Presentation on theme: "Refrigeration & Air Conditioning"— Presentation transcript:
1 Refrigeration & Air Conditioning The purpose of refrigeration is to cool spaces, objects, or materials and to maintain them at a temperature below the temperature of the surrounding atmosphere. In order to produce a refrigeration effect, it is merely necessary to expose the material to be cooled to a colder object or environment and allow heat to flow in its "natural" direction, that is, from the warmer material to the colder material. So, the heat we do not want will be removed, cooling the space or equipment.
2 Objectives Basic operation of refrigeration and AC systems Principle components of refrigeration and AC systemsThermodynamic principles of refrigeration cycleSafety considerationsA. The student will comprehend the basic operation, principle components, and safety considerations related to refrigeration and air conditioning systems.The student will apply correct procedures including comprehension of the thermodynamic principles involved to determine output and efficiency of refrigeration systems.
3 Uses of Systems Cooling of food stores and cargo Cooling of electronic spaces and equipmentCIC (computers and consoles)Radio (communications gear)RadarsESGN/RLGNSonarCooling of magazinesAir conditioning for crew comfort
4 Definition ReviewSpecific heat (cp): Amount of heat required to raise the temperature of 1 lb of substance 1°F (BTU/lb) – how much for water?Sensible heat vs Latent heatLHV/LHFSecond Law of Thermodynamics: must expend energy to get process to workSpecific Heat. Specific heat is the amount of heat required to raise one pound of a substance 1°F at atmospheric pressure. (Notice the difference with the definition of the BTU: the BTU is the heat required to raise the temperature of water, whereas specific heat is for any substance.) British Thermal Unit (BTU). The amount of heat needed to raise one pound of water 1°F at atmospheric pressure.Sensible Heat. Sensible heat is that heat given off or absorbed by a substance which does not cause the substance to change phase. Sensible heat changes are observed as changes in temperature and are measured by a thermometer.Latent Heat. Latent heat is given off or absorbed by a substance that is changing phase. The temperature and pressure remain constant during the phase change until all the substance has been transformed. These temperature and pressure conditions are unique and are called saturation conditions. The latent heat of vaporization (LHV) is the heat required to transform a liquid to a gas at constant temperature and pressure. The latent heat of condensation (LHC) is equivalent in magnitude to the LHV for the substance, but now we are going in the opposite direction in transforming the gas to a liquid. When a liquid is transformed to a solid as in the ice-making process, the liquid gives off its latent heat of fusion (LHF) to form the solid. The LHF to transform a pint of water to 1 lb. of ice at 32°F is 144 BTUs. We would have to add 144 BTUs to every pound of ice to melt it.
5 Refrigeration CycleRefrigeration - Cooling of an object and maintenance of its temp below that of surroundingsWorking substance must alternate b/t colder and hotter regionsMost common: vapor compressionReverse of power cycleHeat absorbed in low temp region and released in high temp region
7 Thermodynamic CycleThe dome-like curve represents saturated conditions for the refrigerant. On the left half of the dome, the refrigerant exists as a saturated liquid and on the right as saturated vapor. Both liquid and gaseous refrigerant coexist inside the dome in saturation. To the left of the dome, the refrigerant is a subcooled liquid and to the right of the dome, it is a superheated vapor.The numbers (1 through 4) represent significant points in the flow of refrigerant as it makes its circuit in the cycle. The refrigerant working fluid undergoes thermodynamic changes between these points.¨ Point 1-2 (Evaporation): Since this is inside the dome, constant pressure (21.5 psia) and temperature (-5°F) are maintained, i.e., saturation. When heat is transferred at saturation, the result is a change in phase.¨ Point 2-3 (Compression): Compressing the gaseous Freon from 21.5 to 141 psia (6.5 to 126 psig) produces a concomitant increase in thermal energy represented by a rise in the enthalpy and the temperature of the Freon from 5° to 125°F. This is the heat of compression resulting from the added energy to the Freon vapor. Compression provides the thermal driving head to sustain the flow of Freon through the cycle.¨ Point 3-4 (Condensation): In passing through the dome from the right side to the left, the refrigerant cools from 125° to 105°F and changes phase from a superheated vapor to a slightly subcooled liquid.Point 4-1 (Expansion): The refrigerant is expanded by passing through an expansion valve where its pressure is reduced from 141 psia to 21.5 psia. In the process of expanding, the Freon cools from 105° to -5°F (cold of expansion) and crosses into the dome where both saturated liquid and gaseous Freon can coexist. About 25% of the fluid vaporizes into a gas during the process.
10 Refrigerant Desirable properties: High latent heat of vaporization - max coolingNon-toxicity (no health hazard)Desirable saturation temp (for operating pressure)Chemical stability (non-flammable/non-explosive)Ease of leak detectionLow costReadily availableCommonly use FREON (R-12, R-114, etc.)
11 Evaporator/Chiller Located in space to be refrigerated Cooling coil acts as an indirect heat exchangerAbsorbs heat from surroundings and vaporizesLatent Heat of VaporizationSensible Heat of surroundingsEvaporationFrom the expansion valve, Freon as a saturated mixture of liquid and vapor passes into the cooling coil, or evaporator, located in the freeze box to be cooled. The cooling coil acts as a heat exchanger. The boiling point of the refrigerant under the low pressure in the evaporator is extremely low - much lower than the temperature of the spaces in which the cooling coils are installed. The temperature differential between the -5°F refrigerant in the coils and the air in the freeze box slightly above 0°F causes heat to be transferred from the freeze box to the refrigerant. It absorbs its latent heat of vaporization from the surroundings, thereby cooling the space. The refrigerant continues to absorb heat until all the liquid has boiled and vaporized. To ensure all the refrigerant changes phase to vapor, the Freon must be slightly superheated. As a rule, 6° to 10°F of superheat is added to the Freon. The refrigerant leaves the evaporator as a low pressure superheated vapor, having cooled the freeze box by absorbing its unwanted heat. The remainder of the cycle is concerned with disposing of this heat and getting the refrigerant back into a liquid state so that it can again vaporize in the evaporator and thus again absorb heat from the freeze box.Slightly superheated (10°F) - ensures no liquid carryover into compressor
13 Compressor Superheated Vapor: Enters as low press, low temp vaporExits as high press, high temp vaporTemp: creates differential (DT) promotes heat transferPress: Tsat allows for condensation at warmer tempsIncrease in energy provides the driving force to circulate refrigerant through the systemCompressionThe low pressure, superheated Freon vapor is discharged from the evaporator to the suction side of the compressor. The compressor is the mechanical unit which keeps the refrigerant circulating through the system by increasing the fluid’s pressure and thermal potential energies. In the compressor (either reciprocating or centrifugal), the refrigerant is compressed from a low pressure vapor to a high pressure vapor, and its temperature rises accordingly from the heat of compression. This increase in energy provides the driving force to allow the Freon to flow through the system.
14 Condenser Refrigerant rejects latent heat to cooling medium Latent heat of condensation (LHC)Indirect heat exchanger: seawater absorbs the heat and discharges it overboardCondensationThe refrigerant must be thermodynamically returned to its starting point as a high pressure (141 psia) and high temperature (105°F) subcooled liquid from a higher temperature (125°F) superheated vapor. There is a significant amount of heat to extract in transforming the Freon from a gas to a liquid in the form of latent heat of condensation (LHC). Since this extraneous heat must be disposed, a sea water heat exchanger is used to absorb the LHC and discharge it overboard. The heat removal from the refrigerant causes it to condense into a liquid at a constant pressure of 141 psia. The refrigerant, still at a high pressure, is now a subcooled liquid ready to commence the process again. From the condenser, the refrigerant flows into a receiver, which serves as a storage place for the liquid refrigerant and as a seal between the high and low pressure sides of the Freon loop. From the receiver, the refrigerant returns to the expansion valve and the cycle begins again.All refrigeration and air conditioning systems follow this simple process no matter what type of refrigerant is used. The operating parameters will change, but it still is the same basic cycle.
15 ReceiverTemporary storage space & surge volume for the sub-cooled refrigerantServes as a vapor seal to prevent vapor from entering the expansion valve
16 Expansion Device Thermostatic Expansion Valve (TXV) Liquid Freon enters the expansion valve at high pressure and leaves as a low pressure wet vapor (vapor forms as refrigerant enters saturation region)Controls:Pressure reductionAmount of refrigerant entering evaporator controls capacityExpansionLiquid Freon enters the expansion valve at high pressure. The refrigerant leaves the outlet of the expansion valve at a much lower pressure and enters the low pressure side of the system. Because the pressure release has decreased the refrigerant’s potential energy, the liquid refrigerant manifests this energy conversion by beginning to boil and to flash into vapor. The Freon is still saturated and at a very low temperature of -5°F entering the evaporator, or chiller, coils. It is now a mixture of liquid and vapor refrigerant. This temperature gives us a thermal differential to cool, or keep cool, a freeze box which must be maintained at 0°F. The refrigerant is now ready to absorb the unnecessary heat from the freeze box by entering the evaporator coils located in the space to be cooled (freeze box).
18 Air ConditioningPurpose: maintain the atmosphere of an enclosed space at a required temp, humidity and purityRefrigeration system is at heart of AC systemHeaters in ventilation systemTypes Used:Self-containedRefrigerant circulatingChill water circulating- Self-contained equipment are sealed refrigerating or freezing systems within a cabinet or housing to perform a specific function or service. These are sometimes referred to as package units.
19 AC System Types Self-Contained System Refrigerant circulating system Add-on to ships that originally did not have AC plantsNot located in ventilation system (window unit)Refrigerant circulating systemHot air passed over refrigerant cooling coils directlyChilled water circulating systemRefrigerant cools chill waterHot air passes over chill water cooling coils- Self-contained equipment are sealed refrigerating or freezing systems within a cabinet or housing to perform a specific function or service. These are sometimes referred to as package units.
21 Safety Precautions Phosgene gas hazard Handling procedures LethalCreated when refrigerant is exposed to high temperaturesHandling proceduresWear goggles and gloves to avoid eye irritation and frostbiteAsphyxiation hazard in non-ventilated spaces (bilges since heavier than air)Handling of compressed gas bottles
22 Questions?Perform sample problems given in Appendix A and assign appropriate problems for student homework.