basics of psychrometrics-f.ppt
- 1. PRESENTED BY : D.BALAJI HVAC CONSULTANT PAST PRESIDENT – ISHRAE CHENNAI CHAPTER SHREE SRB & ASSOCIATES -CHENNAI Mail Id : shreesrb@yahoo.com Mobile : 9600021111
- 2. THE AIR CONDITIONING PROCESSES ARE BEST UNDERSTOOD BY STUDYING PSYCHROMETRY. PSYCHROMETRY IS THE STUDY OF ATMOSPHERIC AIR AND ITS ASSOCIATED WATER VAPOUR. 2 A psychrometric chart presents physical and thermal properties of moist air in a graphical form. INTRODUCTION TO PSYCHROMETRICS
- 4. TEMPERATURE Average kinetic energy of the molecules in a substance. Measure of intensity of Heat in a substance.
- 5. Heat is the total amount of energy possesses by the molecules in a piece of matter. This energy is sum of both kinetic energy and potential energy of the molecules. Heat is a form of energy which transfers from Higher Side to Lower Side. Some Sources of Heat in Air Conditioned Space HEAT
- 6. Modes of Heat Transfer 24 C 35 C Conduction Convection Radiation Heat transfer from molecule to molecule through a substance by chain collision. Heat transfer by the movement of molecule from one place to another. Heat transfer by passing from a source to an absorbent surface without heating the space in between.
- 7. Types of Heat Sensible Heat Latent Heat Electric Bulb The Heat added or removed without change in moisture content of air is termed as Sensible Heat . Purely Sensible !! Zero Latent Heat The Heat added or removed with change in moisture content of air is termed as Latent Heat . Only Heat Addition No Moisture Addition Eg : - Eg : - People Sensible Heat ( normal body temperature 37 C ) + Latent Heat ( Respiration and Perspiration ) Heat Addition Moisture Addition
- 8. British Thermal Unit ( Btu/hr ) One British thermal unit is defined as the amount of heat to be added ( removed ) to raise ( lower ) the temperature of one pound of water by One Degree F. 1 Pound 33 F Sensible Heat = 1 Pound x ( 33 - 32 ) = 1 BTU GOOD DESIGN EXCELLENT PERFORMANCE 5 ISHRAE CHENNAI CHAPTER
- 9. Ton of Refrigeration One Ton of Refrigeration is defined as the quantity of heat added to melt one ton of ice at 32 degree F ( 0 degree C ) to water at the same temperature in 24 hours or vice versa.
- 10. •CFM, or cubic feet per minute - determines the airflow in a space - how much air flows into a space per minute. •CFM determines how efficient your HVAC system is, as a room with little airflow won’t circulate conditioned air , while a room with too much airflow will be drafty. •Approx 2.5 CFM – 3.0 CFM per Sq.Ft. is ideal. CFM – CUBIC FOOT PER MINUTE
- 11. DRY BULB TEMP. WET BULB TEMP. HUMIDITY DEW POINT MOISTURE CONTENT HEATING COOLING HUMIDIFY DE-HUMIDIFY Psychrometric Chart PSYCHROMETRICS
- 12. Dry-Bulb Temperature (DBT) is the commonly measured temperature from a thermometer. 12 DRY BULB TEMPERATURE
- 14. SATURATION LINE IS A TEMPERATURE CONDITION AT WHICH WATER WILL BEGIN TO CONDENSE OUT OF MOIST AIR. IT IS ALSO CALLED THE DEW POINT TEMPERATURE LINE. 14 SATURATION LINE
- 15. WET BULB TEMPERATURE (WBT) IS DETERMINED WHEN AIR IS CIRCULATED PAST A WETTED SENSOR TIP. INTERSECTION OF DBT AND WBT PROVIDES THE STATUS POINT. 15 WET BULB TEMPERATURE
- 19. RELATIVE HUMIDITY The Relative Humidity is the ratio of actual amount of moisture present in one unit volume of dry air to the amount of moisture needed to saturate it at that temperature Dry Air 1 lb Water W lb Mixture ( 1 + W ) lb Relative Humidity = W Ws W : moisture at saturated condition s At Saturated Condition Air is having maximum amount of water particle beyond this moisture in air will condense. X 100 = %
- 20. Relative Humidity (RH) is a ratio between actual moisture content of the air compared with the moisture content of the air required for saturation at the same temperature. RH is affects human comfort significantly. 20 RELATIVE HUMIDITY
- 22. SPECIFIC HUMIDITY The Specific Humidity is the weight of water vapour in grains or pounds of moisture per pound of dry air. Dry Air 1 lb Water W lb Mixture ( 1 + W ) lb
- 25. DEW POINT If moisture condenses on a cold bottle taken from the refrigerator, the dew-point temp of air is above the temp of the bottle.
- 26. DEW POINT •Dew Point- DP-temp when air is completely saturated •DP close to dry bulb temp - relative humidity is high •DP below dry bulb temp - relative humidity is low •Above DP moisture will stay in the air. Below DP water vapor starts to condense out of the air
- 27. Enthalpy is the energy content of dry air & moisture expressed in BTU/LB of dry air. The information is useful is sizing up cooling coils. 27 ENTHALPY
- 29. PSYCHROMETRICS Psychrometrics is the science involving thermodynamic properties of moist air and the effect of atmospheric moisture on materials and human comfort. F G A E C B D H A - Dry Bulb Temperature B - Wet Bulb Temperature C - Relative Humidity D - Humidity Ratio E - Dew Point Temperature F - Enthalpy G - Specific Volume H - Sensible Heat Factor Psychrometric Chart
- 31. PSYCHROMETRICS
- 36. AIR CONDITIONING PROCESS The Apparatus dew point corresponds approximately to the surface temperature of the heat exchanger exposed to the air side and therefore represents the coldest possible temperature that the leaving air could attain.
- 37. BYPASS FACTOR - CHART
- 38. BYPASS FACTOR - EXAMPLE BF = Leaving Air Dry Bulb Temp - Coil Temp Entering Air Dry Bulb Temp - Coil Temp = 60 F - 44 F 80 F - 44 F = 0.44 One way to understand bypass factor is to mentally divide the air stream into two components. The first component is cooled perfectly to the apparatus dew point temperature. The second component is not cooled at all, representing air bypassing the heat exchanger. The bypass factor is ratio of the second component to the total air flow.
- 39. Climate Classification shows the various approximate zones of temperature and humidity on the psychrometric chart. 39 CLIMATE CLASSIFICATION
- 41. Heating & Cooling Process – addition or removal of sensible heat without change in absolute moisture content. But RH changes as temperature changes. 41 SENSIBLE HEAT PROCESS
- 42. Dehumidification by cooling – in cooling coils temp of air reduces and the saturation point (dew point) is reached. Further cooling results in reduction of absolute humidity. 42 DEHUMIDIFICATION
- 43. Comfort Zone – statistical studies have been performed on large numbers of subjects of all ages, sexes and nationalities to arrive at a quantitative description of human comfort. 43 COMFORT ZONE
- 44. PSYCHROMETRICS
- 45. EXAMPLE PROBLEM Example : Given Room Condition of 75 degree F DB and 50% RH. For the air vapour mixture find : •Wet Bulb Temperature •Humidity Ratio •Enthalpy •Dew Point Temperature
- 46. EXAMPLE PROBLEM Solution : Locate the point where the vertical line representing 75 DB and the curve line representing 50% RH intersect. From this point read all the other values. Follow the diagonal line upto the left until it intersects the wet bulb temperature scale read 62.6 degree F as wet bulb temperature. Follow the horizontal line to the right until it intersects the humidity ratio scale. Read 64.8 grains of moisture per pound of dry air as humidity ratio. Revolve a straight edge around the point until its edge intersects the same numerical value on the enthalpy scale at the left and at the bottom of the chart. Read 28.10 Btu / Pound as the enthalpy. Follow the horizontal line to the right until it intersects the dew point temperature scale read 55.1 degree F as Dew Point Temperature.
- 47. EXAMPLE PROBLEM Example : The following readings were taken from an Air Handling Unit in operation. Find out the output capacity of the AHU in TR. Entering Air Dry Bulb Temp = 75.2 Deg F. Entering Air Wet Bulb Temp = 64.4 Deg F. Leaving Air Dry Bulb = 53.6 Deg F Leaving Air Wet Bulb Temp = 53.4 Deg F Air Quantity = 3950 cfm Capacity ( Btu / hr ) = 4.45 x Air quantity x ( H1 – H2 ) Capacity ( Btu / hr ) = 4.45 x 3950 x ( 29.75 – 22.5 ) Capacity ( Btu / hr ) = 128868.8 Capacity ( TR ) = 128868.8 / 12000 = 10.74 TR