Chapter 1 Fundamentals

1. Chapter 1Fundamentals

2. Comfort • HVAC systems are used to provide comfort to its occupants. • Comfort – is the condition that occurs when people cannot sense a difference between themselves and the surrounding air. • 5 requirement of comfort: • Proper temperature • Humidity • Filtration • Circulation • Ventilation

3. Discomfort • Condition that occurs when people can sense a difference between themselves and the surrounding air. • Discomfort can occur when any of the 5 requirements for comfort are not met.

4. Temperature • The measurement of the intensity of the heat of a substance. • Physiological function is a natural physical or chemical function of an organism. The physiological systems play an important part of the regulation of the body temperature. • Normal body temperature is 98.6*F. • Evaporation is the process that occurs when a liquid changes to a vapor by absorbing heat.

5. Temperature • Humidity is the amount of moisture present in the air. • Comfort is usually attained at normal cooling and heating temperatures with a humidity level of 50%. • Humidifier is a device that adds moisture to air by causing water to evaporate into the air. • Dehumidifier is a device that removes moisture by causing the moisture to condensate. • Condensation is the formation of liquid as moisture or other vapor cooled below its dew point. • Dew point is the temperature of air below at which moisture condenses from the air.

6. Filtration • Filtration is the process of removing particulate matter from air that circulates through an air distribution system. • Airborne particulate materials can be unhealthy for building occupants. • Indoor Air Quality (IAQ) is a designation of the contaminants present in the air. • The size of particulates are measured using microns. A micron is equaled to .000039”.

7. Circulation • Circulation is the movement of air. • The air in a space must be circulated or the air will become stagnant and uncomfortable. • Temperature stratification is the variation of temperature in a building space that occurs when warm air rises and cold air falls. Temperature stratification causes discomfort to occupants.

8. Circulation • Air velocity is the speed at which air moves from one point to another. • Air velocity is measured in (fps) feet per second. • Air circulation is a vital element of achieving and maintaining human comfort. • Supply air duct and registers are used to distribute air from the mechanical unit to the occupied space. • Return air ducts and registers are used to distribute air from the controlled space back to the mechanical equipment to be conditioned and re-circulated.

9. Ventilation • Ventilation is the process of introducing fresh air into a building. Is a necessary part for comfort in a building or space. • The fresh air introduced along with the return air from a space dilute the contaminations being returned from the space. • Make-up air is that is used to replace air that is lost to exhaust.

10. Thermodynamics • Thermodynamics is the science of thermal energy (heat) and how it transforms to and from other forms of energy. • First law of thermodynamics – states that energy cannot be created or destroyed but may be changed from one form to another. Exam.-Combustion process - As fuel burns, the elements hydrogen and carbon combine with oxygen in the air and a chemical reaction occurs and converts the chemical energy to thermal energy.

11. Thermodynamics • Second Law of Thermodynamics – Heat flows from a material of high temperature to a material at a lower temperature. • This law applies to all cases of heat transfer. • Example – Air in a furnace is heated by the products of combustion, the heat flows from the warm burner to the cooler air.

12. Heat Measurement • Heat is the measurement of energy contained in a substance and is identified by a temperature or a change of state. • Change of state is the process that occurs when enough heat is added to change it from one physical state to another. • Sensible heat is heat measured with a thermometer or sensed by a person, it doesn’t involve a change of state. • Latent heat is identified by a change of state and no temperature change. The heat added to ice that changes to water. Heat transferred in heating system is usually sensible heat.

13. Heat Measurement • In the U.S. the amount of heat is measured using btu British thermal unit. A British thermal unit is the amount of heat energy needed to raise 1lb. of water 1*F. • Heating equipment is rated using btu. • Ton of cooling is the amount of heat required to melt a ton of ice over a 24 hour period. • 1 ton of cooling = 288,000 btu per 24 hour period.

14. Heat Transfer • Heat transfer is the movement of heat from one material to another • According to the 2nd law of thermodynamics heat transfer will always occur moving from the object with a higher temperature to that of the lower temperature. • Three methods of heat transfer: • Conduction • Convection • Radiation

15. Conduction • Heat transfer that occurs when molecules in a material are heated and the heat is passed from molecule to molecule through the material. • Metal rod is a good example, when you heat one end of the rod the heat transfers throughout by way of conduction.

16. Convection • Heat transfer that occurs when currents circulate between warm and cool regions of a fluid. • For example, as air is warmed by a fire, the warm air rises and then replaced by cool air.

17. Radiation • Heat transfer in the form of radiant energy (electromagnetic waves). • Radiation occurs from object to object directly without the objects touching. • The amount of heat transferred depends on the intensity of the heat and the distance between the heat source and object.

18. Heat Measurement • Weight is the force with which a body is pulled downward by gravity. • Specific heat is the ability of a material to hold heat.

19. Psychrometrics • Is the scientific study of the properties of air and the relationships between them. • Atmospheric air is a mixture of dry air, moisture, and particles. • Dry air atmospheric air – moisture & particles. • Moist air is the mixture of dry air and moisture.

20. Temperature • The most important variable that is measured and controlled in a commercial HVAC system. • The two substances most measured are air and water. • Commonly measured and expressed with either Fahrenheit or Celsius. • Fahrenheit conversion scale: C* = *F-32 1.8 Celsius to Fahrenheit: *F= (1.8 X C*) + 32

21. Humidity Measurement • Humidity is the measurement of moisture in the air. • Humidity represents latent heat and therefore cannot be measured by a thermometer. • Humidity is measured using a hygrometer. • Relative humidity is the amount of moisture in the air compared to the amount that it could hold if it were saturated. • Relative humidity always depends on the temperature of the air.

22. Specific Humidity • Specific humidity is a measurement of the exact amount of weight of moisture in the air. • Specific humidity isn’t dependent on temperature. • A grain is a unit of measurement equal to 1/7000 lb.. • Usually measured not with a meter but using the psychometric chart, which is a graph that defines the properties of the air at various conditions.

23. Wet & Dry Bulb Temperature • Wet Bulb is the temperature of the air taking into account the amount of humidity in the air. • Wet bulb is measured using a sling psychomotor, it has two thermometer with one having a wick (sock). • Dry bulb temperature is the temperature with no reference to moisture in the air.

24. Dew Point Temperature • Is the dry bulb temperature of the air at which the moisture in the air condenses and falls out as dew, rain, ice, or snow. • Is a function of the dry bulb temperature and relative humidity. • Is found by referencing the temperature and relative humidity of the air on a psychometric chart.

25. Enthalpy (h) • Is the total heat contained in a material. • Is the sum of sensible heat and latent heat. • Is expressed in Btu/lb. • Is a better indicator of the outside air than dry bulb temperature. • Outside air enthalpy is often measured and used to determine the suitability of outside air for use in place of mechanical cooling.

26. Pressure • Force created by a substance per unit of area. • May be: • Atmospheric pressure • Gauge Pressure • Absolute pressure • Inches of water column.

27. Atmospheric Pressure • The pressure due to weight of the earth’s atmosphere pressing against an object on the earth’s surface. • At sea level 14.7 lb / sq.in. of air is pushing against every square inch of object on the earth’s surface. • Is relatively constant and varies only a small amount depending on elevation and humidity.

28. Gauge Pressure • Pressure above atmospheric pressure. • Assumes atmospheric pressure is zero. • Expressed in pounds per square inch (psig). • Most common pressure measurement used in HVAC systems.

29. Absolute Pressure • Pressure above a perfect vacuum. • Is the sum of gauge and atmospheric pressures. • Expressed in pounds per square inch absolute (psia). • Inches of water column (wg) is used when indicating very small pressures due to air in ducts or gas pressure in a pipe.