Refrigeration & AC Systems

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Steam Jet Refrigeration Uses principle of boiling water below $100^\circ C$. Boiling occurs at low temperatures when surface pressure is reduced below atmospheric. E.g., $6^\circ C$ at $5 \text{ cm of Hg}$, $10^\circ C$ at $6.5 \text{ cm of Hg}$. Low pressure/high vacuum maintained by throttling steam through jets/nozzles. Refrigerant: Water. High pressure steam from boiler expands in a nozzle. Water vapor from flash chamber is entrained with high velocity steam jet and compressed in a thermocompressor. Kinetic energy of mixture converts to static pressure; mass discharged to condenser. Generally, $1\%$ evaporation of water in flash chamber is sufficient to decrease chilled water temperature to $6^\circ C$. Chilled water circulated by pump; warm water returned from load and sprayed for cooling. Makeup water replaces splashed water and cold water loss. Merits of Steam Jet Refrigeration Flexible operation, easily changed cooling capacity. No moving parts, vibration-free. Can be installed outdoors. Lower system weight per ton of refrigerating capacity. Reliable, low maintenance. Adapted for cold water processing in rubber mills, distilleries, paper mills, food processing plants. Safe refrigerant (water), quick adjustment to load variations, no leakage hazard, suitable for air-conditioning. Demerits of Steam Jet Refrigeration Direct evaporation for chilled water limited by large vapor volume to handle. Requires twice as much heat removal in condenser per ton of refrigeration compared to vapor compression systems. Useful for comfort air-conditioning but not feasible for water temperature below $4^\circ C$. Thermoelectric Refrigeration Direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. Encompasses Seebeck, Peltier, and Thomson effects. Seebeck Effect: Build-up of electric potential across a temperature gradient. Peltier Effect: Continuous heat transport between junctions of two dissimilar conductors when voltage/DC current is applied. Thomson Effect ("Kelvin heat"): Heat release in a material with current applied. Use in Cooling Used when: Heat to be displaced is lower than normal compressor systems. System requires environment unaffected by position/orientation. Object to be cooled is isolated. Exact required temperature not achievable by thermostatic control. Insulated refrigerator cabinets with thermoelectric technology can have variable dimensions (e.g., $5-40 \text{ L}$ capacity: $5-10 \text{ cm}$ thickness; $ TEC operates in a definite temperature difference range. Heat sink is compulsory at the hot end to dissipate heat to the environment. Finned heat sinks inside compartment improve heat transfer to cold side of TEC. Fans transfer heat via convection to dissipate heated/cooled air. Control system allows accurate temperature and cooling capacity control by changing current. Merits of Thermoelectric Refrigeration No moving parts, highly reliable ($10^5$ hrs at $100^\circ C$, longer at lower temps). Ideal for precise temperature control. Can lower temperature below ambient. Heat transport controlled by current input. Operable in any orientation. Compact size, useful for applications with size/weight constraints. Ability to alternate between heating and cooling. Excellent alternative for vibration-sensitive systems. Limitations of Thermoelectric Refrigeration Able to dissipate limited heat flux. Lower COP compared to VC systems. Restricted to low heat flux applications. More total heat to remove than without a TEC. Applications Defense technologies (withstands extreme conditions). Space equipment (zero gravity, not affected by orientation). Inkjet printers (maintains viscosity). Laser diode arrays. Vortex Tube Refrigeration (Ranque-Hilsch Vortex Tube) Simple device for producing cold effect. Invented by G.J. Ranque (1931), developed by Prof. Hilsch. Separates compressed gas (air) into hot and cold streams. Hot end: up to $200^\circ C$. Cold end: down to $-50^\circ C$. Parts Vortex Generator: Creates a high-speed vortex. Nozzle: Tangential entry of high velocity air stream into hot side, gradually converts to spiral form. Can be converging, diverging, or both. Chamber: Facilitates tangential entry. Diaphragm: Cylindrical piece with a small hole at the center. Valve: Obstructs air flow through hot side, controls hot air quantity. Hot Air Side: Cylindrical, various lengths. Cold Air Side: Cylindrical portion through which air passes. Working Principle Pressurized gas (air) tangentially admitted into a swirl chamber, accelerated to high rotation. Vortex flow created; air travels spirally along the periphery of the hot side. Air expands, acquires high velocity due to nozzle shape. Outer shell of compressed gas escapes through conical nozzle at the end. Remainder of gas forced to return in an inner vortex of reduced diameter. Flow restricted by valve. Increased pressure near valve (by partial closing) causes reversed axial flow through core of hot side (high to low pressure). Cold stream escapes through diaphragm hole to cold side; hot air stream passes through valve opening. Energy transfer between inner and outer vortex (inner loses heat), cooling air in core below inlet temperature. Expansion of tube gas decreases temperature at cold end, providing refrigeration. Single stage can reach $124 \text{ K}$, two stage $79 \text{ K}$ (with $300 \text{ K}$ cooling medium, helium at $25 \text{ bar}$). Merits of Vortex Tube Uses air as refrigerant, no leakage problems. Simple design, avoids control systems. Complete absence of moving parts. Lightweight, requires less space. Low initial and working cost where compressed air is available. Low maintenance, no expert attendant required. Applications of Vortex Tube Air suits Aviation Cooling of gas turbine rotor blades Laboratory sample coolers Simultaneous heating and cooling Shrink fitting Cutting tools Spot cooling Cooling enclosures Cooling by Adiabatic Demagnetization Cryogenic technique to cool materials to extremely low temperatures. Aligns magnetic dipoles by magnetizing material, reducing entropy. Isolates material, then removes magnetic field. Removal of field disorders dipoles; adiabatic process (no heat exchange) causes internal energy and temperature to decrease. Based on magnetocaloric effect, achieves temperatures near absolute zero. Uses paramagnetic properties of materials. Ferromagnetic: Strongly attracted by magnet. Diamagnetic: Repelled by magnetic pole. Paramagnetic: Weakly attracted, induces parallel magnetic field. Common paramagnetic salts: ferric alum (ferric ammonium sulfate), gadolinium sulfate. For low temperatures, paramagnetic salt suspended in a tube with low-pressure helium gas for thermal communication with liquid helium bath. Magnetic field strength: $\sim 80 \text{ kilogauss}$ ($1 \text{ gauss} = 10^{-4} \text{ Tesla}$). Temperatures achievable: Liquid air ($90 \text{ K}$), oxygen ($54.3 \text{ K}$), nitrogen ($35.6 \text{ K}$), hydrogen ($14 \text{ K}$), helium ($1 \text{ K}$). Liquid helium bath cooled to $1 \text{ K}$ by pumping; salt approaches this temperature via conduction through exchange gas. Result is adiabatic cooling, not mechanical. Steps Involved Isothermal Magnetization: Paramagnetic material in strong magnetic field at low temperature, aided by helium to conduct away heat. Magnetic field aligns disordered magnetic dipoles, decreasing entropy. Adiabatic Isolation: Material thermally insulated from surroundings (e.g., by removing helium gas). Adiabatic Demagnetization: Magnetic field slowly removed/reduced. Aligned dipoles return to random orientation, increasing entropy. Cooling: Demagnetization is adiabatic, no external heat absorbed. Magnetic dipoles draw energy from material's internal energy to work against external field, causing significant temperature drop. Pulse Tube Refrigeration Used when suitable refrigerants for lower cascades of vapor compression systems are unavailable, hence gas cycle refrigeration. Avoids lubrication problems at low temperatures associated with conventional expansion engines, as it has no moving parts. Proposed in 1961, reported in 1964 by Gifford and Longsworth. Obtains low temperatures with moderate pressure without moving parts, leading to compact and reliable miniature refrigerators. Consists of a closed tube subjected to pressurization and depressurization alternatively. Major Components Vacuum vessel Regenerator Pulse tube Rotary vacuum pump Water jacket Valve mechanism Water tank and pump Compressor Water spray cooler Air reservoir Filter Gear box, DC motor and rectifier Cycle Processes Pressure Build-up and Heat Rejection: Gas (air) compressed, passes through receiver and filter. Regenerator (bronze wire mesh) cools gas, reducing pressure and temperature. Cooled gas sent to pulse tube; compressed gas acts as a piston. Gas piston compresses gas already in pulse tube, increasing its temperature. Hot gas rejects heat to cooling medium, reaching similar temperature. Pressure Release and Heat Absorption: Gas in pulse tube connected to outlet via valve mechanism, releasing gas from pressure build-up. Expansion of tube gas decreases temperature at cold end, handling refrigeration load. Thermodynamics of the Human Body Hot climates: hot-dry (e.g., Rajasthan: DBT $43-49^\circ C$, RH $5-20\%$) and hot-humid (e.g., Assam: DBT $37^\circ C$, RH $80\%$). Human body is homeothermic, maintains relatively constant internal temperature. Thermal machine with $20\%$ efficiency; $80\%$ of heat must be dissipated. Heat dissipated via convection, radiation, and evaporation. Comfort maintained if heat dissipated equals heat produced. Heat Loss Expression $H_m - W_e = H_e + H_c + H_r + H_s$ $H_m$: Metabolic heat produced in body. $W_e$: Useful rate of working. $H_m - W_e$: Heat to be dissipated to atmosphere. $H_e$: Heat lost by evaporation. $H_c$: Heat lost by convection. $H_r$: Heat lost by radiation. $H_s$: Heat stored in body (positive if temperature rises, negative if falls below $36.5^\circ C$). Convective Heat Loss $H_c = UA(T_b - T_a)$ $U$: Heat transfer coefficient on body surface. $A$: Body surface area (e.g., $1.8 \text{ m}^2$ for normal man). $T_b$: Mean body surface temperature. $T_a$: Surrounding temperature. If $T_a > T_b$, heat is gained. Increased $U$ (function of air velocity) increases heat gain. Higher air velocity causes discomfort if $T_a > T_b$. Radiation Heat Loss $H_r = \sigma (T_b^4 - T_a^4) \text{ W/m}^2$ $\sigma$: Stefan-Boltzmann constant. $T_b$: Body temperature. $T_a$: Surrounding temperature. If $T_a > T_b$, heat is gained; if $T_a All bodies emit electromagnetic radiation; intensity and wavelength depend on temperature. Hotter objects emit more radiation, shorter wavelength. Most radiations (except sun) are long wave. Clothing and Radiation Short wave radiation effect differs from long wave. Long wave radiation absorbed by skin regardless of color. Absorption of color wave depends on skin/clothing color. White clothing and fair skin reflect short wave radiation maximally. Various shades of skin/clothing absorb radiation differently. Indoor: skin/clothing color doesn't matter for long wave radiation. Outdoors (direct sun): skin/clothing color is a determining factor for short wave radiation. Evaporation Heat Loss $H_e = K_d A (P_{vs} - P_v) h_{fg} K_c$ $K_d$: Diffusion coefficient of water vapor. $A$: Body surface area. $P_{vs}$: Saturation vapor pressure at skin temperature. $P_v$: Vapor pressure of surrounding air. $h_{fg}$: Latent heat of vaporization ($2450 \text{ kJ/kg}$). $K_c$: Clothing factor. $H_e$ is always positive. $H_e = 0$ when $P_{vs} = P_v$ (saturated air at skin temperature). $H_e$ never becomes negative. Role of Clothing Interferes with air movement, reducing convective heat transfer and evaporation potential. Decreases radiation heat transfer outdoors. Decrease varies $20-50\%$ depending on clothing color. White clothing: best effect. Dark clothing: worst effect. Desert climate: thin, loose-fitting, white/light-colored clothing advisable to reduce convective gain and reflect solar radiation. Humid hot climates: clothing can be disadvantageous due to low evaporative capacity. Should be light, porous, loose-fitting for sweat movement. Human Body Temperature and Heat Storage Weak body: less metabolic heat production. If $H_e, H_r, H_c$ are high and positive, $H_s$ becomes negative. $[(H_e+H_r+H_c)] > (H_m - W_e)$: sick/weak/old feel colder as heat loss exceeds production. Fever: internal body activities increase $H_m$ such that $H_s$ becomes positive. Extreme heat storage limits can cause death. Normal body temperature: $37^\circ C$ ($98.6^\circ F$). Dangerous: $>40.5^\circ C$ ($105^\circ F$) or $ Thermostatic Control Human body maintains $37^\circ C$ by controlling heat transfer modes. $H_e, H_r, H_c$ depend on: dry-bulb temperature, relative humidity, air velocity. Evaporative heat loss increases more than convective heat loss with metabolic rate increase. Comfort air-conditioning balances $(H_e+H_r+H_c) = H_m$ for $H_s = 0$. Metabolic rate $H_m'$ depends on activity. Body Defense Mechanisms Control center (hypothalamus) makes physiological adjustments. Vasomotor Control: Regulates blood supply to skin. Vasodilation increases convective heat transport from body interior to surface. "First law of defense" as it maintains heat balance at low heat load. Sudomotor Control: Regulates sweat production. Only heat transfer mode when radiation and convection are negative (atmosphere hotter than body). Initiates sweat gland activity. Sweating capacity varies with acclimatization. Sweat glands can fatigue, leading to progressive fall in sweat production during severe heat. Heat Stress and Work Limited effective cooling by evaporation due to atmospheric humidity and wind speed. Low evaporative capacity is a major problem in humid heat. Sweat dripping off without evaporation is useless. Working capacity in tropical countries impaired by hot environment. Performance efficiency decreases with increased ambient temperature. Variables influencing thermal balance: air temperature, mean radiant temperature, relative air velocity, activity level, thermal resistance of clothing. Thermal sensation related to thermoregulatory system and discomfort. Comfort: skin temperature $33-34^\circ C$, no sweating. Fnager chart shows preference for lower mean skin temperature and sweating at higher activities. Sedentary ($M=58 \text{ W/m}^2$): no sweat, prefers skin temperature $\sim 34^\circ C$. Higher activities: prefers sweat secretion for $42\%$ of increased heat production. Body Temperature Ranges Hypothermia: $ Normal: $36.5-37.5^\circ C$ ($97.7-99.5^\circ F$) Fever: $>37.5$ or $38.3^\circ C$ ($99.5$ or $100.9^\circ F$) Hyperthermia: $>37.5$ or $38.3^\circ C$ ($99.5$ or $100.9^\circ F$) Hyperpyrexia: $>40.0$ or $41.0^\circ C$ ($104.0$ or $105.8^\circ F$) Effect of Heat on Work Summer atmospheric temperature often higher than body temperature, leading to heat gain by radiation and convection. Only way to dissipate heat in summer is through evaporation of sweat. If evaporative heat loss can't cope, heat storage occurs, body temperature rises. Beyond critical temperature, discomfort and potentially fatal. Physical work in hot environment leads to competitive demands on cardiac output, decreasing work capacity. Work efficiency falls with increased effective temperature. Heat Disorders Prolonged heat exposure beyond critical period causes body mechanism to crumble ("heat stroke"). Heat Stroke: Sudden rise in body temperature above $40.5^\circ C$, loss of consciousness. Most serious, can be fatal. No sweating, dry/warm skin, angry behavior, delirium, convulsions. Victims: overweight, older, heart/respiratory ailments. Immerse in water, hospitalize. Heat Exhaustion: Fatigue in hot climate due to physiological strain. Caused by water shortage (headache, thirst, dry mouth, mild fever up to $102^\circ F$) and/or salt deficiency (not common, $4\%$ of causalities, treated with cool saline drinks/rest). Person feels tired, giddy, nauseated, chilly. Shallow breathing, weak pulse, clammy/pale skin. Dilated blood vessels, insufficient circulation. Heat Syncope: Common ill-effect ($60\%$ of causalities). Person standing in sun becomes pale, blood pressure drops, collapses. Not due to body temperature rise, but reduced blood return to heart, lowering blood pressure and brain blood supply. Recovery within $5-10$ minutes by lying in shade, head slightly down. Heat Cramp or Shock: Severe pain in calf/thigh muscles after prolonged exertion in severe heat. Body loses excessive fluid and salt, causing insufficient circulation and shock. Failure to take salt pills or salty beverages leads to cramps, weakness, headache, fatigue, giddiness. Muscle weakness. Comfort and Comfort Chart Ideal human comfort: heat production equals heat loss. Achieved with proper temperature, humidity, air velocity, and purity. Comfort depends on: eating habits, clothing, duration of stay, age, sex, activity. "Effective temperature" concept measures comfort feeling (warmth/cold) based on air temperature, moisture, air motion. Factors Governing Optimum Effective Temperature Climatic and seasonal differences Clothing Age and gender Activity Duration of stay Air velocity Requirements of Comfort Air-Conditioning Comfort feeling depends on: Supply of $O_2$ and removal of $CO_2$. Removal of body heat dissipated by occupants. Removal of body moisture dissipated by occupants. Provision of sufficient air movement and air distribution. Maintenance of air purity (removing odor and dust). Oxygen Supply Each person requires $0.65 \text{ m}^3/ \text{hour}$ of $O_2$ and produces $0.2 \text{ m}^3$ of $CO_2$. Rise in $CO_2$ concentration indicates $O_2$ consumption. Heat Removal Human body: engine converting thermal energy to mechanical with $20\%$ efficiency. Body dissipates $320 \text{ kJ/hour}$. If $6 \text{ m}^3$ space per person with no heat/air transfer, space temperature rises $\sim 0.15^\circ C$ per $1 \text{ kJ}$ heat, or $48^\circ C/\text{hour}$. Ventilation circulates air to avoid excessive temperature rise, allowing occupants to live and work. Moisture Removal Moisture loss from body: $\sim 50 \text{ g/hour}$ at rest. Body's ability to dispose of heat by evaporation decreases as air humidity increases. High humidity reduces freshness and makes heat disposal difficult. Ventilation system should maintain relative humidity below $70\%$. Air Motion Increased air velocity increases heat transfer. If ambient air is cooler than body, increased velocity reduces discomfort. If ambient air is hotter than body, sensible heat transfer reverses, increasing discomfort. Increased velocity reduces thickness of unsaturated vapor layer near body, aiding evaporation. Evaporative heat loss usually greater than convective heating if dew point temperature is below $30^\circ C$. If air is hot and humid, increased velocity causes discomfort. Air velocity in AC space: Max $6-9 \text{ m/min}$ at $20^\circ C$, $9-15 \text{ m/min}$ at $22^\circ C$. Air motion without proper distribution causes local cooling sensation ("draft"). Velocities $ Velocities $>12 \text{ m/min}$ with $1.5^\circ C$ temperature differential: uncomfortable draft. Purity of Air Quality air: free of odor, dust, toxic gases, bacteria. Evaporation from body adds odor. Smoke is objectionable (bad effects on nose, eye, heart). Removal of toxic gases essential (irritation). Control of bacteria by sterilization important. Biological Requirement of Air Supplied Indoor environment affected by bacteria, fungi, mold, viruses. AC drain pan is breeding source for microorganisms. Indoor conditions often worse than outdoor due to higher population density. Contagious pathogens: human-borne (nosocomial infections, endogenous) and non-contagious (from surroundings). Microbial Ecology Microbes: bacteria, viruses, algae, actinomycetes. Aspergillus spp and Penicillium spp are dominant. Indoor conditions favorable for microbial reproduction. Microbial formation measured as Colony Forming Units (CFU). Spores: single-celled reproductive units (plants, bacteria, fungi, algae). Hardy structures for dispersal. Normal spore level in single-story building: $10-100\%$ of outdoor, up to $200 \text{ CFU/m}^3$. In multi-story buildings: $10-31\%$ of outdoor spore level, depending on ventilation/structure. Air conditioner levels: $20 \text{ CFU/m}^3$ (medium germination), $50-80 \text{ CFU/m}^3$ (mechanical/natural ventilation). $100 \text{ CFU/m}^3$ considered for outdoor. Condenser coils are hosts for microbes. Spores through duct lines can infect occupants. Microbe growth requires moisture and nutrients (available in drain pans, humidifiers, cooling coils). Indoor population density can exceed outdoor due to exponential reproduction in these areas. Respiratory pathogens transmit via direct contact (droplets) or fomites. Eyes and nasal passages vulnerable to fomite transmission. Skin/eye infections, nosocomial infections, open wounds/burns can occur via airborne route. Ocular infection: reddish color in eye due to airborne pathogens. Factors Affecting Transmission of Infection Immunity Duration of exposure Concentration of infectious agents Virulence of infectious agents Breathing rate of occupants Route of infection Calculating AC Tonnage Area Method Multiply room length by width (in feet). Take square root of result. Divide by 10 to get tonnage. Example: $13 \text{ ft} \times 8 \text{ ft} = 104 \text{ sq ft}$. $\sqrt{104}/10 \approx 1.01$. Choose $1 \text{ Ton AC}$. Volume Method Multiply room length, width, and height (in feet). Divide by 1000. Example: $17 \text{ ft} \times 12 \text{ ft} \times 22 \text{ ft} = 4488 \text{ cu ft}$. $4488/1000 = 4.8 \text{ ton}$. AC Tonnage Guidelines (approximate) $0.75 \text{ ton AC}$: up to $90 \text{ sq ft}$ ($27.4 \text{ m}^2$) $1 \text{ ton AC}$: $91-130 \text{ sq ft}$ ($27.7-39.6 \text{ m}^2$) $1.5 \text{ ton AC}$: $131-190 \text{ sq ft}$ ($39.9-57.9 \text{ m}^2$) $2 \text{ ton AC}$: $191-250 \text{ sq ft}$ ($58.2-76.2 \text{ m}^2$) Humidity Control in AC Ideal humidity: $35-55\%$. Dehumidification Mode: Reduces humidity while cooling by cycling compressor. Fan runs at low speed. Built-in Humidifier: Some ACs, heating units, or HVAC systems integrate humidifiers. Humidifier and AC Combo: Some portable ACs can also function as a dehumidifier and fan.