Refrigeration and Air conditioning .pptx

Refrigeration

1. CCSS, NGSS Refrigeration & Air-Conditioning (ME-319)

2. Problem 7 Air enters an air-conditioning system that uses refrigerant-134a at and 70 percent relative humidity at a rate of . The refrigerant enters the cooling section at with a quality of percent and leaves as saturated vapor. The air is cooled to at a pressure of and leaves the system as saturated air. Determine (a) the rate of dehumidification, (b) the rate of heat transfer, and (c) the mass flow rate of the refrigerant.               2

3. Problem 7 (cont’d) Solution Properties at point 1 The total pressure is 1 atm. The properties of the air at the inlet and exit states may be determined from the psychrometric chart. Given: From Chart:   Properties at point 2 Given: From Chart:   mass flow rate of dry air   Since air is cooled to , which is below its dew point temperature, some of the moisture in the air will condense .   3

4. Problem 7 (cont’d) Solution   Rate of Dehumidification .     Rate of heat transfer from atmospheric air to cooling coil Apply energy balance on control volume.         Control Volume Conservation of mass for water   For Dry Air )     Enthalpy of condensate water 4

5. Problem 7 (cont’d) Solution Inlet and exit enthalpies of refrigerant . Mass flow rate of Refrigerant heat lost by the air is gained by the refrigerant, the mass flow rate of the refrigerant becomes       0.603 kg/min             5 400 kPa 400 kPa

6. Air-Conditioning Processes 6 Problems associated with the low relative humidity resulting from simple heating can be eliminated by humidifying the heated air. This is accomplished by passing the air first through a heating section (process 1-2) and then through a humidifying section (process 2-3) . The location of state 3 depends on how the humidification is accomplished. Steam Based Humidification : If steam is introduced in the humidification section, this will result in humidification with additional heating ( ). Water Based Humidification: If humidification is accomplished by spraying water into the airstream instead, part of the latent heat of vaporization comes from the air, which results in the cooling of the heated airstream ( ).   Heating and Humidification

7. Problem 8 Outdoor air enters an air-conditioning system at and percent relative humidity at a steady rate of , and it leaves at and percent relative humidity. The outdoor air is first heated to 18°C in the heating section and then humidified by the injection of hot steam in the humidifying section. Assuming the entire process takes place at a pressure of , determine (a) the rate of heat supply in the heating section and (b) the mass flow rate of steam required in the humidifying section.   7

8. Problem 8 (cont’d) Solution Psychrometric Chart 8

9. Problem 8 (cont’d) Solution Properties at point 1 The total pressure is 1 atm. The properties of the air at the inlet and exit states may be determined from the psychrometric chart. The amount of moisture in the air also remains constants it flows through the heating section , but increases in the humidifying section .   Given: From Chart:   Properties at point 2 Given: From Chart:   mass flow rate of dry air   Properties at point 3 Given: From Chart:   9

10. Problem 8 (cont’d) Solution   Mass flow rate of steam in humidifying section .     Rate of heat transfer from heating coil to atmospheric air         Control Volume       Conservation of mass in humidification section   For Dry Air )   10

11. Air-Conditioning Processes 11 Many air-conditioning applications require the mixing of two airstreams. This is particularly true for large buildings, most production and process plants, and hospitals, which require that the conditioned air be mixed with a certain fraction of fresh outside air before it is routed into the living space. When two airstreams at two different states (states 1 and 2) are mixed adiabatically, the state of the mixture (state 3) lies on the straight line connecting states 1 and 2 on the psychrometric chart, and the ratio of the distances and is equal to the ratio of mass flow rates and .   Mixing of Air Streams  

12. Air-Conditioning Processes 12 Mixing of Air Streams Depending upon the state of the individual streams, the mixing process can take place with or without condensation of moisture. The concave nature of the saturation curve and the conclusion above lead to an interesting possibility. When states 1 and 2 are located close to the saturation curve, the straight line connecting the two states will cross the saturation curve, and state 3 may lie to the left of the saturation curve. In this case, some water will inevitably condense during the mixing process. Without Condensation With Condensation

13. Problem 9 Two airstreams are mixed steadily and adiabatically. The first stream enters at and percent relative humidity at a rate of , while the second stream enters at and percent relative humidity at a rate of . If the mixing process occurs at a pressure of , determine the specific humidity, the relative humidity, the dry bulb temperature, and the volume flow rate of the mixture.         13

14. Problem 9 (cont’d) Solution Psychrometric Chart 14

15. Problem 9 (cont’d) Solution Properties at point 1 Given: From Chart:   Properties at point 2 Given: From Chart:   mass flow rate of dry air in each stream       For Dry Air 15

16. Problem 9 (cont’d) Solution   Specific Humidity of Mixture .     Apply energy balance on control volume.   Conservation of mass for water   Enthalpy of Mixture .       Properties at point 3 from psychrometric chart Given: and From Chart:   16

17. Problem 9 (cont’d) Solution   Volume flow rate of mixture . Conservation of mass for water     17

18. Problem 10 Conditioned air at and percent relative humidity is to be mixed with outside air at and percent relative humidity at . If it is desired that the mixture have a relative humidity of , determine (a) the ratio of the dry air mass flow rates of the conditioned air to the outside air and (b) the temperature of the mixture.         18

19. Problem 10 (cont’d) Solution Psychrometric Chart 19

20. Problem 10 (cont’d) Solution Properties at point 1 Given: From Chart:   Properties at point 2 Given: From Chart:     For Dry Air   Apply energy balance on control volume. ratio of the dry air mass flow rates of the conditioned air to the outside air .       20

21. Problem 10 (cont’d) Solution   Given: Connect straight-line between point 1 and 2, intersection of this line and , provide desired values (heat and mass transfer processes are related in such a way that the mixing process drives on a straight-line on psychrometric chart) Chart:   Properties at point 3 from psychrometric chart   21