell rigid foam that fills the space between the two metal sheets of a cold room panel. Its thermal conductivity is around 0.022 W/mK; the PIR type resists fire better.PrecoolingProduct and storageRemoving the field heat of harvested fruit and vegetables within a few hours, before they go into storage or onto a vehicle. It slows respiration and water loss and extends shelf life.Pressure dropFundamentalsThe pressure a refrigerant loses to friction along the line. Installers usually talk about it in kelvin rather than bar; the accepted upper limit for the suction pipe is about 1–2 K.Pressure equalising valveRooms and panelsA two-way flap vent fitted in the panel that evens out the air pressure difference between the inside and outside of the room by itself. In freezer rooms it is heated; it prevents panels from caving in and the door from locking shut.Pressure relief valveComponentsA valve whose spring gives way and releases refrigerant when pressure reaches the limit the vessel can withstand. It is the last component that prevents a burst when the other protections have failed.Pressure switchComponentsA switch that opens and closes a contact according to pressure. The low-pressure switch stops the compressor when suction pressure falls; the high-pressure switch cuts the system out when condensing pressure climbs to a dangerous level.Preventive maintenanceOperation and measurementCleaning, measuring and adjusting done to a calendar, without waiting for a fault. The condenser is washed, leaks are searched for, terminals are tightened, defrost and oil are checked; the bill goes down and the equipment lasts longer.Product loadHeat loadThe heat removed to bring incoming product down to storage temperature. It is set by the daily intake and the entering temperature.PsychrometricsFundamentalsThe branch of calculation that treats air together with the water vapour in it. It gives the link between temperature, relative humidity, dew point, humidity ratio and enthalpy on the psychrometric chart.Pump-downOperation and measurementWhen the room reaches temperature, the solenoid valve in the liquid line closes, the compressor collects the refrigerant from the evaporator in the receiver and stops on the low-pressure switch. It protects the compressor from drawing in liquid at start-up.PVC strip curtainDoorsClear strips hung in the door opening. The cheapest way to reduce the air coming in while the door is open.
R1234yfRefrigerantsAn HFO gas that breaks down in the air within days and has a GWP of only 4. It has replaced R134a in car air conditioning; it is mildly flammable (A2L) and the main component of low-GWP blends.R134aRefrigerantsThe single-component HFC for above-zero applications. It is common in chillers, milk cooling and car air conditioning; with a GWP of 1,430 it is giving way to refrigerants such as R513A and R1234yf.R22RefrigerantsThe chlorinated gas (HCFC) of old air conditioners and cold stores. Its production and import have been stopped because it damages ozone; GWP 1,810, ODP 0.055.R290 (propane)RefrigerantsRefrigeration-grade propane. An efficient natural refrigerant with a GWP of 3; being flammable (A3), it is used with a limited charge in sealed, leak-tight equipment.R32RefrigerantsThe gas of today’s split air conditioners and heat pumps; a single-component HFC. Its GWP is 675 and its class is A2L, which means it is flammable, if only mildly.R404ARefrigerantsAn HFC blend that was the standard for low- and medium-temperature commercial refrigeration for years. With a GWP of 3,922 it has been banned in new systems; R449A and R448A are taking its place.R407CRefrigerantsA three-component HFC blend that took the place of R22 in air conditioners and chillers. It is non-flammable (A1), has a GWP of 1,774 and needs care because of a glide of up to 7 K.R410ARefrigerantsA high-pressure HFC blend used for years in split air conditioners and heat pumps. Its GWP is 2,088; it is rarely used in cold rooms, and in new equipment it is giving way to R32 and R454B.R448ARefrigerantsA non-flammable (A1) five-component blend charged in place of R404A in cold room units. Its GWP is 1,387 and its temperature glide is 5–6 K.R449ARefrigerantsThe A1 blend that replaces R404A in cold rooms. Its GWP is 1,397 and its temperature glides by about 4.5 K as it evaporates.R454CRefrigerantsA mildly flammable (A2L) blend of R32 and R1234yf with a GWP of 148. In newly built cold room and frozen store units it is regarded as the lasting successor to R404A.R507ARefrigerantsA blend of R125 and R143a in equal weight; an azeotropic HFC with no glide. It does the same job as R404A in frozen applications and is banned in new systems because its GWP is 3,985.R513ARefrigerantsA non-flammable (A1) two-component blend with no glide, used in place of R134a. Its GWP is 631; it is used for above-zero duties.R717 (ammonia)RefrigerantsAmmonia. A natural refrigerant with no greenhouse effect and very high efficiency; because it is toxic and mildly flammable (B2L), it needs a separate machine room and trained operating staff.R744 (carbon dioxide, CO₂)RefrigerantsA non-flammable natural refrigerant with a GWP of 1. It works at very high pressure (about 35 bar at 0 °C) and so needs its own equipment; it is replacing HFCs in supermarket and large warehouse systems.Reciprocating compressorComponentsA compressor that compresses vapour in cylinders with pistons driven by a crankshaft. It covers a very wide range of capacities and temperatures, and the larger models can be repaired on site.RefrigerantRefrigerantsThe substance that circulates inside the refrigeration circuit. It picks up the heat of the room as it evaporates in the evaporator and gives it up outside as it turns back to liquid in the condenser.Refrigerant chargeOperation and measurementBoth the total amount of refrigerant in the circuit and the job of filling that amount into the system. If it is short, capacity drops; if too much is put in, pressure rises. The right amount is found by measuring superheat and subcooling.Refrigerant pipingSystems and equipmentThe suction and liquid lines between the condensing unit and the evaporator. Diameter and length affect capacity.Refrigerant safety class (A1, A2L, A3, B2L)RefrigerantsA short code that says how toxic and how flammable a refrigerant is. The letter is for toxicity (A, B), the number for flammability (1, 2L, 2, 3).Refrigeration capacityHeat loadThe heat a refrigeration unit can remove at a given operating condition. It must cover the heat load plus a safety margin.Refrigeration cycleFundamentalsThe closed circuit in which refrigerant picks up heat in the evaporator and releases it in the condenser, kept going by the compressor and the expansion valve. All mechanical cooling, from cold rooms to air conditioners, runs on this loop.Relative humidityFundamentalsShows what percentage of the maximum water vapour it could hold at that temperature the air is carrying. In a cold room this value decides whether produce wilts, loses weight or rots.Remote monitoring systemSystems and equipmentA system that continuously records room temperature, humidity, door and equipment status with sensors and shows them over the internet. When a value leaves its limits it sends an alarm to a phone.Ripening roomProduct and storageA room that gets fruit picked firm and green ready for the day of sale. Ethylene, carbon dioxide, temperature and humidity are controlled together; it is used for avocados, mangoes, bananas, kiwifruit, tomatoes and citrus.
Safety factorHeat loadThe margin added on top of the calculated load, together with the assumed number of hours the compressor runs per day.Sandwich panelRooms and panelsA prefabricated insulated panel with polyurethane insulation between two metal skins. It forms the walls and ceiling of a cold room.Saturation temperature and saturation pressureOperation and measurementThe temperature at which a refrigerant’s liquid and vapour sit side by side; it changes when the pressure changes. Each pressure has a single saturation temperature, and these pairs make up the refrigerant’s pressure–temperature table.Screw compressorComponentsA high-capacity compressor that compresses vapour continuously between two meshing helical rotors. It is the machine of central systems and large stores.Scroll compressorComponentsA compressor that works with two interleaved spirals. One spiral stays fixed and the other orbits inside it; it has few parts and runs with little noise and vibration.Sensible heatFundamentalsHeat that changes only the temperature of a substance, without changing its state. It is the change a thermometer shows.Service valve (Rotalock)ComponentsA hand-operated valve on the compressor inlet and outlet and on the liquid outlet of the receiver. It shuts the line, provides a gauge connection and makes it possible to change a component without removing the refrigerant.Shared wallRooms and panelsThe single panel shared by two adjacent rooms. One panel is used instead of two separate walls, and its thickness is chosen for the colder room.Shelf lifeProduct and storageThe time a food lasts as both safe and saleable when it is stored under a given condition. Temperature is the deciding factor; every day spent waiting in the store comes off this time.Short cyclingOperation and measurementThe compressor cutting in and out again and again at short intervals. Each start heats the winding, wears the contactor and throws oil into the system before it can return to the crankcase, so compressor life is shortened.Sight glassComponentsA small glass fitting in the liquid line. It shows the refrigerant passing through: bubbles mean the liquid supply is short, and a colour change on the indicator signals moisture.Site surveyProject and quotationMeasuring the place where the room will be built and checking the conditions on site (floor, power supply, unit location).Sliding doorDoorsA door with a leaf that slides along the wall. Used for wide openings with forklift traffic. It needs clear wall beside it to slide into.Solenoid valveComponentsA valve that opens when its coil is energised and closes by spring when the power is cut. In the liquid line it shuts off refrigerant to the evaporator; pump-down control and multi-room systems depend on it.Specific heatFundamentalsThe coefficient that gives the heat to be removed to cool one kilogram of a substance by one degree (kJ/kg·K). Water, at 4.19, has one of the highest values; for ice it is about half that, 2.1.Spiral freezerSystems and equipmentA belt-type blast freezer for continuous lines. The belt winds tier upon tier around a drum inside an insulated enclosure, and the product on it freezes as it travels through fast air at around −35 °C; it needs little floor space.Split systemSystems and equipmentAn arrangement where one condensing unit serves one or a few evaporators. For small and medium rooms.SubcoolingOperation and measurementHow many kelvin the liquid leaving the condenser is colder than the condensing temperature at that pressure. Normal is 3–8 K; it is the best indicator of whether the charge is right.Suction accumulatorComponentsAn upright vessel fitted in the suction line just ahead of the compressor. It holds refrigerant that arrives unevaporated from the evaporator at its bottom and sends only gas on to the compressor.Suction pressureOperation and measurementThe pressure read on the low side of the circuit, at the compressor inlet. The evaporating temperature in the evaporator is found from it using the refrigerant’s table; a deviation from the expected value is the first clue to a fault.SuperheatOperation and measurementHow many kelvin the vapour leaving the evaporator is warmer than the evaporating temperature at that pressure. Normal is 5–10 K, and it is the measurement that says most about the health of the system.
TD (temperature difference)Systems and equipmentThe difference between room temperature and evaporating temperature. A small TD means high humidity in the room.Temperature glideRefrigerantsThe difference between the temperature at which a blended refrigerant starts to boil and the temperature at which it is fully vapour, at the same pressure. About 0.7 K for R404A and 7 K for R407C.Thermal bridgeHeat loadA spot that pierces or interrupts the insulation and passes heat far more easily than the panel around it. Steel columns, bolts, hanger rods, door frames and the floor–wall junction are the best known.Thermal overload relayComponentsIf the motor draws more than the set current for some time, the bimetal strips inside heat up, break the control circuit and the contactor drops out. It protects the winding against overload and burn-out.ThermostatComponentsThe device that reads room temperature through a sensor and switches cooling on and off against the set point. Digital models also manage defrost, the evaporator fan and alarms.Ton of refrigerationFundamentalsA capacity unit found in US catalogues. 1 ton of refrigeration is 12,000 BTU/h, about 3.52 kW; the name comes from one ton of ice melting in a day.Transcritical CO₂ systemSystems and equipmentA refrigeration system that runs on carbon dioxide (R744). On a hot day the discharge side goes above the critical point; the gas does not turn to liquid, it is only cooled in a coil called a gas cooler and gives its heat to the outside.Transmission loadHeat loadThe heat coming in through walls, ceiling and floor. It depends on panel thickness and the inside-to-outside temperature difference.Two-stage compressionSystems and equipmentThe answer for deep-freezing duties that splits the compressor’s work in two. The gas is first raised to an intermediate pressure, cooled there, and then brought up to condensing pressure in a second step.