Process Engineering Beginner's Guide

CHAPTER NINEAIR COOLERS

I worked for American Oil in Whiting, Indiana, in the 1960s. In those days, we used Lake Michigan as a source of cooling water. Of course, we had exchanger tube leaks that allowed hydrocarbons like benzene and xylene to leak into the lake. But as we used to say, “The solution for pollution is dilution.”

In recent times, it’s more cost effective to use air coolers. Most of the newer facilities I work on rely on forced draft air coolers. Forced draft means that the fan is located underneath the tube bundle. Induced draft is when the fan is located above the tube bundle, and cooling air is drawn up through the bundle. Induced draft has certain process advantages over forced draft. Most air coolers, however, are forced draft, so that the fan, motor, and drive belt are readily accessible for maintenance.

MEASURING AIR FLOW

I often feel isolated from the rest of mankind. Apparently I’m the only human who has ever observed that air coolers rarely – if ever – develop their rated air flow. The rated air flow, in pounds per hour (lbs/hr) is listed on the vendor’s air cooler data sheet.

My method of measuring the air flow is:

• Step 1 – Calculate the process side heat duty in BTU per hour. Let’s say I’m condensing 10,000 lbs/hr of butane from 200°F, down to 140°F. The latent heat of condensation of butane is 130 BTU/lb. The sensible heat of butane vapor is 0.60 BTU/LB/°F. Then the process duty is:

• Step 2 – Measure the average temperature rise of the air. To do ...