TEX-MESH Technical Bulletin 101

Performance of TEX-MESH Mist Eliminators

AMISTCO manufactures a line of mist eliminators which spans a wide range of systems. Each model is designed to optimize a set of performance conditions desired by the customer. The mechanical performance of a mist eliminator is described by two curves: Efficiency versus Particle Size and Pressure versus Vapor Load. In addition, corrosion resistance determines the materials of construction. In order to operate at high efficiency, a mist pad must have materials with a very high surface area/volume ratio. Consequently, even materials with a relative slow corrosion rate can have an unacceptably short service life. The key elements of performance of a mist pad are therefore: efficiency, re-entrainment, pressure drop, and corrosion resistance. The curves showing mechanical performance for several models of TEX MESH mist eliminators and materials of construction are provided.

Design Factors
The predominant system property which affects performance is vapor density. For a given vapor density, the correct size for optimum performance depends primarily on the vapor velocity (see Technical Bulletin 102). The liquid density influences the mist eliminator size, but the effect is smaller in magnitude than the effects of vapor density and vapor velocity. The liquid volume rate of flow is important, because it determines the vapor load at which re-entrainment occurs. Finally, pressure drop is important to the mechanical performance of the process in which the mist eliminator operates. In general, however, the pressure drop for a properly designed mist eliminator is usually quite small (less than 3 inches of water).

Mist Collection Efficiency
Typically, the critical performance characteristic is efficiency. In general, a customer wants the highest efficiency possible. However, efficiency is maximized at the expense of pressure drop. As the efficiency curve is shifted to the left (higher efficiency for a given particle size) the pressure drop at design flow increases. Furthermore, service life may decrease if smaller diameter filaments are used in the pad. Selection of the best mist pad therefore depends on optimizing efficiency, pressure drop, reentrainment, corrosion resistance, and cost. For a system that has a small percentage of entrained liquid, and the droplet size distribution is not in the submicron range, TEX-MESH mist eliminators can achieve very high efficiencies at reasonable cost.

The efficiency curves express collection efficiency of a 6-inch thick pad as a function of droplet diameter. To estimate overall collection efficiency, the droplet-size-distribution must be factored into the estimate. Model TM-TFEC (Figure 1A), for example, achieves a removal efficiency of 99.9% at a droplet diameter of 0.5 microns (micrometers). Therefore, if the entrained liquid in a gas stream is composed entirely of droplets greater than 0.5 microns, the overall entrainment removal efficiency of a TM-TFEC mist eliminator, properly sized, will be greater than 99.9%. In most design problems concerning entrained droplets, the actual droplet-size-distribution is based on an "educated guess." Furthermore, the volume flow of entrained liquid is also an estimate. Consequently, mist pad selection depends a lot on experience.

Estimating Droplet Size
A useful heuristic (rule of thumb) in mist pad selection is for droplet size prediction. First, entrainment arising from mechanical processes (boiling, two-phase processes, seal leakage, surface condensation, etc.) typically produces droplets larger than 20 microns.

Second, entrainment arising from chemical processes (reactions, endogenous condensation, etc.) typically produces droplets in the submicron range. Chemically produced droplets should therefore have the opportunity to undergo coalescence before collection using a mist pad.

Collisions between droplets cause coalescence and, hence, a process of droplet size enlargement. Although a mesh pad may have lower than desired efficiency for collection of particles as created, transport of the vapor will cause the droplet-size-distribution to shift toward larger diameters and therefore higher mist pad efficiencies.

Analogy Performance Predictions
A further word about the performance curves. These curves are based on data for an up flow air/water system. Because of differences in the physical properties (densities, viscosities, and surface tension), the actual performance of a given system may be somewhat different from the performance curves. In general, however, many years of experience using curves based on air/water tests has produced considerable confidence in the "analogy" method of performance prediction. That is, most systems (factored for physical properties) are analogous in performance to an air/water system.

Vertical vs. Horizontal
A final word about the performance curves presented here is that they are for up flow conditions at the optimum velocity. Mist pads for horizontal flow may be designed for higher velocity (K factor up to 0.5 ft./sec instead of 0.35 ft./sec for up flow). The resulting collection efficiency will be somewhat higher. Furthermore, for small diameter vessels, crossflow allows better drainage. Consequently, the entrained liquid flux limit (gpm/sqft) may be somewhat higher than for up flow. However, other considerations besides small variations in performance usually dictate the flow orientation of a mist eliminator.
   


TEX-MESH Mist Pad Styles
TEX-MESH mist pads are grouped into three basic types: all metal, all polymeric, and co-knits of metal wire with glass or polymeric filaments. The co-knit models are capable of achieving extremely high efficiencies, but have inherent temperature and fouling limitations. The alphanumeric model number indicates the basic design in terms of materials and/or structure. The following describes the most popular styles of TEX-MESH mist eliminators.


Styles with Metal Wire:

TM- 1112 Stainless steel wire 0.011 inches in diameter manufactured so as to achieve a mesh bulk density of 12 lb/cuft.

TM-1109 Similar to the above unit except having a mesh bulk density of 9 lb/cuft.

TM-1105 Stainless wire 0.011 inch, mesh bulk density 5 lb/cuft.

TM-0609 Stainless wire 0.006 inch, mesh bulk density 9 lb/cuft.

TM-0607 Stainless wire 0.006 inch, mesh bulk density 7 lb/cuft.

Styles with Polymeric Filaments:

TM-PP04 Polypropylene monofilament, mesh bulk density 4 lb/cuft.

TM-TF04 Teflon monofilament, mesh bulk density 4 lb/cuft.

TM-KY08 Kynar monofilament, mesh bulk density 8 lb/cuft.

Styles with Metal Wire Co-knitted with Glass or Polymeric Multistrand Materials:

TM-TFEC Teflon multistrand co-knit designed for maximum efficiency.
Similar mechanical mist collection performance will be observed for: Dacron (TMDAEC), polypropylene (TM-PPEC), and Fiberglass (TM-GLEC). Materials of construction vary to achieve temperature and corrosion resistance constraints.

TM-DAEC Dacron multistrand co-knit designed for optimum capacity and efficiency. Alternate materials are: Teflon (TM-TFEC), polypropylene (TM-PPEC), and Fiberglass (TM-GLEC).

Grids

Grids are typically stainless steel, all plastic, or fiberglass reinforced plastic. Materials of construction are once again dictated by constraints of temperature and corrosion.