What are the steps for starting and stopping blowers and exhaust fans?

1. The sequence for starting and stopping the blower and the exhaust fan is: start the exhaust fan first, then the blower； Stop the blower first, then the induced draft fan. 2. When starting the exhaust fan: fully open the outlet damper of the exhaust fan and the air valve at the inlet of the blower, close the air valve at the inlet of the exhaust fan, and start the exhaust fan ； When starting the blower: After the exhaust fan starts properly, close the blower’s inlet damper and start the blower.

I think there are mainly two points: 1. It is not allowed for positive pressure to build up in the furnace; that is, during startup, exhaust air should be introduced first, followed by forced air； When stopping, blow air in first and then draw air out. 2. Start and stop without load; the corresponding inlet and outlet valves should be closed before starting/stopping.

Start the induced draft fan first, then the blower; if the blower is started first, a positive pressure will be created in the furnace, leading to backfire. When shutting down, stop the blower first and then the induced draft fan, again to prevent a positive pressure from causing backfire in the furnace

1. The task of the heating furnace preheater is to recover as much residual heat from the flue gases as possible, while ensuring normal combustion in the heating furnace, thereby reducing the amount of gas or fuel oil required. Achieve energy savings and reduce production costs. 2. Operating requirements: (1) Maintain a uniform temperature in the furnace, and the furnace should be kept at a slight positive pressure. (2) Ensure good atomization of the burner, that the fuel gas is oil-free and burns completely, with a bright furnace interior and no black smoke. (3) Based on the combustion conditions, and taking into account the temperature of the cold air as well as that after previous heat exchange, adjust the opening degree of the blower outlet (damper) and the inlet damper in a timely manner to improve the heat exchange efficiency of the preheater. (4) Ensure that the temperature of the hot air is between 125 and 135°C, and the temperature of the cold flue gas is around 110°C. 2. Preheater startup steps: (1) First, start the exhaust fan. Pay attention to the combustion condition of the furnace, then slowly close the flue dampers and adjust the flue gas dampers in front of the exhaust fan. (2) Adjust the damper at the burner, and close one quick-open damper. (3) Turn on the blower, and pay attention to the combustion status in the heating furnace to prevent the burner from being extinguished; adjust the size of the air valve according to the combustion conditions. (4) Adjust the appropriate amount of cold air, as well as the amount of flue gas drawn in by the exhaust fan, based on the combustion conditions and the temperatures of the hot air and cold flue gas, to maintain a slight positive pressure in the furnace and ensure that the preheating amount is at an optimal level for heat exchange.

Reply to 2# dongjiang1001: It is correct to create a negative pressure before starting the furnace to prevent backfire. It isn’t necessary to turn on the exhaust fan first; a flue damper can also be used to create negative pressure. If the exhaust fan is turned on first, there is a problem: the flue gas will enter the air preheater right away, and at that time the temperature in the furnace chamber is not high, while the temperature of the flue gas in the air preheater must be at least 130 degrees (the required temperature may vary depending on the equipment). Corrosion due to dew point conditions can occur if the exhaust fan is turned on first. When shutting down the furnace, if the blower is stopped first, on one hand the furnace cools down more slowly; on the other hand, if the exhaust fan continues to operate, it is necessary to ensure that the flue gas temperature remains above 130 degrees to prevent dew point corrosion, and the exhaust fan must be stopped if the temperature drops too low. According to our operating procedures, when starting the furnace, we first turn on the blower and then the exhaust fan; when shutting down the furnace, we first turn off the exhaust fan and then the blower. Start the blower by first closing the inlet damper to reduce the starting load. If the inlet is opened while the outlet valve is closed, pressure buildup will occur, and surge may also happen, as air is compressible; whereas the outlet of a centrifugal pump is closed because liquids are not compressible. Turning on the induced draft fan also means closing the inlet valve. Please note that our fan does not have a return line; it is not possible to open the inlet, close the outlet, or activate the return flow. If it has variable frequency control, I think it’s not a big problem to turn on the inlet or outlet first.

Exhaust fan: Open the outlet damper fully, close the inlet valve of the exhaust fan, and start it； Blower: Close the blower inlet damper and start it.

First, start the blower to ensure that the flue gas temperature is above the dew point corrosion temperature (usually 200°C), and then start the exhaust fan. The opposite is true when it is disabled.

With a waste heat recovery system, the blower must be turned on first, as the heat pipes of the air preheater are sealed. Turning on the induced draft fan first will cause the hot flue gas to vaporize the heat transfer medium, leading to tube rupture. It’s the same principle as a heat exchange reactor. It's just my personal opinion; criticism is welcome.

Is there an authoritative answer? There are all kinds of opinions; give a unified answer

The air preheater should first be connected to the bypass line; once the temperature has risen, then the preheater can be put into use. This does not conflict with creating a negative pressure