Description
20
GA03108_002_C1 - 04/2018 - © Leybold
1.3.3 Maximum Pressure Difference
The maximum differential pressure at which the RUVAC can be operated at is
limited by two factors:
1. The installed motor power
2. The thermal limitations of the pump
The thermal limits of the RUVAC can be exceeded for a limited time if this is
followed by a period of time that allows it to cool down at a rather low pres-
sure.
The percentage of time within a repeating cycle for which the RUVAC is ope-
rated at high differential pressure is called the duty cycle.
If the duty cycle is for example 25 %, the pump runs at high differential pres-
sure for a time period of 1 minute followed by a time period 3 minutes at a
discharge pressure of less than 1 mbar. If a cycle time of 20 minutes is
exceeded, the pump has to be considered in continuous operation.
Further factors can influence the maximum differential pressure as for examp-
le the pump ratio, the rotational speed of the RUVAC, the gas intake tempe-
rature, the ambient temperature and the gas type.
The given values are valid for air with a maximum intake temperature of
40 °C and a maximum ambient temperature of 40 °C.
With the evacuation of vented vacuum chambers in short cycle or load lock
applications the RUVAC must only be operated at max. 60 Hz (air inrush).
Example
Max. allowable differential pressure in duty cycle:
RUVAC WS 2001 at 50 Hz / DV 650
Operation: 10 minutes at high differential pressure
10 minutes at a discharge pressure of less then 1 mbar
Nominal pumping speed DV 650
650 m
3
· h
-1
Pumpe ratio = —————————————————————— =
————————— = 1:3
Nominal pumping speed WS 2001
2050 m
3
· h
-1
Duty cycle is 50 %. The maximum allowable differential pressure resulting
from the table above is
Δ
p
max
= 75 mbar.
See also Section 4.1 for calculating the cut-in pressure.
Short cycle operation below 2 minutes should only be implemented using a
WSU pump; in the case of longer pumpdown times we recommend a WS
pump in combination with a frequency converter.
Do not allow the WSU pumps to operate for longer periods of time at high
pressures. The bypass line has been optimised for rapid pumpdowns and
has not been designed to protect the pump at higher pressures!
