Cylinder Dryers; Unit Heaters - Xylem Bell & Gossett Hoffman Speciality Series Engineering Data Manual

Cylinder Dryers

Cylinder dryers are widely used in the process-
ing industry. Since they are usually rotating in
nature, siphon drainage of the condensate is
involved. Figure 9 shows a typical arrange-
ment. Condensate is drained from the bottom
of the rotating cylinder by a typical siphon
arrangement.
Siphon drained cylinder dryer

Unit Heaters

Unit heaters may be selected to operate over
a wide range of pressures. Operation may
have maintained steam pressure in coils with
a thermostat to control the fan or steam con-
trol may be on-off as heat load is required.
Small low pressure unit heaters up to 15 psi
often use Thermostatic Traps. Large unit
heaters or those operating at higher pressure
may use F & T as first choice and Bucket
Traps as second choice.
When the ambient air may be below freezing
or when outside make-up air is used, a vacu-
um breaker is required to prevent an induced
vacuum from occurring when the steam is
turned off. Induced vacuum causes a reverse
differential pressure across the trap and
holds up condensate in the coils. This is the
major cause of coil freezeup. The trap must
also be able to drain by gravity to assure com-
plete condensate removal.
The recommended safety factor for sizing
traps for unit heaters is 3 times rated
capacity. Low pressure traps may be sized
using SHEMA rating without any additional
safety factor.
Because of their large volume and surface
area, traps for this type of application should
be sized with a substantial safety factor. This
is required to eliminate the air and handle the
large warm-up load. It is not uncommon to
use safety factors of between 5 and 8 for
cylinder dryers.
Figure 9
STRAINER
SWING CHECK VALVE
TRAP
Unit heater
VACUUM
BREAKER
UNIT HEATER
UNIT HEATER
Cylinder
Dryers, Unit
Heaters
TRAP
GRAVITY
RETURN
LINE
31