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Several Factors Affecting the Demister Pad Selection and Installation

Demister pad is a device for gas-liquid separation, where gas passes through the wire pad of the demister pad to remove the entrained mist. Demister pads can be used in many industries, so how to choose an appropriate demister pad? We summarize the following factors that affect the selection of demister pad as listed below.

Material selection

The demister pad material selection mainly depends on the chemical and temperature factors in the service environment. The material selected must be able to withstand the chemical corrosion of the environment, as well as the test of temperature. Common materials are as:

Category	Material
Steel & Alloy	Carbon steel, SUS 304, SUS 321, SUS 316, SUS 316L, SUS 310S, SUS 304M, Nickel, Titanium, Brass, Copper, Cr20Ni80, Cr20Ni50, Inconel, Monel
Nonmetal	PP, PTFE
Steel & Fiber	SUS & Fiberglass, SUS & PP, SUS & PVC, SUS & F46

Mesh type selection

Demister pad consists of bottom grid, knitted mesh, upper grid, connection rods and struts. Knitted mesh is a kind of gas-liquid filter screen made of various materials. Generally, we have 4 types of gas-liquid filter screen for you to choose from: SP (standard type), DP (efficient type), HR (high penetration type) and HR (shock absorber type).

SP type knitted mesh

DP type knitted mesh

HP/HR type knitted mesh

Their specifications are as shown below:
Model	Density (kg/m³)	Free Volume (%)	Surface Area (m²/m³)	Performance
BD-1^*	80	99.0	158	It is used for the case that velocity is big, small pressure drop is required, or collection of big particles or high efficiency is not required. High-treatment style.
BD-2	120	98.5	210	-
BD-3	144	98.2	280	It is extensively used for petrochemical plants. Pressure drop is quite small and efficiency is 90% – 99%. Standard style
BD-4	128	98.4	460	-
BD-5	193	97.5	375	It is used a lot as a standard. It allows 1–5 m/s 98% – 99% efficiency with 5–10 μm particles when it is used as general use. High-efficiency style
BD-6	300	96.2	575	-
BD-7	390	95.0	750	Compared with BD-5, its density is 2 times greater. It is used for the case high efficiency is required. It is necessary especially in case pressure drop is high, but over 90% efficiency at 1 m/sec is required.
BD-8	220	97.2	905	It is used for collecting 1 μm particles. Especially, in the case of low concentration that the quantity of mist (dust) is less than 10 g/m³, it is advantageous as much as the thickness goes up 200–300 m/m.
BD-9	432	94.5	1780	It is used for collecting minute particles of 0.05–1 μm. It can make better effects than BD8. Ultra-high efficiency style.
BD-10	220	97.2	428	-
BD-11	160	96.7	5000	-
^* Density 80 is suitable for both metal materials and plastic materials. Other models are only suitable for metal materials.

Size selection

The size selection of the demister pad includes the selection of diameter and thickness.

The picture shows the front view of the demister pad.

Demister pad front view

The picture shows the top view of the demister pad.

Demister pad top view

Diameter
To determine the nominal diameter of the selected demister pad, you need to determine the operating gas velocity first. The operating gas velocity shall be selected appropriately. If the operating gas velocity is too low, the inertia of the mist in the gas is too small, and it is in a floating state. When the gas passing through the wire mesh demister pads, the mist floating on the air cannot be removed. If the operating gas velocity is too high, and the collected droplets are not easy to fall off from the wire mesh. They will fill the wire mesh, and the trapped droplets are splashed again, and then trapped by the gas, causing a flooding phenomenon, thereby reducing the mist eliminating efficiency. Generally, the operating gas velocity is 60% – 80% of the maximum allowable gas velocity.

Sometimes, the calculated demister diameter is less than that of the vessel, you may use the internal diameter of the vessel as the diameter of the demister to ensure the using performance of the demister.
Thickness
The selection range of the demister pad thickness falls between 100 mm and 200 mm. If the gas to be removed contains a large quantity of droplets, then a thick demister pad is required. Instead, if the gas to be removed contains few droplets, then a thin demister pad is required. Generally, the demister pad with a nominal diameter of 2000 mm below shall have a thickness of 100 mm; the demister pad with a nominal diameter of 2000 mm to 5000 mm shall have a thickness of 200 mm; the demister pad with a nominal diameter of 5000 mm below shall be customized. In addition, what the customer concerns about is the selection of thick demister pad will increase its resistance. In practice, it is proven that it will be no problem when the operating gas velocity falls between 3 m/s and 5 m/s.
Installation way selection
Installation way varies as application scenario varies. The table below shows the simplified diagram of demister pad installation that you can choose from.

Horizontal demister pad fixed from below with hook bolts & clamps

Horizontal demister pad fixed from below with tie wires

Horizontal demister pad fixed from above with hold-down bars/angles

Horizontal demister fixed from below in box housing with treaded rods

Horizontal demister fixed from above with rotating J-bolts

Conical demister 10° upwards fixed from above with spider beams & clamps