HRAC-223 Lecture 3 Air Distribution Systems PDF

HRAC-223 Air Conditioning & Distribution Systems School of Engineering Technology & Applied Science (SETAS) Week 3 Air Distribution Systems  Learning Objectives Characteristics of Air Air Distribution Air properti...

HRAC-223 Air Conditioning & Distribution Systems School of Engineering Technology & Applied Science (SETAS) Week 3 Air Distribution Systems  Learning Objectives Characteristics of Air Air Distribution Air properties Air measurements Occupied Zone Elements of design Primary Air Secondary Air Stratification Comfort in the Occupied Zone Centennial College - A.M.A.T.  Air Distribution The purpose of most air-conditioning systems is to provide comfort and good indoor air quality. A good mixing air-distribution system should provide a steady and almost uniform temperature and air motion in the occupied zone with temperatures and speeds within the acceptable ranges. Poor distribution, insufficient ventilation, stuffiness and uncomfortable air drafts are usually induced by improper airflow patterns resulting from improper design. Material obtained from International Journal of Thermal Sciences Centennial College - A.M.A.T.  Resistance Pressure Air moving through ductwork has to fight against Resistance within the duct. Friction Created by air moving against the walls of the ductwork. Air does not move in a straight line, it has a turbulent flow, churning and mixing. Metal, Fiberglass, and flexible ductwork all have different friction effects. Fittings (90’s, Tees, Reducing Couplings) require extra pressure or velocity to overcome the friction loss. Centennial College - A.M.A.T.  System Pressures Duct system is pressurized by three pressures Static pressure: air pressure against the walls of the ductwork Velocity pressure: pressure generated by the velocity and weight of the air Total pressure Static pressure plus velocity pressure equals total pressure Pt = Pv + Ps  System Pressures (cont’d.) Static Pressure Is the pressure exerted against the walls of the ductwork in all directions like air inside a balloon. The bursting pressure. Can be (+) or (-) Figure 37–10 A manometer connected to measure the static pressure Centennial College - A.M.A.T.  System Pressures (cont’d.) Velocity Pressure Is the pressure in the direction of FLOW. The pushing force. Figure 37–11 A manometer connected to measure the velocity pressure of the air moving in the duct Centennial College - A.M.A.T.  System Pressures (cont’d.) Total Pressure The Total Pressure (Pt) required to move the desired amount of air through ductwork is sum of the Velocity Pressure (Pv) and Static Pressure (Ps) at the point of measure. Pt = Pv + Ps Figure 37–12 A manometer connected to measure the total air pressure in the duct Centennial College - A.M.A.T.  Air Properties Dry Bulb Temperature The temp of the air in °F or °C Dew Point Temperature The temp at which moisture will start to condense out of a given sample of air Relative Humidity Percentage of water vapor in the air I relation to the max it can hold at any given temp Specific Humidity The moisture content of a given sample of air expressed in grains Centennial College - A.M.A.T.  Air Properties Enthalpy Measurement of heat content of a given sample of air expressed in BTU/Lb Wet Bulb Temperature The temp of the air taking into consideration the amount of moisture it contains Specific Volume The amount of space in cubic feet occupied by 1 lb of air Centennial College - A.M.A.T.  Air Measuring Devices Manometers Used to measure duct pressure such as: Static Pressure, Velocity Pressure and Total Pressure Also used to measure the static pressure drop across filter banks which will indicate the need for replacement. U-Tube Inclined Pitot Tube Material obtained from Dwyer Centennial College - A.M.A.T.  Air Measuring Devices Magnehelic Used for direct measurement of pressure across filter banks A magnehelic pressure gauge is a device, similar to a manometer, used to measure positive, negative and differential pressures. Material obtained from Dwyer Centennial College - A.M.A.T.  Air Measuring Devices Anemometer or Velocity Meter Used for Direct measurement of velocity in FPM (Feet per Minute) Provides extremely fast response to air flow conditions in a duct or at a register. Material obtained from Testo Centennial College - A.M.A.T.  Air Measurements Rectangular Duct CFM = Area (SQ. FT) x Velocity (FPM) AREA = L x W ÷ 144 VELOCITY = 4005 x Square Root of Velocity Pressure (FPM) Centennial College - A.M.A.T.  Air Measurements Round Duct CFM = Area (SQ. FT) x Velocity (FPM) AREA = Pie x R² ÷ 144 VELOCITY = 4005 x Square Root of Velocity Pressure (FPM) Centennial College - A.M.A.T.  Air Measurement Calculations 1. A single duct / single zone A/C roof top unit is supplying air to the conditioned space by way of a 24” by 12” supply duct. Calculate the air velocity as well as the volume (CFM) if a pitot tube manometer reads 0.06 inches water column. AREA = L x W ÷ 144 AREA = 24 x 12 ÷ 144 AREA = 2 SQ Feet VELOCITY = 4005 x Sq. Root of Velocity Press. VELOCITY = 4005 x √0.06 VELOCITY = 981 FPM CFM = Area (SQ. FT) x Velocity (FPM) CFM = 2 SQ Feet x 981 CFM = 1962 CFM Centennial College - A.M.A.T.  Air Measurement Calculations 1. A single duct/ single zone A/C roof top unit is supplying air to the conditioned space by way of a 15” round supply duct. Calculate the air velocity as well as the volume (CFM) if a pitot tube manometer reads 0.09 inches water column. AREA = π x R² ÷ 144 AREA = 3.14 x 7.52 ÷ 144 AREA = 1.23 SQ Feet VELOCITY = 4005 x Sq. Root of Velocity Press. VELOCITY = 4005 x √0.09 VELOCITY = 1201 FPM CFM = Area (SQ. FT) x Velocity (FPM) CFM = 1.23 SQ Feet x 1201 CFM = 1473 CFM Centennial College - A.M.A.T.  Rules of Airflow The first basic rule is Newton’s gravitational observations: Hot air rises and cold air falls. The second rule is that air which is in the form of a free jet, as opposed to one that is constrained along a surface, acts pretty much independent of the supply outlet air pattern, following some basic rules of jet theory. Centennial College - A.M.A.T.  Occupied Zone Occupied Zone The region normally occupied by people within a space. Generally considered to be between the floor and (6ft) above the floor and more than (3.3ft) from outside walls/windows or fixed heating, ventilating or air conditioning equipment and (1ft) from internal walls. Material obtained from KE Fibretec Centennial College - A.M.A.T.  Elements of Design Primary Air Is defined as the Air that has yet to be conditioned. Secondary Air Is defined as the air after it has been conditioned by the equipment in place. Material obtained from Price Engineer’s HVAC Handbook Centennial College - A.M.A.T.  Elements of Design Stratification Air stratification is the tendency of two or more airstreams to remain separated. It is always desirable to keep the stagnation layer above the occupied zone in cooling and as near to the floor as possible when heating from above. Material obtained from Price Engineer’s HVAC Handbook Centennial College - A.M.A.T.  Elements of Design Location of supply and return outlets to eliminate short circuiting will increase the ventilation effectiveness. Material obtained from Price Centennial College - A.M.A.T.  Comfort in the Occupied Zone Drafts Are one of the most common complaints in the indoor environment. It may be caused by convective air currents along windows and other cold surfaces providing air movement in the occupied zone of heated spaces. Filtration / Air Quality People feel uncomfortable when they are too hot or too cold or when the air is odorous and stale. Material obtained from Price Centennial College - A.M.A.T.  Comfort in the Occupied Zone Noise Low noise levels from fans and ductwork are desired. Temperature Comfort is best defined as the absence of discomfort. Humidity Although human tolerance to humidity variations is much greater than tolerance to temperature variations, humidity control is also important. High humidity can cause condensation problems on cold surfaces and retards human heat loss by evaporative cooling (sweating and respiration). Material obtained from Price Centennial College - A.M.A.T.  Comfort in the Occupied Zone Stratification Controlling stratification in the occupied zone is critical to maintaining occupant comfort ASHRAE Standard 55 requires that the temperature difference between the head and foot level not to exceed 5.4°F [3 °C] for a standing person and 3.6 °F [2 °C] for a seated person. Material obtained from Price Centennial College - A.M.A.T.