Cycle Time Calculation; To Determine The Backwash Flow Rate; To Determine The Injector Size - Pentair FLECK 2910 NXT Installer Manual

Installer manual Fleck 2910 NXT/NXT2 - System sizing
regeneration of the unit. From this data, determine the required backwash flow rate as well as the
brine draw and slow rinse flow rate. In most cases, the fast rinse flow rate will be the same as the
backwash flow rate, however for certain valve types the fast rinse flow rate will be the same as the
service flow rate.

To determine the backwash flow rate:

Q = Fs x S
backwash backwash
The DLFC installed on the valve has to limit the backwash flow rate to the above calculated flow
rate.

To determine the injector size:

The velocities to be respected for brine draw and slow rinse are given on the resin manufacturer
specifications. Generally speaking, the injector has to allow a flow rate of about 4BV / h
(corresponding to the flow rate of brine being drawn added to the flow rate of raw water passing
through the injector nozzle to create the suction effect).
Q = 4 x BV / h
Inj
Info
This value does not correspond to the brine draw flow rate but to the total flow rate
passing through the injector.
Refer to the injector diagrams at the inlet pressure in order to check if the injector will
give a correct flow rate.
See chapters Salt amount definition [}Page 34] and Injector flow rates [}Page 34].
4.2.5

Cycle time calculation

From this point, the volume of resin, the tank size, the capacity of the softener and the valve
configuration are determined. Next step is to calculate the regeneration cycle duration, which
depends on the valve configuration and once again on the resin specifications.
Info
Preprogrammed cycle times are only factory default programming that need to be
adjusted to fit the system requirements.
For cycle time calculation the valve configuration must be known, which depends on:
• the tank size;
• the resin specifications for the velocity for backwashing the resin bed;
• the velocity of water for brine draw, slow rinse and fast rinse.
Further information needed for cycle time calculation are:
32 / 172
with:
Q : backwash flow rate [m
backwash
Fs : backwash velocity [m/h]
backwash
S: Tank cross section area [m
with:
Q : total flow rate passing through the injector [L/h]
inj
BV: bed volume of resin [L]
Ref. MKT-IM-002 / E - 12.11.2019
3
/h]
2
]