ME 410 MECHANICAL ENGINEERING SYSTEMS LABORATORY MASS & ENERGY BALANCES IN PSYCHROMETRIC PROCESSES EXPERIMENT 3 1. OBJECTIVE The objective of this experiment is to observe four basic psychrometric processes which are heating, cooling, humidification and dehumidification in an air conditioning unit. The air velocity, dry and wet bulb temperatures and the amount of water added/removed will be measured to check the mass and energy balances of these processes. 2. INTRODUCTION The function of an air conditioning equipment is to change the state of the entering air to a desired state by controlling temperature and humidity of the specified space. Air conditioning applications are divided into two types according to their purposes: i) Comfort air conditioning, ii) Industrial air conditioning. The primary function of air conditioning is to modify the state of the air for human comfort. The industrial air conditioning meets the temperature and humidity requirements of an industrial or scientific process. In comfort air conditioning, it is necessary to control simultaneously the temperature, relative humidity, cleanliness and distribution of air to meet the comfort requirements of the occupants. According to the comfort chart given by the American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE), comfort conditions can be obtained at 20-23 C dry bulb temperature (DBT) and (50 ± 20)% relative humidity in winter, and 24-27 C DBT and (50 ± 20)% relative humidity in summer. In order to maintain these requirements, the state of the air is modified in an air conditioning apparatus such that the varying summer and winter loads are balanced. 3. THEORY In air conditioning, the moist air (or simply the air) is taken as a mixture of dry air (a) and water vapor (w) carried with it. At a given total air pressure and temperature, the amount of water vapor that may be contained in the air is limited. The mixture existing at this limit is called saturated air. Any excess water vapor in the air separates itself from the mixture as a liquid (condensate) or solid (ice). 1
The dry bulb temperature (T db ) is the familiar temperature that can be measured by a thermometer or a thermocouple. On the other hand, the wet bulb temperature, T wb, is related to the humidity level. The humidity of moist air may be stated in terms of either relative humidity, Φ or humidity ratio, ω. The psychrometric charts are diagrams giving the relationship between T db, T wb, Φ, ω and h (enthalpy) by assuming an ambient pressure. For example, ASHRAE psychrometric chart no. 5 is for 750 m. elevation (92.634 kpa barometric pressure) which may be used for Ankara (see Fig. 7). Many psychrometric processes may be represented on these charts by straight lines. Wet bulb temperature (T wb ) is the temperature measured when the bulb of a thermometer or the junction of a thermocouple is wetted. For unsaturated moist air, it is less than the dry bulb temperature; the difference being proportional to the relative humidity. In practice T wb is assumed to be equal to the adiabatic saturation temperature, T*, which would be reached if moisture is added in an adiabatic process until the air becomes saturated. Thus, T wb ~ T*. Relative humidity (Φ) and humidity ratio (ω) are defined as, / (1) where P w is the partial pressure of water vapor in air and P ws is the saturation pressure of water at air temperature T. Relative humidity is a dimensionless quantity usually expressed as percentage. The humidity ratio (also called specific humidity), ω, is defined as / (2) where m w is the mass of water vapor in moist air and m a is the mass of the dry air. Using the ideal gas relationship for dry air and water vapor, humidity ratio becomes... (3) The humidity ratio of air at a given P and T may be calculated from the above relationships when T * is known: (4) where., 2
T, T * are the dry and wet bulb temperatures ( C), respectively, h f * h g h fg P w c p * is the specific enthalpy of liquid water at T * (kj/kg w ) g is the specific enthalpy of water vapor at T ( kj/kg w ) g * = (h g - h f ) at T* (kj/kg w ) ws * is the saturation pressure of water evaluated at T * (kpa) a is the constant pressure specific heat of dry air (1.0035 kj/kg a ). Note that * indicates properties which are evaluated at the adiabatic saturation (that is the wet bulb) temperature T *. Enthalpy (h) The enthalpy of the moist air at any state can be read from psychrometric charts or can be calculated as: @ 1.0035 1.8723 2501.4 (5) Sensible Heating or Cooling (Q s ) The sensible heat transfer process is one where only energy is added or removed from the moist air. The dry and wet bulb temperatures, relative humidity change as a result of heat transfer, but there is no change in water vapor content or humidity ratio of the air (see Fig. 1). Fig. 1 Sensible heating and cooling Humidification or Dehumidification The process of adding water vapor to the air is called humidification. Humidification increases the humidity ratio, relative humidity, wet bulb temperature and the enthalpy, but the dry bulbb temperature may slightly change or remains unchanged. The reverse process, which 3
decreases the humidity ratio is called dehumidification. It may be achieved by absorbing the moisture at constant temperature by a desiccant (a drying agent) as shown in Fig. 2 or by cooling the moist air below its dew point temperature as illustrated in Fig. 3 by using refrigeration. Combined Heating and Humidification, or Cooling and Dehumidification The following combined sensible and latent process, shown in Fig. 3 may occur in air conditioning: 1-6: Heating and humidification (common in winter) 1-7: Heating and dehumidification (with a desiccant) 1-8: Cooling and humidification (as in air washers) 1-9: Cooling and dehumidification (common in summer) 1-9 : Cooling and dehumidification (theoretical) In Fig. 3, process 1-9 is actual whereas processs 1-9 is theoretical (ideal). Fig. 2 Humidification and dehumidification concepts Fig. 3 Combined processess Mass and Energy Balances At steady state, the following relations can be obtained from the mass and energy balances for a general process as shown in Fig. 4. The continuity equation for dry air is given by Fig. 4 General control volume 4
and that for the water vapor is The first law of thermodynamics yields where m& & m w. zero during sensible heating or cooling = condensate removed during dehumidification (-) water vapor injected during humidification (+) (6) (7) (8) h w o h g at water vapor temperaturee (96 C) for humidification = h f at T 2 for dehumidification Q & = Rate of heat transfer, (+) for heating, (-) for cooling Note that water boils at about 96 o C in Ankara. The percentage error between the measured and theoretical values can be found by Theoretical Value Measured Value Percentage Error = 100% (9) Theoretical Value Refrigeration Cycle Cooling the moist air with or without dehumidification is usually achieved by using a mechanical refrigeration cycle which includes a compressor, a condenser, an expansion valve (or capillary tube for small systems) and an evaporator. Fig. 5 Refrigeration cycle In the laboratory unit, the compressor is reciprocating type run by an electrical motor which also runs the fan of the air cooled condenser. Figure 5 shows the equipment schematics as well as P-h and T-s diagrams of a typical cycle. In reality, the compression processs will be 5
irreversible and there will be pressure losses through the evaporator, the condenser and the connecting pipes. The isentropic efficiency of the compressor is defined as: (10) The parameters that are important include the compressor discharge temperature (T 2 ), cooling capacity, power input and coefficient of performance of the cycle which may be defined as : (11) Because of the irreversibility of the expansion valve and also the other parts, the COP becomes less than the ideal value of a reversible (Carnot) cycle, Fig. 8 is the P-h diagram for the refrigerant, R-12. (12) 4. Experimental Setup The schematic layout of the set-up is shown in Fig. 6. The main parts of the set-up are as follows: i. Preheaters : Three electrical heaters to heat the air entering ii. Boiler : To supply steam for humidifier. It is composed of a stainless steel container and three electrical heaters, which are dipped into the water iii. Cooling Coil : To cool the air with or without dehumidification iv. Rotating vane anemometer : To measure air flow rate in feet per minute v. Reheaters : Two electrical heaters after the cooling coil which reheats the cooled air before delivery to the space, if required vi. Compressor-Condenser unit : To complete the refrigeration cycle vii. Fan : For air circulation viii. Thermocouples and thermometers : For measuring dry and wet bulb temperatures 6
PROCEDURE : Before the Experiment: Check all the thermocouples and thermometers, they should show the same dry bulb and wet bulb temperatures at all locations. Start the boiler and wait until the thermometer shows 96 C. Then turn OFF the power to the boiler, to be restarted for humidification. During the Experiment: Turn the fan ON and note down the air flow. Use heating, cooling, humidification and dehumidification as required. Make the necessary measurements and note them down on the enclosed Data Sheet. At least 10-15 minutes should pass to reach a steady state after any modification on the operation is made. Measurement steps during the experiment: Start your alarm clock to measure the condensed water and water level change in the boiler (5 min). During this 5 min. duration, read the wet bulb and dry bulb temperature values for each state, read the temperature values related with the refrigeration cycle, read the pressure values related with the refrigeration cycle. After 5 min measure the amount of the collected condensed water and boiler level. Measure the air velocity. After the Experiment: (1) Plot the process lines on psychrometric chart. (2) Estimate the T wb at section 4, based on state 5 and the processes between states 4 and 5 (Hint: use the psychrometric chart). (3) Find h, Φ and ω from chart and from equations (1) to (5). Compare the results. (4) Make necessary calculations for m& a, m& w and Q & at each section. Compare the theoretical energy and mass changes with measured ones. (5) Draw the refrigeration (R-12) cycle on the P-h diagram provided (Fig. 8) and estimate power input to the compressor, (, refrigerant flow rate ( m& r ), isentropic efficiency ( ) and the COP. 7
RESULTS AND DISCUSSIONS Questions for Further Discussion: i. Why using the sea level psychrometric chart for Ankara is incorrect? Estimate the error in humidity ratio and enthalpy at some selected moist air states. ii. Estimate the heat lost or gained from the duct surfaces. Will the omission of this cause significant errors in the energy balances? (U sur = 1.7 W/(m 2. C)) (ONLY FOR THE LONG REPORT) Section Preheater Evaporator Reheater Total Lateral Area (m 2 ) 0.72 0.6 2.28 6 iii. Comment on taking electrical heaters consumption as constant. Estimate the variation in electrical energy supplied to these heaters if the resistance is known within ± 20%, and voltage varies within ± 5%. (ONLY FOR LONG REPORT) 8
5 1 Discharge d Inlet Mixer Rotating vane anemometer 4 Evaporator 3 Mixer Steam Injection Mixer 2 Fan Reheaters (3.6 kw) T.E.V PreHeaters (2.88 kw) Feed Water Drier Condensate Boiler Compressor condenser unit Liquid Receiver 1.44 kw 2.5 kw 1.44 kw Fig. 6 Schematic drawing of the experimental set-up 9
ME 410 EXPERIMENT 3 Mass & Energy Balances in Psychrometric Processes DATA SHEET Lab Group : Date: AMBIENT Pressure 92 kpa Temperature mv (11th T.C) AIR FLOW (ft/min) Preheaters: 0.72 kw each (x3) Duct Area: 0.0875 m² Reheaters: 0.72 kw each (x2) Boiler Cross Section: 0.3 m x 0.4 m Temperature Section Dry Bulb Wet Bulb TC Readings at Section 4 (Dry-bulb) TC No. (mv) TC No. (mv) TC No. (mv) TC No. (mv) 1 & 2 1 2 9 14 3 3 4 10 15 5 7 8 12 16 13 18 Tavg Energy Input at Preheaters Energy Input at Reheaters ENERGY VALUES kw kw High side pressure of compressor Low side pressure of compressor Temperature at condenser inlet Temperature at evaporator outlet REFRIGERATION CYCLE Psi (Red Gage) Psi (Blue Gage) mv (20th TC) mv (21st TC) Measurement Time Change in boiler level Amount of condensate WATER MEASUREMENTS min mm ml Conversion Factors: 1 Psi =6.895 kpa 1 ft/min 0.00508 m/s T ( C) 23.46xT(mV) + 2.35 10
ME 410 EXPERIMENT 3 OUTLINE FOR RESULTS Table-1 Enthalpy (h), humidity ratio (ω) and relative humidity (Φ) values for each section T db T wb From Chart From Equations Deviations ( % ) Section ( o C ) ( o C ) h (kj/kg) ω (gr/kg) Φ (%) h (kj/kg) ω (gr/kg) Φ (%) h ω Φ 1 & 2 3 4 5 Table-2 Results of energy and mass balance calculations States 2 & 3 3 & 4 Process Preheating+ Humidification Cooling+ Dehumidification Measured Values Theoretical Values % Deviations Q & m (kw) m& w (kg/s) Q & m (kw) m& w (kg/s) Q & m m& w 4 & 5 Reheating :.. kg/s :.. kg/s :.. kw :.. COP :.. 11
Fig. 7 ASHRAE Psychrometric Chart 12
Fig. 8 P-H Chart 13