CALIFORNIA STATE UNIVERSITY, LONG BEACH ECHANICAL AND AEROSPACE ENGINEERING DEPARTMENT Thermal Engineering Laboratory Exp#6-Analysis of an Air Conditioning System Object: To investigate different air-conditioning processes including heating, cooling, humidification and dehumidification and to use the Psychrometric chart as applied to humidity measurement and control. Background Theory: The transfer of heat from lower temperature regions to higher temperature ones is called refrigeration. It is the cooling or maintenance of a space or a body at a temperature below the equilibrium temperature it would normally resume. Generally the term refers to cooling by interposing a system which absorbs heat from the space or body at a cold temperature, then rejects it to the surrounding at a higher temperature with a net input of work energy. Several systems are used to accomplish this. Three of those are: The vapor-compression cycle Cools by compression and expansion of a vapor The absorption cycle Cools by absorption of a vapor in a liquid (most commonly, ammonia in water) The hot-junction/cold-junction electrical system Provides cooling effect using two bimetal junctions, where current flow in one direction (DC) causes junction A to heat up and junction B to cool. Reverse the current and A will cool and B will heat up Devices that produce refrigeration are called refrigerators, and the cycles on which they operate are called refrigeration cycles. The working fluids used in refrigerators are called refrigerants. This experiment will investigate the vapor-compression refrigeration cycle, which resembles the Rankine cycle with an expansion valve substituted for the turbine. This cycle, usually referred to as the refrigeration cycle may be evaluated as a heat engine with net work input and a cooling effect as the desired output. Vapor-compression refrigeration is the most widely used method for air-conditioning of large public buildings, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. 1 The vapor-compression refrigeration uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. Figure 1 depicts a typical, single-stage vapor-compression system. Figure 1: Vapor-compression Refrigeration All such systems have four components: a compressor, a condenser, a Thermal expansion valve (also called a throttling valve), and an evaporator. Circulating refrigerant enters the compressor in the thermodynamic state known as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well. The hot, compressed vapor is then in the thermodynamic state known as a superheated vapor and it is at a temperature and pressure at which it can be condensed with typically available cooling water or cooling air. That hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and the rejected heat is carried away by the cooling medium. The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated. 2 The cold mixture is then routed through the coil or tubes in the evaporator. A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser. To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor. The idealized refrigeration cycle, like other ideal cycles has four processes: 1. 2. 3. 4. Isentropic compression Constant pressure heat rejection Isenthalpic (constant enthalpy) expansion (throttling) Constant pressure heat absorption Unlike other ideal thermodynamic cycles, process 3, the expansion process, is not reversible. Figure 2 shows the schematic and the T-s diagram for an ideal vapor-compression cycle Figure 2-Schematic and T-s diagram for an ideal vapor-compression refrigeration cycle 3 The processes are also shown on the P-h diagram in figure 3. Figure 3- the P-h diagram for an ideal vapor-compression refrigeration cycle A consequence of the second law of thermodynamics is that heat can not be transferred from a colder to a warmer space without a net heat input of energy. The coefficient of performance (COP) is the ratio of heat so moved to the input energy required to move it. COP= (Heat added or removed)/ Work input Results of First and Second Law Analysis for Steady-Flow: Component Compressor Condenser Throttle Valve Evaporator Process s = Const. P = Const. ∆s > 0 P = Const. First Law Result h3=h4 4 Therefore: The cooling capacity of refrigeration systems is often defined in units called “TONs of refrigeration”. The most common definition of that unit is: 1 TON of refrigeration is the rate of heat removal required to freeze a ton (i.e., 2000 pounds) of water at 32 °F in 24 hours. Based on the heat of fusion for water being 144 Btu per pound, 1 ton of refrigeration = 12,000 Btu/h = 12,660 kJ/h = 3.517 kW. Most residential air conditioning units range in capacity from about 1 to 5 Tons of refrigeration. Air conditioning: Air conditioning is the removal of heat from indoor air for thermal comfort. In another sense, the term can refer to any form of cooling, heating, ventilation, or disinfection that modifies the condition of air. An air conditioner (often referred to as AC or air con.) is an appliance, system, or machine designed to change the air temperature and humidity within an area (used for cooling as well as heating depending on the air properties at a given time), typically using a refrigeration cycle but sometimes using evaporation, commonly for comfort cooling in buildings and motor vehicles. Important Terms used in Air conditioning studies: Dry Bulb Temperature, DBT is that of an air sample, as determined by an ordinary thermometer, the thermometer’s bulb being dry. On the standard Psychrometric chart this is shown horizontally along the abscissa. Wet Bulb Temperature or Saturation Temperature, WBT, is that of an air sample after it has passed through a constant-pressure, ideal adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel. In practice, this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air. Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. The two most common measurements are relative humidity and specific humidity. Relative humidity is a term used to describe the amount of water vapor in a mixture of air and water vapor. It is defined as the ratio of the partial pressure of water vapor in the air-water mixture to the saturated vapor pressure of water at those conditions. The relative humidity of air 5 depends not only on temperature but also on pressure of the system of interest. Relative humidity is normally expressed as a percentage and is calculated by using the following equation. It is defined as the ratio of the partial pressure of water vapor (H2O) mixture to the saturated vapor pressure of water in the at a prescribed temperature. Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, a rise in relative humidity also increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin as the relative humidity rises. For example, according to the Heat Index, a relative humidity of 75% at 80°F (27°C) would feel like 83.574°F ±1.3 °F (28.652°C ±0.7 °C) at about 44% relative humidity. Specific humidity is the ratio of water vapor to dry air in a particular mass, and is sometimes referred to as humidity ratio. Specific humidity ratio is expressed as a ratio of mass of water vapor, , per unit mass of dry air .That ratio is defined as: The dew point is the temperature to which a volume of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. Condensed water is called dew when it forms on a solid surface. Saturation Vapor Pressure Ps (N/m2, Pa) The pressure at which the vapor phase of a material is in equilibrium with the liquid phase of the same material. The saturation vapor pressure varies with temperature. In the case of saturated air (air saturated with water vapor), the saturation vapor pressure is the pressure (at a specific temperature) when the rate of evaporation of water equals the rate of condensation of water, and is also the point at which the relative humidity is 100%. The equations relating relative and specific humidity, temperature (wet and dry bulb), pressure (air, vapor) and enthalpy are quite tedious and inconvenient. For this reason a graph of the thermodynamic parameters of moist air at a constant pressure was developed relating all the relevant variables. This graph is called a Psychrometric chart and is extremely useful for designing and evaluating air-conditioning and cooling tower system. The versatility of the Psychrometric chart lies in the fact that by knowing two independent properties of some moist air (at a constant known pressure), the other properties can be determined. Changes in state, such as when two air streams mix, can easily be graphically modeled using the correct Psychrometric chart for the location’s air pressure or elevation relative to sea level. For 6 locations at or below 2000 ft (600 m), a common assumption is to use the sea level Psychrometric chart. The most common chart is the “ω-t” (omega-t) chart in which the Dry Bulb Temperature (DBT) appears horizontally as the abscissa and the humidity ratios (ω) appear as the ordinates. In order to use a particular chart, for a given air pressure or elevation, at least two of the six independent properties must be known (DBT, WBT, RH, Humidity Ratio, Specific Enthalpy, and Specific Volume). 7 RA2 Air-Conditioning unit The changes of air condition that may be investigated with the RA2 are:  Heating of air  Cooling of air  Humidification of air  Dehumidification of air with cooling The properties of air that may be measured directly by the RA2 sensors and controls are:  Air velocity  Relative humidity  Temperature (at multiple locations)  Power input (electrical) to each heater unit (preheat, reheat and boiler) The constants assumed by the software for calculations are:  Heat capacity ratio (g or k) for air: 1.41 @20°C [ratio, dimensionless]  Heat capacity ratio (g or k) for water: 133 @20°C [ratio, dimensionless]  Acceleration due to gravity (g): 9.81 [m/s²] -1 -1  Ideal gas constant (R): 8.314472 [J K mol ] -1 -1  Constant pressure specific heat (cp): 1.005 @20°C [kJ kg K ] -1 -1  Constant volume specific heat (cv): 0.715 [kJ kg K ] Variables that cannot be measured by the RA2 and must be input from additional measurements are:  Ambient (atmospheric) pressure Using Calculations instead of Psychrometric Chart to Determine Air State The standard method of determining the parameters required to analyze HVAC systems is to use the Psychrometric chart as described above. However these parameters can also be calculated. This section describes the formulae used in the RA2 software to determine the air state. Saturation Pressure and Partial Pressure of the Water Vapor The maximum saturation pressure of the water vapor in moist air varies with the temperature of the air vapor mixture and can be expressed as: pws = e(77.3450 + 0.0057 T – 7,235 / T) / T8.2 (1) where pws = water vapor saturation pressure (Pa) e = the constant 2.718……. T = temperature of the moist air (K) Equation (1) represents the curve on the Psychrometric chart at 100% RH. 8 Relative Humidity (RH) is defined as the partial pressure of the water vapor, divided by the partial pressure of saturated air at the same temperature. RH = pw / pws x 100% (2) From equations (1) and (2) the partial pressure of the water vapor can be calculated if the temperature and RH are known. Humidity Ratio The humidity ratio can be determined from the partial pressure of water vapor and air: x = 0.62198 pw / (pa – pw) (3) where pw = partial pressure of water vapor in the moist air (Pa) pa = atmospheric pressure of the moist air (Pa) Thus from equations (1), (2) and (3), the humidity ratio (x) (i.e. the abscissa of the Psychrometric chart) can be determined from the temperature and RH measurements. Other Calculations Required Calculating Mass Flow Rate From the continuity equation: where A and B are two points along the duct. In the experiments that follow, the letter subscripts refer to the positions along the duct as shown below: Thus, for a simple duct, the mass flow rate is constant through the duct. 9 The air flow rate (F) is measured by the air speed sensor at position D. The volume flow rate can be calculated to be F.A m3/s, where A is the cross section area of the duct. Therefore the mass flow rate can be expressed as: where v = specific volume of moist air per mass unit of dry air and water vapor (m3/kg) F = Flow rate of the air (m/s) A = Area of the duct (m2) The Specific Volume, v, (Inverse Density) of the air is determined from the Psychrometric chart, by plotting the dry bulb temperature and measured RH at the air flow sensor position. Note on Flow Rate Measurement: the above assumes the flow rate is constant throughout the duct, which of course is not the case. Flow rates near the walls and in the corners will be much lower than in the center. The RA2 measures the flow rate at the center of the duct. Therefore a factor needs to be applied to calculate the average flow. On RA2 this factor has been found empirically to be approximately 0.6. Sensible Air Heating Sensible Heating is heating that does not involve a change of phase (e.g. evaporation) of any of the materials involved. Similarly sensible cooling of air does not involve any condensation. The sensible heat of a material is the heat energy of the air that may be gained or lost through convection and conduction. The sensible heat is a result of the material’s specific heat capacity, its mass, and its temperature compared to some defined datum or reference temperature (e.g. measured using a standard scale of temperature such as Kelvin, Fahrenheit or Celsius, all of which use fixed reference points). The term ‘sensible heat’ rather than simply ‘heat’ is used in order to distinguish it from latent heat. From first law of thermodynamics: W (work transfer rate) is zero 10 Therefore the effective heating (or cooling) of the air between positions A and B can be expressed as: Individual enthalpy can be determined from the Psychrometric chart. Alternatively, the change in enthalpy may be calculated as cpa(TB – TA) + x cpw (TB – TA) where TA is the initial temperature of the air TB is the temperature of the air after heating cpa is the specific heat capacity of air at constant pressure cpw is the specific heat capacity of water vapor at constant pressure x is the humidity ratio Note on Latent Heat: Latent heat is the heat energy required for a material (e.g. water) to undergo a change of phase (e.g. evaporation from liquid to vapor). For example, a mass of liquid water will not immediately and completely change phase to water vapor as it reaches the evaporation temperature of 100°C, but requires additional heat input for the entire mass to evaporate. The temperature of the water will remain at 100°C (the temperature of the phase change) until the change is complete. The heat that must be added to enable the phase change, which does not result in a change of temperature, is the latent heat. The RA2 provides the facilities to investigate latent heat as the input power to the humidifier is measured. Also it is possible to collect the condensate from the chiller over a period of time. However detailed analysis of this non-sensible heating and cooling is beyond the scope of the standard experiments for the RA2. This would make an ideal topic for project work. Energy Balance and Heating Efficiency Efficiency = sensible air heating/ Electrical Heater power Note: In an HVAC system it is quite possible to obtain ‘efficiencies’ of >100% as heat may be gained from the surroundings as well as lost. It is more correct to term efficiency investigations as an ‘Energy Balance’. 11 Description: The Armfield RA2 Unit represents a model of an Air Conditioning system by demonstrating the effects of essential Air Conditioning processes: cooling, heating, humidifying and dehumidifying. The effect and relationships of the primary processes involved in air handling systems can be investigated. The RA2 Unit is designed so that students can simulate different environments and perform measurements to allow Psychrometric data analysis. The RA2 is a bench-top unit which comprises of a square ventilation duct mounted on a mild steel support frame. The duct is made of clear acrylic so all components are clearly visible: air fan, air preheater, humidifier tube, chiller/dehumidifier heat exchanger and air re-heater. The duct consists of 4 main parts: Left-Hand (LH) assembly, Right-Hand (RH) assembly, Fan assembly and Louvre assembly. An axial fan moves the air to be conditioned through the duct. Heating elements are used to heat the air. Humidification is provided by steam delivered through a tube from a boiler. The refrigerating capacity is generated by an evaporator (heat exchanger) which is connected to the refrigeration unit. The refrigeration unit and boiler are located underneath the duct. Temperature and humidity sensors record the temperature and relative humidity at every stage of operation. The air flow rate is determined using an air velocity transmitter. An acrylic Louvre is located at the exit from the duct. 1 Axial Fan The axial fan moves the air through the duct. The speed of the fan may be controlled to give different air flow rates. The fan must be on when both the pre-heater and re-heater are on to avoid heat damage to the acrylic duct during operation. The fan is protected with a guard, which prevents objects from reaching the blades. Front view of fan assembly Pre-heater and Re-heater The pre-heater comprises two electric elements of 200W each, for a total power of 400W. It is located downstream of the fan in order to preheat the air flowing through the evaporator. In the second part of the duct, after the evaporator, there is a re-heater (200W) which can be used to reheat the cooled or cooled and dehumidified air. The elements are arranged at an angle to give efficient heat transfer to the air stream. Air sensing thermostats are incorporated in the duct above the heater elements to provide overheat protection. Heating Coils 2 Evaporator The refrigerating capacity of approximately 500W at 20°C is generated by an evaporator, which is part of a compact refrigeration system. The refrigeration unit is used to cool and dehumidify the air stream. The evaporator consists of a direct-expansion coil operated with a thermostatic expansion valve. The evaporator is clearly visible within the ventilation duct, and the rest of the refrigeration unit- the condensing unit- is placed just underneath the duct. Air passing across the evaporator fins is cooled as the refrigerant flowing through the tubes absorbs heat and is boiled (evaporated). Refrigerant flowing through the coil tubes is controlled by a thermostatic expansion valve mounted at the inlet to the evaporator coil. This valve automatically feeds just enough refrigerant into the coil for the refrigerant to be completely converted (boiled) from liquid to gas. The valve is controlled by a temperature-sensing bulb mounted on the coil outlet (suction) connection. The evaporator itself is complete with an angled draining tray at the bottom. During the dehumidification experiment, condensate can be collected and measured with a graduated cylinder. Evaporator Assembly Refrigerant This equipment includes a sealed unit containing refrigerant R134a (Also known as: HFC-134a; 1,1,1-2 Tetrafluoroethane; Norflurane; Norfluran). This is a common refrigerant introduced to replace CFC (chloro-fluoro-carbon) refrigerants such as R-12. R134a is colourless, nonflammable and non-corrosive with a very faint odour, and is safe under normal use as described in this manual. See the safety section at the front of this manual for additional information. 3 Condensing Unit The Condensing Unit, located below the ventilation duct, incorporates a compressor and a condenser. The compressor is used to compress gaseous refrigerant leaving the evaporator, and in the fan cooled condenser the refrigerant gives away the heat gained in the evaporator. The Condensing Unit also incorporates a refrigerant collector, filter/dryer, sight glass and high/low pressure cut-out for safety purposes. Refrigeration Unit Assembly Humidifier Humidification is provided by a water boiler of 5L total volume. Steam is generated when the water is boiled using the electric element, (2kW). The boiler is made of plastic and includes a tube which delivers steam to the air duct. It also includes a drain valve, and can be refilled manually through the filler cap and refill lance. Distilled water is recommended in order to avoid scaling of the vessel and duct. The boiler incorporates a cut-out switch, which prevents the electrical element from overheating if the water level falls too low. If this occurs, wait 2 minutes and refill boiler, the cut off will self- reset and steam can be produced again with 5 -12 minutes. Power to the boiler heaters can be remotely controlled and monitored using the Armfield RA2 Software. 4 Air Velocity Sensor The air velocity in the duct is measured by the air velocity transmitter. This operates on the hot film anemometer principle, using special thin film. It has very good accuracy at low air velocities. The working range is 0–10m/s and the response time can be up to 4 seconds at constant temperature. Therefore it is important to obtain steady conditions in order to have stable velocity measurement. Steady state in the system is usually obtained after about 15 minutes. The velocity transmitter is mounted in the duct in the best position to measure the average air velocity. Care should be taken to ensure the correct angle between the sensor head and the air flow. Air Velocity Sensor Temperature / Relative Humidity Sensor Temperature and Relative Humidity (T/RH) sensors are located at every stage of operation. There are 4 T/RH sensors in total: at the duct inlet, before the evaporator, after the evaporator and at the duct outlet. Temperature and Relative Humidity is measured by the sensor. The RH sensor is a water resistant type so that it can operate in the range from 10 to 100% Relative Humidity. Temperature/Relative Humidity (T/RH) Sensor Block 5 Data Logger/Equipment Controller and Software The Armfield RA2 Air Conditioning Unit is designed to be operated using the RA2-306 software supplied with the equipment. The RA2 Air Conditioning Unit must therefore be connected to a suitable PC running the RA2-306 software (or an equivalent program created by the student). The RA2 software also allows data logging of experimental results, and performs some standard calculations on the data. Unit Dimensions: Length – 170 cm Depth – 44 cm Height – 60.5 cm 6 Exp#6-Analysis of an Air Conditioning System o o Investigate different air-conditioning processes including heating, cooling, humidification and dehumidification Learn how to use the Psychrometric chart as applied to humidity measurement and control o o o Refrigeration refers to the transfer of heat from lower temperature regions to higher temperature ones It is the cooling or maintenance of a space or a body at a temperature below the equilibrium temperature it would normally resume Generally the term refers to cooling by interposing a system which absorbs heat from the space or body at a cold temperature, then rejects it to the surrounding at a higher temperature with a net input of work energy o o o The vapor-compression cycle Cools by compression and expansion of a vapor The absorption cycle Cools by absorption of a vapor in a liquid (most commonly, ammonia in water) The hot-junction/cold-junction electrical system Provides cooling effect using two bimetal junctions, where current flow in one direction (DC) causes junction A to heat up and junction B to cool. Reverse the current and A will cool and B will heat up Devices that produce refrigeration are called refrigerators, Heat pumps or AC units o The cycles on which they operate are called refrigeration cycles o The working fluids used in refrigerators are called refrigerants o o o The most widely used method for airconditioning of large public buildings, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles Uses a circulating liquid refrigerant as the medium which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere Main components: • Compressor • Condenser • Thermal Expansion Valve • Evaporator o Compressor: o Condenser: Refrigerant enters the compressor as a saturated vapor and is compressed to a higher pressure, resulting in a higher temperature as well. Refrigerant then leaves the compressor as a superheated vapor and it is at a temperature and pressure at which it can be condensed with typically available cooling water or cooling air That hot vapor is routed through a condenser where it is cooled and condensed into a liquid by flowing through a coil or tubes with cool water or cool air flowing across the coil or tubes. This is where the circulating refrigerant rejects heat from the system and rejected heat is carried away by the cooling medium. o Thermal Expansion Valve (Throttling Valve) The condensed liquid refrigerant, in the thermodynamic state known as a saturated liquid, is next routed through an expansion valve where it undergoes an abrupt reduction in pressure. That pressure reduction results in the adiabatic flash evaporation of a part of the liquid refrigerant. The auto-refrigeration effect of the adiabatic flash evaporation lowers the temperature of the liquid and vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated. The cold mixture is then routed through the coil or tubes in the evaporator o Evaporator A fan circulates the warm air in the enclosed space across the coil or tubes carrying the cold refrigerant liquid and vapor mixture. That warm air evaporates the liquid part of the cold refrigerant mixture. At the same time, the circulating air is cooled and thus lowers the temperature of the enclosed space to the desired temperature. The evaporator is where the circulating refrigerant absorbs and removes heat which is subsequently rejected in the condenser and transferred elsewhere by the water or air used in the condenser. To complete the refrigeration cycle, the refrigerant vapor from the evaporator is again a saturated vapor and is routed back into the compressor. Processes are: 1. Isentropic compression 2. Constant pressure heat rejection 3. Isenthalpic (constant enthalpy) expansion (throttling) 4. Constant pressure heat absorption Unlike other ideal thermodynamic cycles, process 3, the expansion process, is not reversible o o A consequence of the second law of thermodynamics is that heat can not be transferred from a colder to a warmer space without a net input of energy The coefficient of performance (COP) is the ratio of heat removed to the input energy required to move it COP= (Heat added or removed)/ Work input o o Defined in a unit called “TONs of refrigeration“ TON of refrigeration is the rate of heat removal required to freeze one ton (i.e., 2000 pounds) of water at 32 °F in 24 hours. Based on the heat of fusion for water being 144 Btu per pound 1 TON= 12,000 Btu/h = 12,660 kJ/h = 3.517 kW o Most residential air conditioning units range in capacity from about 1 to 5 TONs of refrigeration. o o o Air conditioning is the removal of heat from indoor air for thermal comfort. The term can refer to any form of cooling, heating, ventilation, or disinfection that modifies the condition of air. An air conditioner (often referred to as AC unit) is an appliance, system, or machine designed to change the air temperature and humidity within an area ❖Used for cooling as well as heating depending on the air properties at a given time ❖Typically using a refrigeration cycle   Dry Bulb Temperature (DBT) is the temperature determined by an ordinary thermometer. On the standard Psychrometric chart this is shown horizontally along the abscissa Wet Bulb Temperature or Saturation Temperature, WBT, is that of an air sample after it has passed through a constant-pressure, ideal adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel. In practice, this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air. a term used for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. The two most common measurements are relative humidity and specific humidity. o o o o The amount of water vapor in a mixture of air and water vapor Defined as the ratio of the partial pressure of water vapor in the air-water mixture to the saturated vapor pressure of water at those conditions: Normally expressed as a percentage Depends not only on temperature but also on pressure of the system of interest. o o o The ratio of water vapor to dry air in a particular mass Also referred to as humidity ratio Expressed as a ratio of mass of water vapor per unit mass of dry air .That ratio is defined as: The temperature to which a volume of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. Condensed water is called dew when it forms on a solid surface. o o o o A graph of the thermodynamic parameters of moist air at a constant pressure relating all the relevant variables extremely useful for designing and evaluating air-conditioning and cooling tower system. By knowing two independent properties of some moist air (at a constant known pressure), the other properties can be determined Changes in state, such as when two air streams mix, can easily be graphically modeled using the correct Psychrometric chart for the location’s air pressure or elevation relative to sea level o Sensible heating o Humidification refers to the process of heat exchanged by a body or thermodynamic system that changes the temperature, and some macroscopic variables of the body, but leaves unchanged certain other macroscopic variables, such as volume or pressure is the process in which the moisture or water vapor or humidity is added to the air without changing its dry bulb (DB) temperature is called as humidification process. This process is represented by a straight vertical line on the Psychrometric chart starting from the initial value of relative humidity, extending upwards and ending at the final value of the relative humidity. In actual practice the pure humidification process is not possible, since the humidification is always accompanied by cooling or heating of the air. Humidification process along with cooling or heating is used in number of air conditioning applications. o o o In heating and humidification psychrometric process of the air, the dry bulb temperature as well as the humidity of the air increases. The heating and humidification process is carried out by passing the air over spray of water, which is maintained at temperature higher than the dry bulb temperature of air or by mixing air and the steam. When the ordinary air is passed over the spray of water maintained at temperature higher than the dry bulb temperature of the air, the moisture particles from the spray tend to get evaporated and get absorbed in the air due to which the moisture content of the air increase. At the same time, since the temperature of the moisture is greater than the dry bulb temperature of the air, there is overall increase in its temperature. During heating and humidification process the dry bulb, wet bulb, and dew point temperature of the air increases along with its relative humidity. The heating and humidification process is represented on the psychrometric chart by an angular line that starts from the given value of the dry bulb temperature and extends upwards towards right (see the figure below). o o o o One of the most commonly used air conditioning application for the cooling purposes In this process the moisture is added to the air by passing it over the stream or spray of water which is at temperature lower than the dry bulb temperature of the air During the cooling and humidification process the dry bulb of the air reduces, its wet bulb and the dew point temperature increases, while its moisture content and thus the relative humidity also increases. Also, the sensible heat of the air reduces, while the latent heat of the air increases resulting in the overall increase in the enthalpy of the air. Cooling and humidification process is represented by an angular line on the psychrometric chart starting from the given value of the dry bulb temperature and the relative humidity and extending upwards toward left. o o o The process in which the moisture or water vapor or the humidity is removed from the air keeping its dry bulb (DB) temperature constant This process is represented by a straight vertical line on the psychrometric chart starting from the initial value of relative humidity, extending downwards and ending at the final value of the relative humidity. Like the pure humidification process, in actual practice the pure dehumidification process is not possible, since the dehumidification is always accompanied by cooling or heating of the air. The process in which the air is cooled sensibly and at the same time the moisture is removed from it is called as cooling and dehumidification process. Cooling and dehumidification process is obtained when the air at the given dry bulb and dew point (DP) temperature is cooled below the dew point temperature During the cooling and dehumidification process the dry bulb, wet bulb and the dew point temperature of air reduces. Similarly, the sensible heat and the latent heat of the air also reduce leading to overall reduction in the enthalpy of the air. The cooling and dehumidification process is represented by a straight angular line on the psychrometric chart. The line starts from the given value of the DB temperature and extends downwards towards left The process in which the air is heated and at the same time moisture is removed from it is called as heating and dehumidification process. This process is obtained by passing the air over certain chemicals like alumina and molecular sieves. These elements have inherent properties due to which they keep on releasing the heat and also have the tendency to absorb the moisture. These are called as the hygroscopic chemicals During the heating and dehumidification process dry bulb temperature of the air increases while its dew point and wet bulb temperature reduces. On the psychrometric chart, this process is represented by a straight angular line starting from the given DB temperature conditions and extending downwards towards right to the final DB temperature conditions EXP#6-Air-Conditioning Analysis Objective: To investigate different air conditioning processes such as heating, cooling and possibly dehumidification Experimental Procedure: Equipment Set-up: 1. Turn on the unit. 2. Connect the PC to the equipment using the USB provided, turn it on and open the RA2 software. The software should indicate ‘IFDVCM (No.): OK’ in the bottom right of the software window, and the red and green USB indicator lights on the electrical console should be illuminated. 3. Check that the RCCD (circuit breaker) on the electrical console is in the up (OFF) position. 4. Check that the sensor readings in the software indicate reasonable values 5. Select and load Exercise E: Enthalpy and Project Work 6. Click on the “View diagram” icon in top menu 7. Click on the “Start COM Session” icon and follow the instruction 8. Find the atmospheric pressure from online sources and enter the value in the box for “Atmospheric Pressure” 9. Click on “Power On” in “Controls” on the left side of the screen Procedure: Run 1: 1. Set the fan to 60% 2. Set the Preheat control to manual and 30% (Check that the preheat element on the mimic diagram changes to red indicating that the heater is in operation) 3. Turn the cooling on 4. Wait 5 minutes for T3 to stabilize 5. Open the reheat control and set T4 at 2-3 degrees greater than T3. (Check that the reheat element on the mimic diagram changes to red indicating that the heater is in operation) 6. Wait for the system to stabilize (T4 must be close to the set temperature at this point) 7. Once the system’s stabilized select the “GO” icon to record data at an interval of 10 seconds for the duration of 1 minute (Use the “Configure the Data Sampling” icon for the proper setting) 8. Click on “Save the results to file”, name the file, and save it as an excel file Run 2: 9. Increase the Preheat control to 50% 10. Allow the system to stabilize and repeat the experiment Run 3: 11. Increase the Preheat control to 70% 12. Allow the system to stabilize and repeat the experiment Note: A full report is not required for this lab. Instead, each member of a group must perform the calculations for one run (a different pre-heat setting), submit the detailed calculations and briefly answer the questions in the “Analysis” Calculations: 1. At one preheat setting (30%, 50%, and 70%), use the average T and RH (relative humidity) at each of the four positions, point 1 (fan inlet), point 2 (before the evaporator), point3 (after the evaporator), and point 4 (duct exit) to locate the points on the Psychrometric and label each point with its number 2. Using the Psychrometric chart, estimate the wet-bulb temperature, absolute humidity (aka Humidity Ratio (x), and the Specific Enthalpy (h) at each point. Also, find the dew point temperature at point 2 (before the evaporator) and the specific volume at point 4. Show the results in a table 3. Calculate the density of air at point 4 4. Use the average velocity and the cross section area of the duct (0.0412 m2) to calculate the volumetric flow rate of air in m3/s 5. Calculate the mass flow rate of air in kg/s 6. Use the enthalpy at points 1,2,3 and 4 to determine: i. ii. iii. Heat added to the air by the pre-heater Heat removed from the air by the evaporator Heat added to the air by the re-heater 7. Determine the cooling capacity of the unit in TON 8. Calculate the COP of the refrigeration unit if the power input to the compressor is 0.2 hP (Note: 1 hP= 745.7 Watts) 9. From the change in Humidity Ratio across the evaporator, and the mass flow rate of air, calculate the rate of dehumidification in g/s Analysis: 1. Connect the points on the Psychrometric charts and describe the type of processes that occur between each two points (e.g. 1-2, 2-3, and 3-4) 2. Are the processes as expected? Explain 3. How does the relative humidity change with temperature? Are the changes as expected? 4. Is dehumidification expected to occur in process 2-3? (Hint: compare T3 with Tdew at point Lab 6 data and report • • • • • • Data for experiment 6, Air Conditioning analysis, is now posted. As was explained in class, the report for this lab is INDIVIDUAL and DOES NOT REQUIRE all components of a complete report. Each member of a group must select a different run of the experiment and submit an individual report consists of ONLY the following parts: Title page (Must include the title of the experiment and Run#, Student’s name, group# ,section enrolled, and submission date) Raw data (Raw data can be copied from beachboard but the selected run must be highlighted) Detailed calculations (this part can be handwritten) Answer to the questions in the “Analysis” part of the handout (MUST BE TYPED) A copy of the Psychrometric chart showing all points of analysis (marked and labeled with numbers) Use the following table to present properties of air found at each point using the psychrometric chart: o 3 T (oC) Twet (oC) X RH(%) h (kJ/kg) Tdew ( C) V(m /kg) (gH2O/kgair) Point 1 NR Point 2 NR NR Point 3 NR Point 4 NR NR NR= NOT REQUIRED The report for this lab is due Monday, May 6. This report MUST be submitted individually . Like the other reports, an electronic copy must be submitted to BeachBoard. The hard copy will be collected in class
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