In Earlier Times, one could hire people to blow cool air off a river using palm leaves.However, keeping someone by your desk is expensive and impractical, and mostbosses won’t let you work by the river or lake.
Luckily, during the industrial revolution, humans discovered the internal combustionengine, and later a derivative, the fan.However, unless there is a body of water close by (such as Deep Water Coolingdiscussed later), what can you use as a cooling source?First some definitions…
What is Refrigeration? Refrigeration is the process of removing heat from one substance and transferring it to another substance.The term refrigeration is commonly associatedwith something cold. A householdrefrigerator, for example, keeps food cold. Itaccomplishes this task by removing heat fromthe food. Therefore, refrigeration involves theremoval of heat. The word cold describes astate of low heat content.
What is Heat?Heat is energy transferred from one body to another bythermal interaction. It is not a property of a system orbody, but instead is always associated with a process ofsome kind.Heat intensity is measured by its temperature, commonly in either degreesFahrenheit (°F) or degrees Celsius (°C). If all heat were removed from an object, thetemperature of the object would decrease to -459.6°F [-273.2°C]. This temperatureis referred to as “absolute zero” and is the temperature at which all molecularactivity stops.The quantity of heat contained in an object or substance is not the same as itsintensity of heat. For example, the hot sands of the desert contain a larger quantityof heat, but a single burning candle has a higher intensity of heat.
What is Temperature?Measure of the amount of Thermal Energy in a given area. These two different masses of water contain the same quantity of heat, yet the temperature of the water on the left is higher. Why? The water on the left contains more heat per unit of mass than the water on the right. In other words, the heat energy within the water on the left is more concentrated, or intense, resulting in the higher temperature. Note that the temperature of a substance does not reveal the quantity of heat that it contains.
Measuring Heat Quantity: 1 kg of 1 Ib of Water Water 1 Kcal 1 Btu 21oC 22oC 60oF 61oFIn the English system of units, the quantity of heat is measured in terms ofthe British Thermal Unit (Btu). The Btu is defined as the quantity of heatenergy required to raise the temperature of 1 lb of water by 1°F.Similarly, in the metric system of units, the quantity of heat is measured interms of the kilocalorie (kilogram-calorie or kcal). The kcal is defined asthe amount of heat energy required to raise the temperature of 1 kg ofwater 1°C.
Principles of Heat Transfer: • Heat energy cannot be destroyed • Heat always flows from a higher temperature substance to a lower temperature substance • Heat can be transferred from one substance to another
Heat Energy Cannot be Destroyed: To produce cooling, heat must be removed from the substance by transferring it to another substance. The first principle to discuss regarding heat transfer is that heat energy cannot be destroyed; it can only be transferred to another substance. This is commonly referred to as the principle of “conservation of energy.” Ice cubes are typically placed in a beverage to cool it before it is served. As heat is transferred from the beverage to the ice, the temperature of the beverage is lowered. The heat removed from the beverage is not destroyed but instead is absorbed by the ice, changing the ice from a solid to a liquid.
Heat Flows from Hot to Cold: The second principle is that heat naturally flows from a higher temperature substance to a lower temperature substance; in other words, from hot to cold. Heat cannot flow from a cold substance to a hot substance. Consider the example of the beverage and the ice cubes. As long as the temperature of the beverage is higher than the temperature of the ice cubes, heat will always flow from the beverage to the ice cubes.
Method of Heat Transfer - Conduction: The third principle is that heat is transferred from one substance to another by one of three basic processes: • Conduction • Convection • and Radiation. Conduction is the process of transferring heat through a solid by way of diffusion and collisions of particles.
Method of Heat Transfer - Convection: Convection is the process of transferring heat as the result of the movement of a fluid. Convection often occurs as the result of the natural movement of air caused by temperature (density) differences.
Method of Heat Transfer - Radiation: Radiation is the process of transferring heat by means of electromagnetic waves, emitted due to the temperature difference between two objects. An interesting thing about radiated heat is that it does not heat the air between the source and the object it contacts ; it only heats the object itself.
Rate of Heat Flow:In refrigeration, as in heating, emphasis is placed on the rate of heat transfer, thatis, the quantity of heat that flows from one substance to another within a givenperiod of time. This rate of heat flow is commonly expressed in terms of Btu/hr;the quantity of heat, in Btus, that flows from one substance to another over aperiod of 1 hour.Similarly, in the SI metric system of units, the rate of heat flow is expressed interms of kilowatts (kW), which are equivalent to kJ/sec. Kilowatts describ thequantity of heat, in kJ, that flows from one substance to another over a period of 1second.
Rate of Heat Flow:In the English system of units, there is a larger and more convenient measure ofthe rate of heat flow. It is called a ton of refrigeration. One ton of refrigerationproduces the same cooling effect as the melting of 2000 lb of ice over a 24-hourperiod.When 1 lb of ice melts, it absorbs 144 Btu. Therefore, when 1 ton (2000 lb) of icemelts, it absorbs 288,000 Btu (2000 x 144). Consequently, 1 ton of refrigerationabsorbs 288,000 Btu within a 24-hour period or 12,000 Btu/hr (288,000/24).So, 1 ton of refrigeration is defined as the transfer of heat at the rate of 12,000Btu/hr.
1 kg of Water How Refrigeration Works – Change of State: 1 Kcal 100oC 100oC 1 kg of 1 kg of Water Water 79 Kcal 21oC 100oC 244 Kcal 100oC 100oCThis question is best answered by examining the effects of heat transfer on water.Consider 1 kg of 21°C water. By adding or subtracting 1 Kcal of heat energy, thewater temperature is raised or lowered by 1°C.Therefore, adding 79 Kcal to 1 kg of 21°F water raises its temperature to 100°C.Although this is the boiling temperature of water at atmospheric pressure, adding1 more Kcal will not cause all of the water to evaporate.In fat, 244 kcal (1023 kJ) must be added to 1 kg of 100°C water to completelytransform it to 1 kg of steam at the same temperature.
How Refrigeration Works – Change of State: 1 kg of Steam -244 Kcal 100oC 100oCConversely, when 1 kg of 100°C steam condenses, it gives off 244.5 kcal (1023 kJ) ofheat energy in the process. After the steam condenses completely, the removal ofmore heat will begin to lower the temperature of the water below 100°C.The quantity of heat that must be added to the water in order for it to evaporatecannot be sensed by an ordinary thermometer. This is because both the water andsteam remain at the same temperature during this phase change.This kind of heat is called latent heat, which is dormant or concealed heat energy.Latent heat is the energy involved in changing the phase of a substance—from aliquid to a vapor in this example.
How Refrigeration Works – Change of State: 1 kg of Steam -20 Kcal 100oC 80oCIn contrast, sensible heat is heat energy that, when added to or removed from asubstance, results in a measurable change in temperature.Refrigerants can absorb a significant amount of heat when they change phase;much more than if they just change temperature. Different substances havedifferent specific temperatures at which these phase changes occur, and differentquantities of heat are required for this change to take place. They also havedifferent capacities for absorbing heat. This capacity is a property of the substancecalled specific heat.
How Refrigeration Works – Change of State: “Cold” Refrigerant Airflow Liquid “Warm” Refrigerant Vapour EvaporatorA rudimentary refrigeration system could be constructed using a drum of liquidrefrigerant at atmospheric pressure, a coil, a collecting drum, and a valve toregulate the flow of refrigerant into the coil.Opening the valve allows the liquid refrigerant to flow into the coil (Evaporator) bygravity. As warm air is blown over the surface of the coil, the liquid refrigerantinside the coil will absorb heat from the air, eventually causing the refrigerant toboil while the air is cooled. Adjustment of the valve makes it possible to supply justenough liquid refrigerant to the coil so that all the refrigerant evaporates before itreaches the end of the coil.
How Refrigeration Works – Change of State: Evaporator Refrigerant Liquid Refrigerant ? VapourOne disadvantage of this system is that after the liquid refrigerant passes throughthe coil and collects in the drum as a vapor, it cannot be reused. The cost andenvironmental impacts of chemical refrigerants require the refrigeration process tocontinue without loss of refrigerant.There needs to be a magic box that converts the refrigerant vapours back to liquid.
How Refrigeration Works – Change of State: Refrigerant - Heat Refrigerant Vapour LiquidHeat flows from a higher temperature substance to a lower temperaturesubstance. In order to remove heat from the refrigerant vapor, it must transfer thisheat to a substance that is at a lower temperature. Assume that the refrigerantevaporated at -41.4°F [-40.8°C]. To condense back into liquid, the refrigerant vapormust transfer heat to a substance that has a temperature less than -41.4°F [-40.8°C].
How Refrigeration Works – Lowering Temperature:One way to change the temperature is to adjust the pressure on the vapour.Gay-Lussacs_law holds that the pressure of a gas of fixed mass and fixed volume isdirectly proportional to the gas absolute temperature. Simply put, if a gastemperature increases then so does its pressure, if the mass and volume of the gasare held constant. The law has a particularly simple mathematical form if thetemperature is measured on an absolute scale, such as in kelvins. The law can thenbe expressed mathematically as:This law holds true because temperature is a measureof the average kinetic energy of a substance; as thekinetic energy of a gas increases, its particles collidewith the container walls more rapidly, thereby exertingincreased pressure.
How Refrigeration Works – Lowering Temperature:Example:If the refrigerant is at -40oC (233 Kelvin) andwe wish to increase the temperature to -10oC(263 Kelvin) and the current pressure is 14psi;P1T2 = P2T1P2 = (14psi)*(263K)/ (233K)P2 = 15.8psiTo raise the temperature 30C, raise thepressure by 1.81psi (assuming the volume andmass remain the same).
How Refrigeration Works – Compressor: Evaporator Refrigerant Liquid Refrigerant Vapour Compressor Once the refrigerant leaves the Evaporator as vapour, it flows through the Compressor which compress it to a higher pressure (and therefore also a higher temperature).
How Refrigeration Works – Condenser: Evaporator Refrigerant Liquid Refrigerant Vapour Condenser Compressor After the Refrigerant leaves the compressor as a high- temperature-high-pressure vapour, it is passed through a series of coils called the Condenser located outside the cooling area, where outside air is blown past to cool the refrigerate enough to return it back to its liquid state.
How Refrigeration Works – Expansion Device: Expansion Device Evaporator Refrigerant Liquid Refrigerant Vapour Condenser Compressor Finally, an expansion device is used to create a large pressure drop that lowers the pressure, and correspondingly the temperature, of the liquid refrigerant. The temperature is lowered to a point where it is again cool enough to absorb heat in the evaporator
How Refrigeration Works – Recap:0. Refrigerant starts as cool liquid at low pressure.1. Refrigerant runs through Evaporator coil to cool area, gains heat, boils into vapour.2. Refrigerant runs through Compressor, becomes high press./high temp vapour.3. Refrigerant runs through Condenser coil, Heat transfer to outside air.4. Refrigerant runs through Expansion Device, becomes low press./low temp liquid.
Deep Water Cooling“Enwaves three intake pipes draw water (4 degrees Celsius) from 5 kilometres off theshore of Lake Ontario at a depth of 83 metres below the surface. Naturally cold watermakes its way to the Citys John Street Pumping Station. There, heat exchangersfacilitate the energy transfer between the icy cold lake water and the Enwave closedchilled water supply loop.The water drawn from the lake continues on its regular route through the John StreetPumping Station for normal distribution into the City water supply. Enwave uses onlythe coldness from the lake water, not the actual water, to provide the alternative toconventional air-conditioning.”
Questions• 1. If one was to increase the pressure in a closed refrigeration cycle, what happens to the temperature?• 2. When the refrigerant passed through the Evaporator, the temperature of the refrigerant goes down and the surrounding area is cooled (T/F)• 3. There are 12,000 Btu in a ton of cooling (T/F)• 4. The SI unit of pressure is “Pressure per square inch (psi)” (T/F)• 5. Convection is heat transfer through liquids (T/F)• 6. The Evaporator comes before the expansion device, and after the compressor. (T/F)• 7. The relationship between pressure & temperature is described by: a) Guy-Lussac’s Law b)Gay-Lussac’s Law c) Ideal Gas Law d) Boyle’s Law e) none of the above