By the end of this chapter, you will be able to:
Mastering these skills will help you become a trusted and capable technician in the refrigeration and air conditioning trade.
Refrigeration systems are vital across diverse Kenyan sectors, from preserving vaccines at county referral hospitals to maintaining fresh produce in agricultural cooperatives. Understanding the physics principles underlying refrigeration enables technicians and users to maintain these systems effectively, ensuring optimal performance and energy efficiency. This chapter explores the fundamental physics concepts, including gas laws and heat transfer mechanisms, that govern refrigeration maintenance.
A refrigerator moves heat by changing the pressure, volume and temperature of a refrigerant gas, so the behaviour of gases explains almost every reading a technician takes on a refrigeration system. Gauge pressures, discharge temperatures and the pressure drop seen during a nitrogen pressure test all follow from a small set of laws. In a hospital the same principles apply to a vaccine refrigerator, a blood bank refrigerator and a laboratory refrigerator, where a fault in pressure or temperature control can spoil stored products.
The gas laws describe how the pressure (P), volume (V) and absolute temperature (T) of a fixed amount of gas are linked. They are written for an ideal gas, so they are good approximations for air and nitrogen and a rough guide for refrigerant vapour, which departs further from ideal behaviour near saturation, where the vapour is close to turning into liquid. Two rules apply to every calculation:
Boyle's law states that, at constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume: P1 × V1 = P2 × V2. Halving the volume doubles the absolute pressure. This is the principle of the compressor: the piston or scroll reduces the space occupied by the refrigerant vapour, and its pressure rises.
Worked example: a compressor draws in 0.50 m³ of refrigerant vapour at 150 kPa (absolute) and squeezes it to 0.20 m³ with the temperature held constant.
P2 = P1 × V1 ÷ V2 = 150 × 0.50 ÷ 0.20 = 375 kPa (absolute)
In a real compressor the vapour also heats up as it is compressed, so the delivery pressure is higher than the constant-temperature figure. Boyle's law gives the first estimate, and the combined gas law below gives the closer one.
Charles' law states that, at constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature: V1 ÷ T1 = V2 ÷ T2. A gas that is warmed expands, and a gas that is cooled contracts. In refrigeration this explains why the volume of vapour entering the compressor changes with the suction temperature, and why the air in a cold room expands when warm air enters through an open door.
Worked example: 2.0 m³ of air in a cold room at 20 °C (293 K) is warmed at constant pressure to 50 °C (323 K).
V2 = V1 × T2 ÷ T1 = 2.0 × 323 ÷ 293 = 2.20 m³
The same mass of gas now needs about 10 percent more space. A technician who ignores this when sizing ducts or choosing a suction line will under-estimate the flow of vapour at higher temperatures.
Dalton's law states that the total pressure of a mixture of gases is the sum of the partial pressures that each gas would exert if it alone filled the space: P total = P1 + P2 + P3 + … . In refrigeration the law is used in two ways:
Worked example: the saturation pressure of a refrigerant at the measured condensing temperature is 1,000 kPa (absolute), but the high-side gauge shows 1,150 kPa (absolute). The excess is 1,150 − 1,000 = 150 kPa, which points to non-condensable gas. The remedy is to recover the charge, evacuate the system properly and recharge.
Gay-Lussac's law states that, at constant volume, the pressure of a fixed mass of gas is directly proportional to its absolute temperature: P1 ÷ T1 = P2 ÷ T2. A sealed pipe charged with dry nitrogen for a pressure test shows the law clearly.
Worked example: refrigerant pipework is charged with dry nitrogen to 1,000 kPa (absolute) at 20 °C (293 K). By afternoon the pipe has warmed to 40 °C (313 K).
P2 = P1 × T2 ÷ T1 = 1,000 × 313 ÷ 293 = 1,068 kPa (absolute)
A change of this size is caused by temperature, not by a leak, so a technician records the temperature with the pressure when judging a test. The law applies to a gas such as nitrogen. A cylinder of liquid refrigerant behaves differently: its pressure follows the saturation pressure at its temperature, and a refrigerant cylinder must never be heated.
The combined gas law joins the three laws when pressure, volume and temperature all change: P1 × V1 ÷ T1 = P2 × V2 ÷ T2. It gives a closer estimate of the temperature rise during compression.
Worked example: 0.80 m³ of air at 100 kPa and 300 K is compressed to 0.20 m³ at 500 kPa.
T2 = T1 × P2 × V2 ÷ (P1 × V1) = 300 × 500 × 0.20 ÷ (100 × 0.80) = 375 K, which is 102 °C
The compression heats the gas by about 75 K. This is why the discharge line of a compressor is hot, and why a very high discharge temperature signals a problem such as a high compression ratio or a restricted condenser.
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Create a free accountThis chapter explored the application of physics principles to the maintenance of refrigeration systems, focusing on fundamental gas laws such as Boyle's, Charles', and Dalton's laws. It examined heat transfer mechanisms including sensible and latent heat, unit conversions, temperature enthalpy relationships, and the role of thermal insulation. Various refrigeration cycles were discussed, highlighting vapour compression, absorption, evaporative cooling, and the Peltier effect. The chapter covered different refrigerants, emphasizing the transition from fluoro-carbons to hydro-carbons for environmental reasons. Safe handling practices were detailed, addressing toxicity, cold burns, and ozone layer protection. The process of recycling fluoro-carbons was also outlined. Applications of refrigeration in hospital settings were described, including vaccine storage, blood banks, laboratories, pathology, and kitchen food storage. Key components of refrigerators such as the evaporator, condenser, compressor, expansion valve, reservoir, fans, ducts, and filters were identified, followed by an overview of refrigeration system installation. Finally, pipe work processes including cutting, debarring, bending, joining, soldering, brazing, flared, and swaged joints were explained, concluding with essential maintenance procedures for refrigeration equipment.
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