Fluke 922 Fluke Low Pressure Differential Meters - HVAC Pressure Applications - Page 2

How pressure sensing works The Fluke 922 is a low pressure differential meter used in traditional manometer or micromanometer applications. It has a range of +/-16" wc, resolution of 0.001" wc, and over-pressure protection of 10 psi at each high and low pressure port. With a single tube connected to its high or low port, the meter will display positive or negative pressure relative to the ambient pressure at the meter's open port. For a pressure differential at two remote points, use tubing to connect the higher pressure sensing point to the "+" port and the lower pressure sensing point to the "-" port. The meter uses a variety of sensing probes and fittings to access the type and location of pressure measurement points. These can be as simple as a straight metal tube for checking draft, a "Tee" for tapping into pressure sensing tubing connected to a pressure switch, a Pitot tube for sensing total pressure, a Pitot-Static tube for sensing both total and/or static pressure, or a static pressure tip. Pressure applications Draft pressures: Draft pressures are negative relative to ambient pressures at the burner. Measuring and controlling draft is a critical step in combustion tests and influences net thermal efficiency of the appliance. Draft is created by vertical vents or powered sidewall vent systems connected to fossil fuel equipment. Like too much air, excessive draft through a heat exchanger increases the velocity of the combustion products containing heat, thereby reducing the contact time in the heat exchanger. Follow guidelines from the manufacturer or standards authority. Over-fire draft: Power burners typically have specifications related to over-fire draft, which relates to the velocity of combustion products through the heat exchanger. This over-fire draft typically is slightly negative (-0.01" wc to -0.02" wc) but, depending on burner type and application, may be slightly positive (0.01" wc to 0.02" wc). Follow guidelines from the manufacturer or standards authority. Draft inducer pressure: Category I fan-assisted gas appliances utilize a combustion air inducer to create draft through the appliance and deliver the combustion products to a negative pressure vent. A pressure switch is normally connected to the inlet side of the inducer to ensure a minimum draft has been established before allowing an ignition sequence to begin or continue. For testing, a "Tee" taps into the pressure sensing tubing, to monitor the draft created by the inducer. This is an important value to benchmark at appliance installation. While the pressure switch ratings list cut-in and cut-out (or differential) pressures, without a benchmark you can't evaluate changes within the appliance or vent over time. Follow guidelines from the manufacturer. Pressure effects and measurements Pressure is an integral part of HVAC equipment design, system design, function, application, control and diagnostics. Pressure determines • cleanliness of airside devices (pressure drop across filters, evaporators, etc.) • boiler steam temperature • the saturation temperature of a fluid (boiling temperature of a liquid, the condensing temperature of a vapor) • the direction of fluid flow (fluids flow from higher to lower pressures) • fluid volume flow rate (pressure drop across an orifice) • whether or not a control or safety switch will make or break (high/low pressure switches, draft proving switches) • the availability of a fluid to perform work (oxygen/nitrogen tank pressures, gas supply pressures, duct static pressures) Pressure also • senses zone device positions, modu- lates VFD drives, bypass dampers and valves • affects solubility of air in water • contributes to flue gas velocity (stack draft) Airflow Total Pressure Airflow Static Pressure Pitot-Static Tube Airflow Total Pressure Airflow Airflow Static Pressure Pitot Tube Static Tip Airflow Options for sensing probes and fittings when accessing pressure measurement points.  Fluke Corporation HVAC pressure applications with the Fluke 922