Before the invention of mechanical refrigeration over 150 years ago, postharvest cooling of fresh produce was difficult, expensive, often inadequate, and accomplished by harvesting winter ice and storing it for use during the warm summer season. Ancient records suggest that by 1000 BC, the Chinese, Persians, and others were harvesting snow and ice and storing it in cellars that were insulated with hay or straw for use during the warmer months. By the 1830s, large commercial firms in the United States were beginning to harvest winter ice from rivers in New England (see Figure 4-1), and shipping it south in specially built, insulated ships to Charleston, Savannah, and Havana to service the growing banana and meat packing trades.

In 1755, William Cullen of Scotland developed the first mechanical refrigeration machine. Cullen used an air pump to create a vacuum over a flask of ether. The ether boiled, which extracted heat from the ether and flask. Water vapor from the surrounding air condensed on the outside of the flask and then froze into ice. Around the same time, Benjamin Franklin and others in America were experimenting with similar concepts. In 1805, Oliver Evans, an American inventor, described a closed vapor-compression cycle system that produced ice. In 1834, Jacob Perkins built and patented the first continuously working refrigeration system. In 1842, John Gorrie, a Florida physician, built a working ice making system that inspired global efforts at mechanical refrigeration (see Figure 4-2). Gorrie’s actual design was not commercially successful.

Despite experiments with mechanical refrigeration, the use of winter ice increased from the 1850s for both rail and ocean shipments into the early 1900s. By the 1880s, frozen beef and mutton were kept cool with ice and shipped from South America and Australia to Europe. After 1890, issues with ice from streams polluted with waste and sewage forced a change in the industry to artificial ice that was made primarily with ammonia cycle mechanical refrigeration. In the United States, where there was wide availability of commercial refrigeration, Chicago and the Midwest became the center of meatpacking, as the central valley of California, the Rio Grande Valley, and south Florida became the centers of national vegetable and fruit production. Fresh produce from these areas was shipped nationwide in specially constructed ice-cooled rail cars.

Figure 4-1. Harvesting cut blocks of ice from a northeast river.

Source: M. Boyette.

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Figure 4-1. Harvesting cut blocks of ice from a northeast river.

Source: M. Boyette.

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Figure 4-2. Dr. John Gorrie (1803 – 1855).

Source: National Statuary Hall, Washington DC — 1914.

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Figure 4-2. Dr. John Gorrie (1803 – 1855).

Source: National Statuary Hall, Washington DC — 1914.

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There are a number of ways to produce artificial cooling. The most important is with a mechanical vapor compression system. Very simply, a compressible gas is contained inside a cylinder equipped with a piston. When the piston compresses the gas, the volume decreases proportionally, while the pressure and temperature increase. When the compressed gas cools back to its original temperature and the pressure is then released to allow the fluid to expand, the expansion causes the temperature of the gas to drop. This process is adiabatic cooling. If the fluid condenses into a liquid during the compression cycle, the heat transferred out through the walls of the cylinder will be the sensible heat of the gas plus the heat of condensation. When the condensed fluid is pumped to another location and then allowed to expand and evaporate, it cools the surroundings via a heat exchanger and collects the required heat of evaporation. In this way, the system can be considered a pump for heat.

Just as water naturally flows downhill, heat energy moves from warm to cool. Similarly, a refrigeration system is a device for pumping heat “uphill” to a warmer environment just as a water pump is a mechanical device for pumping water uphill against a head pressure. For example, a mechanical refrigeration system removes the field heat from hot produce inside a refrigerated room held at 40°F to the ambient outside, which may be 90°F+. In this way, heat is pumped uphill from cold to hot.

By definition, any device or process that converts one type of energy to another can be called an engine. A primitive fire starter, shown in Figure 4-3, which converts mechanical energy to heat via friction, is a heat engine. Likewise, the fire that is eventually started is also a heat engine since it converts the chemical energy stored in the wood to heat and light.

As shown in Figure 4-4, force that is applied to the piston causes it to move (work = force × distance). The pressure increases, the volume decreases, and the molecules of the compressed fluid are forced closer together, which results in the production of heat. Consequently, mechanical energy is converted to heat energy. When the fluid is allowed to cool to ambient and the pressure is released, the fluid cools to below ambient. This is the key mechanism of mechanical refrigeration.

Compressing a gas (such as air) and allowing it to cool and then releasing the pressure will yield air with a temperature below ambient. Due to mechanical inefficiencies, the total refrigeration produced will be less than the equivalent amount of work input. However, if the process of compressing and cooling the gas results in its condensation into a liquid, the amount of heat removed will be greater that the equivalent amount of work input. The changes of phase allow the fluid to carry a much greater amount of heat as it passes through the circuit.

Enthalpy (H) is a property equivalent to the total heat of a system. Enthalpy is equal to the system’s internal energy plus the product of its pressure and volume (P·V). In a closed system where matter cannot escape, the heat absorbed or released equals the change in enthalpy.

H = E + PV

Figure 4-3. A heat engine.

Source: M. Boyette.

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Figure 4-3. A heat engine.

Source: M. Boyette.

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Figure 4.-4. Another type of heat engine.

Source: NC State University.

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Figure 4.-4. Another type of heat engine.

Source: NC State University.

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A refrigerant is the compound or mixture of working fluids that acts as the heat carrier in a mechanical refrigeration system. The search for an improved refrigerant has been a major focus of the research and development of mechanical refrigeration from its beginning. As the science of refrigeration has matured, the definition of a better refrigerant has changed. The ability to change from a gas to a liquid at a reasonable temperature and pressure is fundamental. Early on, sulfur dioxide, methyl chloride, or ammonia were used. All were toxic and corrosive, and have disappeared slowly from use with the introduction of chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC). The CFC’s and HCFC’s were used widely because they were non-flammable, non-toxic, and non-corrosive. However, the CFC’s and HCFC’s have been phased out in recent years because of their potential harm to the environment. Newer formulations now have much lower potential for environmental harm.

Adiabatic means that no heat enters or leaves the system. In the commercial expansion valve shown in Figure 4-5, the fluid enters the system at point 1 under high pressure and leaves it at point 2 under low pressure, while the mass flow rate is steady. The Joule-Thomson effect involves a steady adiabatic flow of a fluid through a flow resistance (valve, porous plug, or any other type of flow resistance). This process is very important because it is at the heart of refrigeration processes. In operation, the warm compressed liquid refrigerant passes through the flow resistance of an expansion valve to an area of lower pressure where it flashes (boils) to become a cold vapor. The cold vapor is then passed through a heat exchanger where it extracts heat from the surrounding air. The warmed vapor is then compressed, passed through a second heat exchanger where it is cooled before sending it again through the expansion valve. This is a closed system.

Figure 4-5. Example of a commercial expansion valve. The valve is operated automatically by the gas-filled copper bulb that controls the fluid flow through the orifice.

Source: M. Boyette.

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Figure 4-5. Example of a commercial expansion valve. The valve is operated automatically by the gas-filled copper bulb that controls the fluid flow through the orifice.

Source: M. Boyette.

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A refrigeration system is a type of heat pump. In some ways, a refrigeration system is like the liquid cooling system of an automobile. In an automobile engine, the water pump moves a mixture of water and glycol (coolant) through cavities in the engine block where it picks up the waste heat from the combustion in the engine. The water then passes through a water-to-air heat exchanger (radiator), while heat is released to the atmosphere. The cooled water is then recirculated back through the engine block because it is a closed system. The heat that is moved (pumped, transferred) from the engine to the atmosphere is only the sensible heat carried by the coolant because there is no change of phase.

Likewise, in a closed-cycle refrigeration system (see Figure 4-6), the compressor works on the refrigerant, which increases the pressure of the gas. As the pressure of the gas increases, the temperature of the gas also increases as predicted by the ideal-gas law. This high-pressure, high-temperature gas then enters the condenser coil which is exposed to the atmosphere. Heat flows through the coil from the high-temperature gas to the lower-temperature ambient air. (This is similar to heated coolant passing through an automobile radiator.) This loss of heat causes the high-pressure gas to condense to liquid. The work done on the gas by the compressor (causing an exothermic phase transition in the gas) is converted to heat given off in the ambient air. In addition to the sensible heat, there is also a change of phase (the gas is liquefied), which allows the refrigerant to carry a much greater amount of heat per unit weight.

The now cooled liquid then passes into an accumulator (a tank that acts as a kind of surge bin or capacitor) before passing through the expansion valve and coil inside the insulated space. The liquid is at a low pressure (as a result of the expansion) and is lower in temperature (cooler) than the air inside the refrigerated space. The compressor and the expansion valve are functionally exact opposites.

Since heat is transferred from areas of greater temperature to areas of lower temperature, heat is absorbed (from inside the refrigerated space) by the evaporating (phase changing) refrigerant, which causes the temperature inside the refrigerated space to be reduced. The absorbed heat begins to break the intermolecular attractions of the liquid refrigerant, which allows the endothermic vaporization process to occur. Compared to the gain in sensible heat, the latent heat gain is much greater. The warmed gas now passes back to the compressor, which begins the cycle once again.

Figure 4-6. A typical refrigeration system.

Source: M. Boyette.

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Figure 4-6. A typical refrigeration system.

Source: M. Boyette.

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Another way to understand the refrigeration cycle is to view a diagram of enthalpy versus pressure of the refrigerant as shown in Figure 4-7. The process begins at B′ because compressors cannot compress incompressible liquids – which will stall or break. Point C is not 100% gas but in the transition state has some liquid. The compressor cannot handle liquids. By making the evaporator coil a little bigger (which decreases the ∆T), the refrigerant can absorb more heat from the refrigerated space after it passes the saturated point B. This is known as super-heating the refrigerant, which occurs after the saturated point. The temperature rises to T4 and the process moves to Point B′. The temperature is usually higher than the saturated temperature by 10°F to 15°F. It is very important to make sure that only gas goes to the compressor.

Notice that point B and point C (the compression process) both have the same amount of enthalpy (E2). However, in reality, the compressor is an electro-mechanical device with an efficiency of less than 100%. Typically, this would be about 80%, which means that only 80% of the mechanical energy used to operate the compressor will actually be used to compress the refrigerant. The remaining 20% is lost in a form of heat. The First Law of Thermodynamics says that energy cannot be created or destroyed but only changed in form. The remaining 20% heat will go into heating the refrigerant and increasing its sensible heat (T1 to T2). That is the reason that Point C′ has a higher enthalpy (E5) than Point B′ (E4.)

In addition, by making the condenser a little larger, more heat can be rejected from the refrigerant from the saturation point D to point D′, which has a lower temperature (T3) and lower enthalpy (E3). This is called sub-cooling. The function of sub-cooling is to allow the evaporator to start from the saturation point A′ rather than the transient point A. This will increase the amount of heat the evaporator will absorb (A′-B′), which is larger than from point A to point B.

Note that:

• Plotting the refrigeration cycle on the pressure-enthalpy graph can be a very important tool for trouble shooting the cycle by knowing the extent of super-heat and the sub-cool. From this, you can observe how much heat the compressor adds and then determine the level of mechanical efficiency.
• The amount of heat we reject from the condenser (C′-D′) is equal to the amount of heat that we absorb from the evaporate (A′-B′), plus the little heat (E4 to E5) that the compressor adds to the system and
• The ratio of (A′-B′) / (C′-D′) is the refrigeration cycle efficiency (RCE) which is always lower than 1.

Many Ways to Measure Efficiency. The refrigeration cycle allows us to move heat uphill from one place to another against its natural flow. The price of this is the cost of electricity to run the compressor motor. Coefficient of performance (COP) is used frequently with heat pumps like refrigeration systems. The COP ratings range from 1.5 to 4 with performance depending on the ∆T between inside and outside temperatures. For example, when it is 95°F outside and 40°F inside (∆T= 55 F°), a refrigeration system must work much harder to pump heat uphill than when it is 65°F outside (∆T= 25 F°). A system with a COP of 3.4 delivers 3.4 units of output for every 1.0 unit of input.

Figure 4-7. Pressure-enthalpy graph for a typical refrigeration system.

Source: M. Boyette.

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Figure 4-7. Pressure-enthalpy graph for a typical refrigeration system.

Source: M. Boyette.

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Window air conditioners are rated in tons. A ½ ton unit certainly does not appear to weigh 1000 lb. The explanation is very simple. Natural ice harvested in blocks from rivers was sold by the ton. The users of block ice were familiar with the cooling capacity of a ton of ice. When mechanical refrigeration came on the market in the early 1900s, it was natural to size capacity in tons of ice equivalent. Buyers wanted to know how many tons of ice the system would produce in a day. Since the heat of fusion of ice is 144 Btu/lb., a ton of ice melted per day would yield the cooling equivalent of (144 Btu/lb.) (2000 lb./day) = 288,000 Btu/day or 12,000 Btu/hr. Thus, a window air conditioning unit rated at ½ ton is capable of extracting ½ (12,000) = 6000 Btu per hour, which is sufficient to keep a small room comfortably cool on a hot summer day.