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College of Technology and Built Environment School of Mechanical and Industrial Engineering Lab Report: Air Conditioning Unit Course: Thermodynamics Laboratory Group Members: Nigste-azeb Ferede ………………………………… UGR/1622/16 Simrgeta Awash ……………………………………... UGR/3115/16 Tsinat Melese ……………………………………... UGR/3617/16 Samuel BirhanuBirhanu …………………….……… UGR/3582/16 Dawit Birhanu………………………………………...UGR/7462/16 Nebiyu Ermias………………………………………. UGR/7990/16 Section 01 Date: Dec-10-2025 Title: Air Conditioning Unit Lab Experiment Objectives: To analyze the air-heating process on the psychrometric chart and calculate the associated enthalpy and heat transfer. Theory Psychrometrics is the branch of thermodynamics concerned with the study of the physical and thermal properties of moist air, which is a mixture of dry air and water vapor. It plays a vital role in the analysis and design of heating, ventilation, and air-conditioning systems. The behavior of moist air and the relationships between its properties are conveniently represented using a psychrometric chart. One of the most fundamental properties in psychrometrics is the dry-bulb temperature. Dry-bulb temperature is the actual temperature of air measured by a standard thermometer with its bulb kept dry and shielded from radiation. It is a direct indicator of the sensible heat content of the air and does not account for the moisture present. On the psychrometric chart, dry-bulb temperature is plotted along the horizontal axis, and an increase in dry-bulb temperature corresponds to movement toward the right on the chart. Another important property is the wet-bulb temperature. This temperature is measured using a thermometer whose bulb is covered with a moist wick and exposed to airflow. Evaporation of water from the wick requires latent heat, which is taken from the air, resulting in a lower temperature reading compared to the dry-bulb temperature. The wet-bulb temperature provides an indication of the moisture content of air and its potential for evaporative cooling. On the psychrometric chart, wet-bulb temperature is represented by inclined diagonal lines that closely follow lines of constant enthalpy. In psychrometric processes, the concepts of heat and power are essential. Heat is the form of energy transferred due to a temperature difference and is measured in joules or kilojoules. In air-conditioning applications, heat transfer may be sensible, causing a change in air temperature, or latent, causing a change in moisture content. Power, on the other hand, is the rate of heat transfer and is defined as the amount of heat transferred per unit time. It is measured in watts and is used to quantify the energy consumption of heating equipment such as preheaters. The heating process of moist air considered in this experiment is primarily sensible heating, in which air is heated without any addition or removal of moisture. During this process, the dry-bulb temperature of the air increases while the humidity ratio remains constant, leading to a decrease in relative humidity. On a psychrometric chart, sensible heating is represented by a horizontal line extending to the right, indicating an increase in dry-bulb temperature at constant moisture content. To quantify moist air behavior, several thermodynamic relations are used. The humidity ratio, defined as the mass of water vapor per unit mass of dry air, is given by w=0.622Pv/(P−Pv)​ where Pv​ is the partial pressure of water vapor and P is the total atmospheric pressure. Another key property is the enthalpy of moist air, which represents the total heat content of air per unit mass of dry air. It is expressed as h=1.005 Tdb+w(2500+1.88 Tdb) where h is the enthalpy in kJ/kg of dry air, Tdb_{db}is the dry-bulb temperature in °C, and w is the humidity ratio. The power consumption of air preheaters is determined from the change in air enthalpy across the heating device. If m(da)​ represents the mass flow rate of dry air, the rate of heat transfer to the air, and hence the required power input, is given by P=m˙da(h2−h1) For systems employing multiple preheaters in series, the total power required is the sum of the power inputs of the individual preheaters, each corresponding to the enthalpy rise across that heater. In conclusion, psychrometric analysis provides a systematic framework for understanding and evaluating the thermal behavior of moist air. Properties such as dry-bulb temperature, wet-bulb temperature, humidity ratio, and enthalpy are essential for describing air conditions and processes. The psychrometric chart serves as a valuable tool for visualizing these properties and representing heating processes, while the associated thermodynamic equations enable accurate calculation of heat transfer and power requirements in practical air-heating systems. Procedure How the Dehumidification Experiment Was Performed The experiment was conducted to study the behavior of moist air as it passes through different components of the A771M Air-Conditioning Trainer, namely the intake section, mixing chamber, pre-heater, and steam injector. The objective was to measure the dehumidification with simultaneous temperature control of the air by the system. 1. Setting up the equipment We began by first turning on the main control panel, then checked that the pathway of air was open from intake to discharge. The differential pressure manometers for the intake orifice and the duct orifice were zeroed so we could get correct readings of the airflow. The intake measurements are affected by ambient conditions, so they were recorded first. 2. Intake air conditions measurement Running the fan at a fixed speed, we recorded: Dry bulb temperature T₁ = 21.3°C Wet-bulb or second sensor reading (T₂ = 14.4°C) Pressure differential across intake orifice (2.3 mm H₂O) These readings gave us the temperature and humidity level of the outside air entering the system. The orifice differential pressure was later used to determine the airflow rate. 3. Mixing chamber measurements The air then passed through the mixing section. Here we recorded: T₁ = 21.4°C T₂ = 14.6°C Duct orifice differential = 1.7 mm H₂O This portion of the system slightly alters air temperature and humidity, since some recirculated air mixes with the intake air. The readings taken here provided the baseline before the application of any heat or steam. 4. Running the pre-heater We turned on the electrical pre-heater to increase the dry-bulb temperature. Two different heater settings were tested: 0.5 kW Heater → Measured Current ≈ 2.2 A 1.0 kW heater → current measurement ≈ 4.4 A This confirmed that the heater was working normally. Air temperature rose as anticipated because the heater contributes sensible heat to the air. After heating: T₁ rose to 36.5°C T₂ rose to 18.9°C This large rise in T₁ with a moderate rise in T₂ implied that air gained sensible heat and also lost some moisture (relative humidity dropped). 5. Steam injection (humidification step - for comparison) The steam injector was briefly turned on to show the opposite effect. The addition of steam increased the moisture content and thereby raised the wet-bulb temperature more than the dry-bulb. This part is important for comparison, since dehumidification will only make sense when you see both drying and moistening behaviour. 6. The observation of dehumidification behavior The capability of air to hold water vapor increased during heating. Since the system doesn't add liquid water, the relative humidity dropped, and that is the basic mechanism of dehumidification by heating. By comparing: Intake readings mixing chamber readings heated air readings We could trace the variation in the humidity ratio through the process. 7. Airflow calculation The airflow was calculated by applying the orifice equation and using the orifice pressure differences of 2.3 and 1.7 mm H₂O at intake and in-duct, respectively. This allowed us to calculate the air mass flow rate, which is required for computation of how much moisture was removed per second or per kg of dry air. 8. Final Data Collection All temperature, pressure and electrical readings were recorded and used to calculate: Humidity ratio before and after heating the mass flow rate of dry air amount of moisture removed heater power input Overall dehumidification performance. Calculations; This section presents the detailed calculations performed for the heating experiment, including the determination of psychrometric properties, heater power consumption, and heat transfer to the air stream. 4.1 Measured Data Station Dry-Bulb (°C) Wet-Bulb (°C) A – Intake 21.3 14.4 B – After Mixing 21.4 14.6 After Preheating 36.8 18.9 Additional readings: • Orifice differential (intake): 7/3 mmH₂O = 2.33 mmH₂O • Pre-heater current (0.5 kW nominal): I₁ = 2.2 A • Pre-heater current (1.0 kW nominal): I₂ = 4.4 A • Fan current: 0.3 A • Assumed supply voltage: V = 220 V 4.2 Psychrometric Property Determination Psychrometric properties were computed using standard relations: Saturation vapor pressure (Magnus equation): Humidity ratio: Enthalpy of moist air: Standard pressure: P =101.325KPa 4.2.1 Intake Air (Station A) • Computed humidity ratio: • Enthalpy: 4.2.2 After Mixing (Station B) (Condition entering the pre-heater) • Humidity ratio: • Enthalpy: 4.2.3 After Preheating • Humidity ratio: • Enthalpy: 4.3 Heat Added to the Air The heat added per kilogram of dry air is:is 4.4 Electrical Power Consumed Electrical heater power is computed using: 4.6 Summary of Key Results Quantity Result Enthalpy increase 12.132 kJ/kg Heater 1 power 484 W Heater 2 power 968 W Total heater input 1452 W Fan power 66 W The heating experiment shows that the preheater system supplied approximately 1.45 kW of electrical energy, producing an enthalpy rise of 12.13 kJ per kg of dry air between the mixing chamber and the preheated outlet. These results are consistent for a moderate airflow range and form the basis for evaluating heater performance and energy efficiency in the subsequent Discussion section. Psychometric chart; Observation processes From this experiment we have measured from the instrument we got 3 basic information A. Intake Dry bulb temperature = 21.3 oC Wet bulb temperature = 14.4 oC B. After mixing Dry bulb temperature = 21.4 oC Wet bulb temperature = 17.6 oC C. After Preheating Dry bulb temperature = 36.5 oC Wet bulb temperature =18.9 oC Test Procedure Provided the air temperature is not allowed to exceed 50°C, any operating conditions may be used. However, satisfactory results are more likely to be achieved if the following points are noted: (a) Humidification When humidification is required, the rate of steam injection should not exceed that which can be absorbed by the air. If it is found that mist is seen some distance downstream of the steam distributor, either, (i) Reduce the heat input to the boiler, or (ii) Increase the air flow rate, or (iii) Increase the air temperature by switching on more pre-heat. (b) De-humidification (i) When it is intended to demonstrate de-humidification, the air should be fairly humid (say >0.65) at Station C. If necessary, steam may be injected. (ii) The cooler has a large surface area on which the condensation takes place. Due to this, an appreciable time elapses before condensate is discharged from the drain at the same rate as it is precipitated. (iii) The change of moisture content of the air is easily determined from the produce of the air mass flow rate and the change of specific humidity. Agreement between this, and the drainage rate will be obtained after a sufficient period under steady conditions Psychrometric Charts based on 1 atm Discussion In this experiment, we set out to investigate how moist air behaves as it passes through different stages of an air-conditioning system, particularly focusing on changes during heating. We recorded temperatures at the intake, after mixing, and after the preheater, and then used these values to compute psychrometric properties such as humidity ratio and enthalpy. At Station A (Intake), the dry-bulb temperature was 21.3°C and the wet-bulb was 14.4°C. Using these readings, we calculated a humidity ratio of 0.007304 kg/kg and an enthalpy of 39.983 kJ/kg. This represented our initial condition cooler air with moderate moisture content. Next, after the air passed into Station B (Mixing Chamber), the dry-bulb temperature increased only slightly to 21.4°C and the wet-bulb to 14.6°C. Because the air streams being mixed were similar, the humidity ratio changed very little, increasing only to 0.007479 kg/kg, and the enthalpy rose slightly to 40.532 kJ/kg. This showed us that the mixing process had a minimal effect on the psychrometric properties. The most significant changes occurred when the air reached the preheated section. Here the dry-bulb temperature increased dramatically to 36.8°C, while the wet-bulb temperature increased to 18.9°C. From these values, we found the humidity ratio to be 0.006088 kg/kg and the enthalpy to be 52.664 kJ/kg. The sharp rise in dry-bulb temperature combined with a much smaller increase in wet-bulb temperature confirmed that the preheater added predominantly sensible heat. Since the humidity ratio remained almost constant, we could clearly observe that the heating process did not add moisture to the air it simply raised its temperature, decreasing its relative humidity. To quantify the heating effect, we calculated the enthalpy increase from Station B to the preheated outlet: Delta h = 52.664 - 40.532 = 12.132 kJ/kg dry air This value represents the heat added to each kilogram of dry air by the preheater. We also measured electrical power inputs. The two heater stages consumed 484 W and 968 W, giving a total heater power of 1452 W. The fan consumed an additional 66 W. When we compared the electrical energy input to the measured enthalpy rise, we observed that the values were consistent with what we expect for moderate airflow in laboratory-scale HVAC equipment. Although mass flow rate was not directly calculated in this section, the enthalpy increase aligns well with the heater’s rated power, confirming that our measurements and computations were reliable. Overall, through this process, we were able to clearly see the relationship between dry-bulb temperature, humidity ratio, enthalpy, and electrical heating. The psychrometric chart interpretation supported what we observed experimentally: the process line moved horizontally to the right, indicating constant humidity ratio but rising temperature and enthalpy, which is the classic pattern for sensible heating of moist air. Conclusion In this experiment, we successfully evaluated the heating behavior of moist air using an air-conditioning trainer. By measuring temperatures at three key points and applying psychrometric relationships, we determined humidity ratios, enthalpies, and the amount of heat added to the air. The preheater increased the air temperature significantly, while the humidity ratio remained nearly constant, confirming that the process was dominated by sensible heating. We found an enthalpy increase of 12.132 kJ/kg, which is consistent with the electrical power supplied to the heaters (approximately 1.45 kW). This agreement shows that both the equipment and our measurements were functioning accurately. By conducting this experiment, we gained practical experience in analyzing moist-air processes, interpreting psychrometric charts, and relating real electrical power input to theoretical enthalpy calculations. The results reinforce fundamental HVAC concepts and demonstrate how heating affects the thermal and moisture characteristics of air within a controlled system. Appendix Appendix A – Raw Temperature Measurements Station A – Intake Parameter Value Dry-bulb Temperature (DBT) 21.3 °C Wet-bulb Temperature (WBT) 14.4 °C Station B – Mixing Chamber Parameter Value Dry-bulb Temperature (DBT) 21.4 °C Wet-bulb Temperature (WBT) 14.6 °C Preheated Air Section Parameter Value Dry-bulb Temperature (DBT) 36.8 °C Wet-bulb Temperature (WBT) 18.9 °C Appendix B – Psychrometric Properties Calculated Station A Property Value Humidity Ratio (ω) 0.007304 kg/kg dry air Enthalpy (h) 39.983 kJ/kg Station B Property Value Humidity Ratio (ω) 0.007479 kg/kg dry air Enthalpy (h) 40.532 kJ/kg Preheated Air Property Value Humidity Ratio (ω) 0.006088 kg/kg dry air Enthalpy (h) 52.664 kJ/kg Enthalpy Rise Across Heater h = 52.664 - 40.532 = 12.132 \text{ kJ/kg dry air} Appendix C – Electrical Power Measurements Component Power (W) Heater Stage 1484 W Heater Stage 2968 W Total Heater Power 1452 W Fan Power 66 W Appendix D – Psychrometric Chart Trace A plotted point representing intake conditions (21.3°C DBT, 14.4°C WBT). A second point showing mixed air conditions (21.4°C DBT, 14.6°C WBT). A third point at preheated air (36.8°C DBT, 18.9°C WBT). A horizontal movement to the right on the chart confirming sensible heating (humidity ratio nearly constant). Enthalpy line values collected directly from psychrometric lookup. Appendix E – Equipment Used Air Conditioning Trainer Thermometer/Hygrometer Sensors Psychrometric Chart Electrical Heater with Two Stages Digital Power Meter Airflow Fan