Heat Exchangers: Is Mist a Must?

An air-fin exchanger is a cross-flow exchanger on the air side. Even with multiple tube passes, getting close approach temperatures is difficult. Air-fin exchangers often pinch out against air inlet temperature on very hot days (see: “Cope with Condenser Constraints”). Against a pinch, higher air flow rates give little benefit — the only effective technique to improve air-fin performance may be to drop the air temperature. Spraying water into the air can do this.

The objective isn’t to put bulk water on the exchanger but instead to create a mist in the air that leads to humidification cooling. So, let’s look at when a mist is useful; how fine a mist is needed; and how to make the mist.

The goal is to create humidification cooling when the water evaporates. Psychrometric charts detail the difference between various relative humidity levels as air becomes more saturated. The charts include dry bulb and wet bulb temperatures. The dry bulb temperature is the starting air temperature. The wet bulb temperature is the achievable cooling by saturating the air. A quick glance at a psychrometric chart shows very little temperature drop is possible once air gets to ~85% relative humidity. To cool air at 115°F to 110°F requires starting with 70% humidity and increasing the humidity to 83%. Considering a 5° cooling as the minimum performance to make the expense worth the effort, a site should have a relative humidity of 70% or less. Checking the required temperature drop against site conditions will clarify if mist cooling might make sense.
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The benefit of humidification comes from evaporating water upstream of the air-fin — by creating a mist immediately underneath the air-fin. Only a short distance is available for the water droplets to evaporate before they enter the exchanger. Based on observation of operating misting systems, my own rule-of-thumb is to get a spray pattern that creates a nominal 50-µ (or smaller) droplet at 3 ft below the air-fin bundle.

EVAPORATION PLOT

Figure 1. Larger droplets require more distance to evaporate.However, rather than just relying on that rule-of-thumb, let’s delve into the issue a bit more. Figure 1 shows expected distance for a droplet to evaporate versus droplet size. It’s based on 115°F air at 70% relative humidity with a face velocity of 10 ft/sec into the exchanger. The analysis involves too many assumptions to list here but, rest assured, I took care to ensure the simplifications aren’t all in one direction. The analysis starts with Beard and Pruppacher’s work on droplet evaporation (“A Wind Tunnel Investigation of the Rate of Evaporation of Small Water Drops Falling at Terminal Velocity in Air,” J. of Atm. Sci., November 1971) and modifies its assumptions to better fit the conditions of typical hot (above 100°F) air-fin operation.Figure 1 indicates that droplets 60 µ and smaller should evaporate in 3 ft. However, spray nozzles don’t create uniform-size droplets but a distribution of droplet sizes. These droplet sizes most commonly are characterized by Sauter Mean Diameter (SMD), which is the diameter whose ratio of volume to surface area equals that of the entire droplet distribution. Other useful parameters include the peak diameter (PD), which is the droplet size that matches the peak in the droplet size distribution, and the mass median diameter (MMD), which is the diameter that has 50% of the total volume smaller than this size. For most sprays, the SMD is 80–84% of the PD. The MMD is larger still. A few small but large particles contain much of the mass in the spray.For an SMD of 50 µ, the PD is 50/0.8 = 62.5 µ. Figure 1 shows that the distance required for this diameter is 3 ft. Less than 50% of the mass evaporates at that point. Achieving the target air cooling may require a ratio of roughly 3:1 of sprayed water to minimum water. The excess water enters the exchanger and rapidly vaporizes.How do we create such a fine mist? Either air atomizing or fine spray nozzles might meet the requirements. Air atomizing nozzles generally create the smallest droplets. They use a gas stream to physically break a liquid stream into droplets. This requires adding an air system as well as a water distribution system. A fine spray nozzle uses pressure drop to do the job. It comes in two versions: one forms spray directly while the other creates the spray by bouncing a jet of liquid on a surface. In either case, what makes a fine spray is high pressure drop and a small nozzle. Droplet size distributions are extremely difficult to predict. What you need are data. Work with the nozzle vendor, explain your objective and circumstances, and get a nozzle that’s been thoroughly tested. In any case, expect a high pressure drop. Some units require pressure drops of up to 400 psi to achieve small droplet sizes.

ANDREW SLOLEY is a Contributing Editor to Chemical Processing. You can e-mail him at [email protected]