14 Turning VanesWhen airflow changes direction in a duct that lacks turning vanes, the walls of the duct must absorb the sudden impact of the air in order to reorient the airflow to the direction desired. Turning vanes assist the airflow in making a smoother and more gradual change in direction, resulting in less of an impact, and thus less force transferred (as airflow velocity increases, this effect becomes more pronounced). While the turning vane surfaces do add a small amount of friction, the amount of energy lost to friction from the vanes is nothing compared to the energy lost in the impact resulting from the airflow taking an abrupt or significant change in direction. Figures 1.1 and 1.2 below illustrate the airflow resistance that occurs in a 90° square elbow with and without turning vanes.
15 Turning VanesAn elbow with the turning vanes is 800% more efficient than the same elbow without the vanes. If the owner desires a less expensive installation, the designer may specify radius elbows without turning vanes. A radius elbow without turning vanes is still highly efficient, and is much easier and cheaper to fabricate and install. The diagram illustrates airflow in a radius elbow.
16 Rectangular 90 Degree Ells 90 degree turns in forced air ducting are to be avoidedwhenever possible. However, if structurally there is nochoice, there are ways to minimize impact.The square ell is very poor for airflow.The square ell can be improved substantially with theaddition of interior turning vanes. These can befabricated at any sheet metal shop.If your duct system requires 90 degree ells, insist thatboth inside and outside throats be radiused.
17 Rectangular 90 Degree Tees A tee is usually utilized in a duct system when the main trunks must be located in the hallway, andthe AH/furnace must be located remotely in a mechanical room or closet.A plenum ell is usually used in conjunction with a tee. Proper sizing of the trunkline between thetwo will make or break the performance of your comfort system.
18 Ductwork Transitions Square to Rounds Ductwork square to rounds, as the name suggests, convert square or rectangular ductwork to round wherever such a transition is required. This may be to connect rectangular ductwork to ventilation equipment designed for a circular connection, or to change the duct shape to pass through a confined space. In confined areas such as ceiling voids (the space between the suspended and structural ceiling of a building) it's common to see ductwork runs comprised of both rectangular and round spiral duct - the main duct and branches are rectangular as the equivalent sized round duct may not fit into the ceiling void. Square to rounds provide the means to make this transition.
20 Ductwork ReducersA duct reducer allows for a smooth transition from one Duct size to another.
21 Thermostats A furnace is controlled by a thermostat. A thermostat is an automatic sensing device that sends a signal to activate a furnace at a temperature set by the owner.The thermostat is usually located on an inside wall because the outside temperature may affect its accuracy if located on an exterior wall.
26 Heat Loss Heat loss is the amount of heat that passes through the exposed surfaces of aStructure.To maintain a constant indoortemperature the total heat loss through walls, ceilings, and floors must be balanced by heat delivered to each room.
27 Heat Loss Heat escapes in two ways: Transmission through walls, ceilings, and floors.Infiltration through cracks around openings.
28 Heat Loss Factors that affect heat loss: Infiltration Infiltration is the amount of heat lost through spaces aroundwindows and doors.R-FactorResistivity also called the R-factor or R-value is the ability of amaterial to resist the transfer of heat or cold.U-FactorThe U-factor is the number of Btus transmitted in one hour throughone square foot of a building material for each degree oftemperature difference.The U-factor can be determined by taking the reciprocal of theR-factor of the material (1/Rtotal)
29 Heat Loss Factors that affect heat loss: BTU (British Thermal Unit) Quantity of heat needed to raise one pound of water one degreeFahrenheit.BTUH (British Thermal Unit per Hour)The rate of heat flow for the amount of BTU’s transmitted througheach square foot of surface area in one hour for each degree oftemperature.
30 Heat Loss Calculating heat loss (BTUH): BTUH = (total area (ft2) x (U-factor) x (the design temperature difference)BTUH = A x U x (t inside – t outside)The design temperature difference is the difference between the insidedesign temperature and the outside design temperature.The inside design temperature is between 65º and 75º with 70º used inmost calculations.The outside design temperature can be found in the ASHRAE manual.The outside design temperature for the Pittsburgh area is 5º (air port).
31 Heat Loss Calculating heat loss (BTUH): Infiltration is calculate by multiplying the room volume x the U-factor(0.018) x the design temperature difference.The U-factor (0.018) is a constant equal to the specific heat of air(0.24 BTU/lb) x the density of air (0.075 lb/ft3).