Pulse jet valves are used in filtering systems of dust collectors, gas turbines, and desulfurization equipment. They are commonly used to clean filters such as cartridge filters, envelope filters, ceramic filters, and sintered metal fiber filters. Here, we discuss how these valves work in a typical application for dust collector systems.
Dust Collectors Overview
Dust collectors are systems used to either remove granular solid pollutants or recover valuable solid or powder from exhaust gases prior to venting to the atmosphere. A dust collector usually consists of a blower, dust filter, a filter cleaning system and a dust removal system. One of the most efficient and cost-effective dust collectors available is a fabric collector commonly known as a baghouse.
In a baghouse, particle-loaded gas is passed through fabric bags depositing dust on the outside surface of the bag (Figure 1). To keep the pressure resistance of the baghouse within a pre-set range, and thus ensure the dust collecting efficiency of the baghouse, bags are cleaned constantly. The most commonly used filter cleaning method is called pulse jet or pressure jet.
In this method the dust is removed by a high-pressure blast of air that enters the top of the tubular bag filter (1), see figures 1 and 2. A pulse jet valve (2) feeds pulsing air through a blow pipe (3) which consequently feeds nozzles located above each bag (1). This will create a shock wave that passes through the bag and causes the bag to expand which shatters the dust cake accumulated around the bag. Although this pulse of compressed air temporarily stops the flow of air through the filter tube bag, due to its rapid release, the flow of particle-loaded gas into the baghouse is not interfered. Therefore, pulse jet cleaning baghouses can operate continuously.
The shock wave travels down and up the tubular filter bag in around half a second. Therefore, the pulsing air must be strong enough to travel the total length of the bag and fracture the dust cake. The necessary fast opening and closing of the dust collector valve (2) is reached by using a Pulse Jet Solenoid valve (2). Using a pulse jet solenoid valve enables controlling of the pressure drop across the filter as well as the pulse sequence.
Operating Principle
Pulse jet valves are indirect operated solenoid valves especially designed for dust collector systems. When the solenoid is energized, trapped air above a diaphragm is quickly exhausted causing a high pressure difference across the diaphragm. Thereby the diaphragm is suddenly opened. When the solenoid is de-energized, air escapes through a hole to the chamber above the diaphragm which balances the pressure and instantly closes the valve.
The fast opening and closing of the valve is of great importance for effective cleaning of filters and economical consumption of compressed air. This is made possible by keeping the weight of the moving parts small and therefore having a small inertia. These valves have very high kV values. The maximum flow rate is reached when the air velocity becomes sonic (344 m/s).
Selection Criteria
Main parameters that affect the selection of pulse jet valves are as follows:
Tank volume: The amount of air volume stored in the supply tank which in turn depends on valve size.
Tank pressure: The air pressure in the tank which is also the inlet pressure of valve.
Max allowable pressure: Maximum line or system pressure for safe operation.
Electrical pulse length: The time during which the valve is energized.
Total pulse length: The time between when the valve opens and when it closes.
Peak pressure: The maximum pressure reached at the end of the blow pipe which creates the shock wave.
Pressure drop tank: The difference between the tank pressure before and after the shock wave. To maintain sonic flow in the blow pipes, it is necessary to limit pressure drop to a maximum of 50% of the absolute tank pressure. In an installation, it is the easiest way to reduce the electrical pulse time if the pressure drop is too high.
Performance ratio: The ratio between tank pressure and peak pressure in percentage. The amount of this parameter depends on the kV of the valve and the opening time.
Volume per pulse: The volume of air at atmospheric pressure passing through the valve per pulse. The greater this parameter, the better filter bags are cleaned, and more bags can be cleaned per valve.
These types of solenoid valves come with different choice of material of construction namely of body and of seal material. All materials of construction have specific properties that make them suitable for different applications. It is essential to choose the appropriate body and seal material for your media.