Positive pressure or negative pressure?
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The pressure at the blower outlet is around 800 Pa, while the negative pressure in the furnace chamber is around -60 Pa. The question is: there is a pressure gauge located behind the main air damper of the heating furnace; should this gauge indicate positive pressure or negative pressure? Why?
I think it’s positive pressure; the air requires about 400 Pa to pass through the burner, so -60 + 400 = 340 Pa
Where is the damper for the main air valve of the heating furnace you are referring to?
Blower ----> Main air damper ----> Air dampers for each burner ----> Burner chamber; the pressure gauge is located about 10 cm behind the main air damper
Could you go take a look at that pressure gauge?
The actual pressure at the site is negative, but the gauge designed for it has a range suitable for positive pressure; so the question is why? Is it a design flaw or an issue with the actual operation?
Blower ----> Main air damper ----> Air dampers for each burner ----> Burner chamber; the pressure gauge is located about 10 cm behind the main air damper. The positive pressure at the blower’s outlet is dynamic pressure; no matter how much air is required inside the furnace, there will always be positive pressure. However, when the amount of air delivered by the blower is less than what is needed for combustion in the furnace, the air coming out of the blower will exhibit negative pressure beyond the distance covered by this dynamic pressure. If the blower delivers enough air yet the gauge still shows negative pressure, it indicates that there is high resistance in the air dampers, and the blower’s actual energy is being used for compression, resulting in a reduced actual amount of air delivered, which in turn causes the gauge to show negative pressure
The pressure behind the damper of the main air valve in the heating furnace = furnace pressure + pressure drop across the burner + pressure drop in this section of the air duct. The pressure drop across the burner and in this section of the air duct are related to the flow rate of the combustion air, which in turn is related to the heat load of the heating furnace. In particular, for domestically produced natural ventilation burners, the pressure drop of the burner is generally low, especially at low loads. It is estimated that when the design was finalized, only the burner pressure drop under maximum operating conditions was taken into account; as a result, a gauge showing positive pressure was selected behind the main air damper of the heating furnace. This leads to negative pressure at this location when the heating furnace operates at low load. Furthermore, the load performance curve of the burner may also be a theoretical value, and it doesn’t receive much attention. Generally, people prefer that the pressure drop on the air side of the burner be as low as possible, so that the pressure required by the blower can be reduced.
1 The pressure gauge is too close to the damper; as air passes through the damper, vortices are generated, so the measuring gauge should be at least a few equivalent diameters away from the damper. And the structural design of the baffle also has an impact. All of these will affect the measurement. 2 As for whether it is actually positive pressure or negative pressure? It mainly depends on whether the loss in pressure occurs at the burner or on the damper plate