Understand the cold facts about subcooling

June 4, 2008

Systems sometimes can surmount major layout errors

Figure 1. Oversized condenser ensures that more than enough surface area is available for subcooling.

Units that work despite apparent violations of fundamental practices teach us many important lessons. Here we’ll examine hot-vapor-bypass pressure controls that succeed against all odds. Such control on a distillation column requires that liquid entering the overhead receiver be subcooled, so vapor can maintain a pressure blanket with no net loss of vapor overhead.

One dependable way to get the distillate liquid subcooled is to oversize the condenser, thus ensuring that excess surface area is available for subcooling. The condenser may be mounted above or below the receiver, as long as the liquid from the condenser seals into the liquid pool in the drum (Figure 1). Putting the liquid line into the bottom of the drum guarantees it. Pressure balance between the condenser and the overhead receiver (controlled by the hot-vapor-bypass valve) forces the liquid into the drum.

Now, let’s look at two systems that work in spite of major layout errors. You never should build either system deliberately but you may well run into one. So, let’s see why each manages to work.

Figure 2. Subcooling should be impossible but may occur because of restricted flow from the condenser.

Figure 2 shows an exchanger mounted above the drum with a free-draining line into the drum. Normally, with gravity flow to drain liquid from the condenser to the drum we’d expect that subcooling is impossible. Yet this system has been shown to work. But how?

If its outlet nozzle doesn’t let enough liquid pass, the condenser can maintain a liquid level despite the piping layout for a gravity drain. Such flow limitations can stem from selecting too small a nozzle for the service or from restrictions, e.g., caused by buildup of corrosion products. As long as some liquid level remains on the tubes, it’s conceivable that the liquid leaving the exchanger could be subcooled. So, here, two mistakes balance out.

Or consider what can happen when using an air-fin exchanger as the condenser. Figure 3 shows an end-on view of liquid stratification inside the tubes in the air-fin. For wide-temperature-range condensing mixtures, low velocity in the tubes may lead to liquid separating from the vapor. Then external air may subcool the liquid on the bottom of the tubes. If mixing is limited in the air-fin header box, the liquid may enter the overhead drum subcooled. The same problem may occur if the air-fin has multiple tube passes. Phase separation in the header boxes may allow for some flooded tubes and some unflooded tubes in the exchanger.

Figure 3. Liquid that has separated from the vapor is susceptible to subcooling by external air.

The air-fin situation requires an unusual combination of exchanger design, tube velocity and condensation curve characteristics. However, it can happen.

A more common problem of the stratified air-fin is that once the heavier part of the liquid leaves the vapor, the remaining vapor may no longer be condensable. Many condensers of all types have failed to work due to this variant of vapor blanketing the exchanger surface.

Again, never count on making one of the flawed subcooling approaches work — but understand why they may.