SPX Hankinson HHE Series Instruction Manual page 8

3.4 Automatic Purge Saving System
(Refer to Figure 3-4a for Models 40 through 3000 and Figure 3-4b for
Models 4100 through 5400.)
Assume tower A is on-line drying while tower B has just gone off-line
to be regenerated. At the beginning of tower B's regeneration cycle a
thermistor temperature measurement is made at position B1. After the
tower has been regenerated, another measurement is made at B1. The
drop in temperature sensed during regeneration is an indirect measure
of the water vapor content of the inlet air. The Automatic Purge Saving
System's microprocessor then uses this information to calculate an
allowable temperature rise in the bed during the drying cycle.
When tower B goes back on-line, a temperature probe at position B2
measures the initial bed temperature at this point and then monitors
the bed until the calculated temperature rise occurs. The temperature
rise occurs as heat of adsorption is released during the drying process.
The time for the temperature rise to occur depends on flow rate. At
100% flow the temperature rise takes 5 minutes, at 50% flow it takes
10 minutes.
NOTE: If after 30 minutes, the bed temperature has not risen to the
calculated value, the dryer will automatically switch towers.
When the calculated temperature rise is reached, the towers switch
with tower A now drying and tower B being regenerated. Tower B
regenerates for 3.9 minutes, re-pressurizes, and remains idle until it
is called upon for the next drying cycle.
TOWER
4A
9A
TOWER 4B REGENERATING
8
1
TOWER
9A
10A
TOWER 4B REGENERATING
1. Tower Pressure Gauges
2. Purge Pressure Gauge
3. Inlet Shuttle or Switching
Valves
4. Desiccant Drying Towers
5. Shuttle or Check Valve(s)
6. Adjustable Purge Rate Valve
7. Purge Orifice(s)
Process Stream
Purge Stream
— 6 —
OUTLET
11
5
1 1
8
7 7
6 2
A2 B2
TOWER
4B
3
A1 B1
9B
INLET
10A 10B
FIGURE 3-4a
Models 40 through 3000
TOWER 4A DRYING
OUTLET
5A 5B
8
11
6
2 5D
5C
1
7
A2 B2
TOWER
4A 4B
A1 B1
3A 3B 9B
10B
INLET
FIGURE 3-4b
Models 4100 through 5400
TOWER 4A DRYING
8. Safety Valve(s)
9. Purge and Repres-
surization Valves
10. Purge Mufflers
11. Moisture Indicator
A & C Left Tower Suffix(es)
B & D Right Tower Suffix(es)