The height of a trayed column is calculated by multiplying the number of (actual) stages by the tray separation. Tray spacing can be determined as a cost optimum, but is usually set by mechanical factors. The most common tray spacing in 24 inchesit allows enough space to work on the trays whenever the column is big enough around (> 5 ft diameter) that workers must crawl inside. Smaller diameter columns may be able to get by with 18 inch tray spacing. .