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Air Distribution. Lesson Objectives. Understand: The importance of proper air distribution and air flow; Air flow measurement; Causes of low air flow; The use of manufacturers data for system checks;. Introduction. You’re in the business of heat transfer.
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Lesson Objectives • Understand: • The importance of proper air distribution and air flow; • Air flow measurement; • Causes of low air flow; • The use of manufacturers data for system checks;
Introduction • You’re in the business of heat transfer. • Refrigerants changing state makes this possible. • Refrigerants evaporate and absorb heat. • Refrigerants condense and reject heat. As an HVACR professional, you are engaged in the business of heat transfer. Whether they know it or not, your customers hire you to move heat from one location to another. In the summer, your job is to make sure that the systems you service remove unwanted heat from indoor conditioned spaces and reject it into the outdoor air. In the winter, the systems must remove heat from the outdoor air or soil and reject it into the indoor air to keep your customers warm.
Air Distribution • Low airflow or air that is too cold may not provide enough heat to boil all of the refrigerant: • Liquid refrigerant reaches the compressor. • Too much airflow or air that is too warm may increase the load on the compressor and reduce dehumidification. One of the most important system performance checks that you can carry out is air flow measurement. The relationship between air flow and the refrigeration cycle is critical. Circulating air is the source of heat needed to boil liquid refrigerant in the evaporator. If the air flow is too low, not enough heat will be available to evaporate the entire amount of liquid refrigerant fed into the coil by the metering device. Likewise, if the air entering the evaporator is too cool, not all of the liquid fed into the coil will be evaporated. All of the refrigerant entering the compressor must be in the vapor state. If the evaporator is unable to boil off the entire amount of liquid refrigerant for any reason, liquid can enter the compressor and damage mechanical components. Problems also arise on the other end of the spectrum. If the air flow across an evaporator is excessive, too much heat will be added to the refrigerant. System pressures will rise, resulting in operating conditions that can easily overload a compressor motor. As you can see, it is very important for the volume and temperature of the air entering an evaporator coil to be correct.
Airflow • 400 CFM/ton: • Typical A/C. • 450 CFM/ton: • Typical heat pump; • High sensible heat. • 350 CFM/ton: • Higher levels of outdoor air; • High latent loads (dehumidification).
Airflow Measurement • External Static Pressure: • Compare to manufacturers’ tables; • Measured with a “diaphragm-type” pressure gauge: • “Magnehelic”; or • Electronic. • Static pressure. • Velocity pressure . • Diagram on next page. A number of methods are available to service technicians for verifying air flow across the evaporator coil. One of these methods involves measuring the external static pressure (ESP) developed by the furnace or air-handler blower. Static pressure is the force of air exerted in all directions (bursting pressure) on the inside surface of ductwork. It is a measure of potential energy (pushing outward against the duct walls) and is used to determine the total resistance to air movement imposed by the ductwork system. Velocity pressure is a measure of kinetic energy, or the pressure exerted by moving air. An electronic meter or diaphragm-type differential pressure gauge calibrated in inches of water column (also called inches of water gauge, abbreviated in. w.g.) is used to measure air pressure within ductwork. The image on this page shows a popular gauge used for this purpose. External static pressure is the sum of the absolute values of static pressures in the supply and return ductwork of an operating system when measured near the furnace or air-handler unit. The measured ESP is used together with blower tables supplied by the equipment manufacturer to determine the air flow being delivered by the indoor section of the system. The diagram on the following slide shows how ESP is measured.
External Static Pressure Measurement In the example illustrated in the drawing shown, the supply static pressure (SSP) is equal to 0.4 in. w.g. above atmospheric pressure and the return static pressure (RSP) is equal to 0.1 in. w.g. below atmospheric pressure. The preferred static pressure for the return ductwork used in a residential heat-pump system is 0.08 in. w.g. Any improvements made in the aerodynamics of existing return ductwork that reduce the RSP will increase air flow through the supply ducts. Ductwork improvements that have the most dramatic effect on the performance of existing systems are typically upgrades to the return ductwork. The plus and minus signs (+ and –) are ignored and the values of the two pressures are added together to determine the external static pressure. In this example, the ESP is equal to 0.5 in. w.g. (0.4 + 0.1). For systems with cooling capacities of 5 tons or less, you should not measure an ESP greater than 0.5 in. w.g. If you do, the ductwork is imposing too much resistance to air flow, and modifications should be made to improve the aerodynamic performance of the ductwork system. An ESP reading of less than 0.5 in. w.g. is preferred to reduce the fan power required to deliver the rated air flow. Many blowers do not deliver the rated air flow (400 cfm per ton of capacity) when the ESP measures more than 0.5 in. w.g.
Airflow Measurement • Temperature-rise method. • Calculate Btus from electric heater: • Amps X volts X 3.413 Btus/watt = Btus. • For gas, clock meter to calculate input X efficiency. • Measure temperature difference between supply and return: • Use one thermometer; and • Be out of the line-of-sight of the heater. • CFM = Btus ÷ (TR X 1.08).
Air flow volume also can be determined by measuring the heat rise across the electric heater of an air handler. This method employs use of the sensible heat equation and can be performed by taking the following steps: • Operate only the electric furnace (the heat-pump compressor must not be operating) for 20 minutes to reach steady-state operation. • Using a single thermometer to avoid variations in readings, measure the entering and leaving air temperature. Take care to prevent the thermometer from being in the line-of-sight of the heater. An electronic temperature meter is preferred for accuracy. Subtract the entering air temperature from the leaving air temperature to determine the heat rise. • Measure the voltage impressed on the indoor section and the unit’s current draw. No allowance need be made if the blower motor is in the airstream. • Determine the power input (watts) to the electric furnace by multiplying the voltage times the current. • Convert the power input to heat output (Btuh) by multiplying watts times 3.413. • Divide the Btuh output by the product of the heat rise and a multiplication factor of 1.08 to calculate the air flow in cfm. • Take time to work through the following example to see how the process works. As an example, we will use an electric furnace operating with an impressed voltage equal to 230 V and drawing 40 A of current. The entering air temperature is 65°F and the leaving temperature is 95°F, which yields a temperature rise equal to 30°F. The air flow volume is determined here as follows: • Btuh = Amps X Volts X 3.413. • Btuh = 40 X 230 X 3.413. • Btuh = 31,399.6 • Btuh = 31,400. • CFM = Btuh ÷ (supply air temperature – return air temperature) X 1.08 • CFM = 31,400 ÷ (95˚F – 65˚F) X 1.08 • CFM = 31,400 ÷ (30 X 1.08) • CFM = 31,400 ÷ 32.4 • CFM = 969.1 • CFM = 970.
Duct Design • Proper duct design prevents high pressure drops that cause low airflow: • Use ACCA Residential Duct Systems Manual D; • Registers and diffusers—face velocity of no more than 700 ft/min; • Supply trunks 700 ft/min (900 max); • Branch 600 ft/min. • Return trunks 600ft/min (700 max); • Branch 400 ft/min. • Return grill face velocity 500 ft/min max; and • Filter grill 300 ft/min max.
Causes of Low Air Flow • Duct systems; • Dirty air filters; • Dirty coils; • Closed or restricted diffusers; • Blower: • Wrong speed tap; • Bearing wear; • Low voltage; and • Dirty fan blades.
Heat-pump Air Flow • Low airflow will also effect the heat pump in the heating mode: • High head pressure; • Lower heat output; and • Higher operating costs. • Inadequate air flow volumes also have a dramatic impact on heat-pump systems operating in the heating cycle. In the winter, the indoor coil functions as the condenser and adequate air flow is critical for many reasons. Low air flow volume through the indoor coil reduces the amount of heat delivered to the conditioned space. This results in the need for more auxiliary heat than would otherwise be necessary to maintain space temperature. Low air flow across the indoor coil also causes refrigerant circuit pressures to rise. Elevated pressures can increase operating costs and overload the compressor.
Air Balance • It also is important for the air flow to be balanced properly. A system may be operating with the correct total air flow volume and still not satisfy a customer’s comfort needs. For example, the correct air flow into each individual room of a home may not occur if all of the balancing dampers are completely open. Air follows the path of least resistance, just as electricity does. When all balancing dampers are open wide, some rooms may receive more air flow volume than needed, while other rooms receive less than they should be getting. This situation causes the deprived rooms to experience inadequate cooling in the summer and inadequate heating in the winter, especially during periods of extreme weather. • How important is a properly designed and installed ductwork system? All too often, refrigeration-cycle components fail due to improper air flow. It is equally common for technicians to mistakenly condemn those components that are performing poorly due to improper air flow. Before you make any attempt to evaluate the refrigeration cycle, first verify that all heat transfer coils are clean, the air filters are clean, the blower wheel is clean, the blower motor is running at the correct speed, and the air flow volume is adequate. Only then is it appropriate to take the heat measurements needed to evaluate the vapor-compression system.
