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Basics of Psychrometrics Practical Heat Load Calculation

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Basics of Psychrometrics Practical Heat Load Calculation Webinar 30 April 2020 Vikram Murthy ASHRAE Mumbai Chapter Sessions ►Basics of Psychrometrics ►All about Heat ►Practical Heat Load (Cooling Load) Calculation ( Using the E 20 , CLTD - Cooling Load Temperature Difference Method ) Psychrometric basics Psychrometrics ► A hundred and eighteen years ago, Willis Carrier, developed a method that allows us to visualize two of the variables -- the combination of air temperature and humidity that exist in a space. The tool he developed is called the Psychrometric Chart. ► Psychrometrics, which Willis Carrier developed, is the study of the mixture of dry air and water , and is the scientific basis of Air conditioning . Willis Carrier Willis began his first job at the Buffalo Forge Company . Solving a Problem at the Sackett Wilhelm Lithographing Company in Brooklyn, he formulated the Laws of Psychrometrics . Willis Carrier laid down the Equations of Psychrometrics in 1902 . The Carrier Company he founded developed the Centrifugal Chiller and the Weathermaker, that we call an Air Handling Unit or AHU . Purpose Of Comfort Airconditioning ►To Cool or Heat ►To Dehumidify or Humidify (remove or add moisture) ►To remove odours ►To remove particulate & microbial pollutants Definitions Of Air ► Air is a vital component of our everyday lives. ► Psychrometrics refers to the properties of moist air. ► Dry air ► Moist air ► Moist air and atmospheric air can be considered to mean the same Units ► We work in INCH POUND system of units, IP units. (The other unit system in use is SI units) Units of length: ft, inches. Units of area: sq.ft Units of volume: cu.ft. Weight: pound, lb. Moisture: grains. 7000 grains = 1 lb. Units ► Temperature: Deg F Ice = 32 deg F (0 deg C) ► Boiling water = 212 deg F (100 deg C) Body temp.: = 98.6 deg F ( 37 deg C) Karachi Summer temp = 99 deg F (37.1 deg C) Heat: (sensible and latent) Btu Specific Heat: btu/lb per deg F Specific Heat of dry air: Btu/lb per deg F Specific Heat of water vapour: Btu/lb per deg F W = humidity ratio, lbs of water per pound of dry air Units ► Rate of heat flow: Btu/Hr 1 watt = 3.41 BTU/Hr 1 kW = 3410 BTU/Hr 1 H.P. = 2545 BTU/Hr 1 Ton of Refrigeration = 12,000 BTU/Hr K value: BTU/Hr/Sq.ft/Inch thickness/deg F U value: BTU/Hr/Sq.ft/deg F Air quantity: cuft per minute, Cfm Psychrometry ►Air conditioning, by its very name means treating air. ►How would Air behave when it is subjected to cooling, heating, humidifying or de- humidifying processes. ►A study of the properties of Air at normal atmospheric pressure. ►Such a study is what is called Psychrometry. Psychrometry ►Psychrometry is the science of studying the thermodynamic properties of moist air and the use of these properties to analyze conditions and processes involving moist air. Psychrometry (from the Greek word : psukhros which means cold) , is the study of moist air (which is mostly oxygen, nitrogen and water vapor) and of the changes in its condition. an energy or heat graph ►Any point on the psychrometric chart represents air in a specific condition containing a certain amount of heat. The following can be determined by using a Psychrometric Chart : ►dry-bulb temperature ►wet-bulb temperature ►relative humidity (RH) ►humidity ratio ►specific volume ►dew point temperature ►enthalpy Dry Bulb Temperature ►air temperature ►indicated by a thermometer ►measured using a normal thermometer ►degrees Fahrenheit (oF) ►an indicator of heat content ►Constant dry bulb temperatures ►appear as vertical lines Dry Bulb Lines ►Any vertical line is a line of constant temperature. ►condition of air represented by any point on this line will have the temperature corresponding to this vertical line. ►the temperature as recorded by a thermometer which is dry. Dry Bulb Temperatures Dry-bulb Temperature - The temperature of air as registered by an ordinary thermometer. The horizontal X-axis denotes dry bulb temperature (DBT) scale. Vertical lines indicate constant dry bulb temperature. DBT is the air temperature measured in °C or °F and determined by an ordinary thermometer. Typical DB Line Humidity Ratio / Absolute Humidity Y-axis indicates humidity ratio or absolute humidity, which is the weight of the water, contained in the air per unit of dry air. This is often expressed as pounds of moisture per pound of dry air. Humidity ratio is found on the vertical, y-axis with lines of constant humidity ratio running horizontally across the chart. Humidity Ratio / Absolute Humidity The Y axis shows the water vapor component and is generally shown in lbs of water vapor. Sometimes the vapor content is also shown in grains of water vapor. One pound of water vapor =7000 grains of water vapor Moisture is indicated in either Lbs of water vapor or grains of water vapor, per pound of dry air Typical Absolute Humidity Line Wet Bulb Lines ►There are number of parallel slant lines which are called wet bulb lines. ►temperature of the air as recorded by a thermometer with a wet wick on its bulb. ►air having a certain wet bulb temperature will have a definite heat content although its dry bulb temperature may be anything. Wet Bulb Lines Wet Bulb Temperatures Wet Bulb Temperature (WBT) is defined as the temperature at which water, by evaporating into air, can bring the air W e to saturation at the same t B ul b temperature Li ne Inherent in this definition is an assumption that no heat is lost or gained by the air. MEASURING THE WET BULB TEMPERATURE The wet-bulb thermometer is wrapped in a cotton wick; when the wick is completely wet, swing the thermometer around, and the water evaporating at the wick pulls the wet- bulb thermometer’s temperature down in direct proportion to the water content of the air around it. The drier the air, the more water evaporates at the wick and the lower the wet-bulb temperature gets MEASURING THE WET BULB TEMPERATURE The wet-bulb thermometer tells us the relative humidity-the moisture content of the air compared with how much moisture it can hold. When the dry- and wet-bulb temperatures are equal it means that the air is holding as much moisture as it possibly can- i.e. air is at 100% relative humidity. Relative Humidity Lines ►When the air contains its maximum moisture content, we call it saturated air. ►when it contains anything less than this maximum limit then it is not saturated air. ►We, therefore, say that such air is 50% saturated or 60% saturated. ►the percentage saturation is "relative humidity" Relative Humidity The Condition of Air at Point T is plotted on the chart and its saturated moisture content is then 2 checked We find that the T 1 saturated condition moisture content is indicated by Point 2 The moisture condition at condition T is indicated by Point 1 Relative Humidity The relative Humidity of 2 air at condition T is the ratio of Moisture content T at saturation, to the 1 Moisture condition of air at the specific condition RH = Specific Moisture value ( Point1) Specific Moisture value ( Point 2 Relative Humidity Relative Humidity, is an expression of the moisture content of a given atmosphere as a percentage of the saturation humidity at the same temperature. The RH lines are shown on the chart Saturation Line ►The curved line on the extreme left-hand side of the chart is what is called the saturation line. ►condition of air represented by any point on this line is said to be saturated air. ►the air is having the maximum possible content in it. It cannot hold any further moisture. Relative Humidity The air is 100% saturated when the moisture content in the air is at its maximum possible and the saturation line is shown on the chart Dew Point ►The Dew Point is the temperature at which water vapor starts to condense out of the air. ►Move horizontally on the psychrometric chart and read the temperature where you intersect the saturation line. ►It is the moisture content which determines the dew point. Dew Point Dew Point Temperatures When air, at a certain dry bulb temperature and relative humidity, is cooled up to saturation condition, from point R to Saturation, it reaches its DEW POINT CONDITION R Condensation occurs on surfaces, which are at or below the dew-point temperature, and which are in contact with the air DEW POINT at condition R Dew Point ► If the dew-point temperature is close to the air temperature, the relative humidity is high. ► if the dew point is well below the air temperature, the relative humidity is low. ► If moisture condensates on a cold bottle from the refrigerator, the dew-point temperature of the air is above the temperature in the refrigerator. ► The Dew Point is given by the saturation line in the psychrometric chart. Enthalpy ►Wet Bulb Lines as lines of constant heat content of air. ►Enthalpy is just another term used in place of "heat content". Enthalpy ►At any temperature there is a limit to the maximum moisture holding capacity of air. ►At higher and higher atmospheric pressure, the moisture holding capacity at any given temperature becomes less and less. ►The enthalpy of moist and humid air consist of sensible heat and latent heat. ENTHALPY Enthalpy (E) is the heat energy content of moist air. It is expressed in Btu per Enthalpy scale pound of dry air and represents the heat energy due to temperature and moisture in the air. Lines of constant enthalpy run diagonally downward from left to right across the chart ( As shown).
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