Given the dosing regimes have been determined for level, frequency and duration, with the seawater system flow rate known then the hypochlorite demand in each instance can then be calculated from the following simple formulae.
For each hypochlorite demand the following equation applies
NaOCl kg/h = Seawater Flowrate m3/h x mg/l (NaOCl)
1000
Alternatively
NaOCl kg/h = Seawater Flowrate m3/h x ppm (NaOCl) x SG
1000
Note 1
mg/l = ppm x SG
If mg/l and ppm are treated as equivalents then errors can occur in the weight calculations which on a marginally designed system could lead to failure to meet guarantee points during commissioning.
As stated above it is an industry convention that, unless otherwise specified, continuous dosing will be suspended during shock dosing periods. Therefore the daily continuous demand will be for 24 hours minus the sum of the duration of the shock dosing periods, in the example utilised it is 2 hours hence the continuous dosing daily demand is calculated against the hourly demand for a period of 22 hours.
Electrochlorination packages benefit from continuous operation and to achieve this mode of operation the media mass balance is averaged over a daily period. Therefore the sum of the daily shock dosing and continuous dosing demand is averaged over 24 hours to determine the required kg/h capacity of the ECP.
Example 1
Continuous Dosing 20,000m3/h x 1mg/l = 20kg/h NaOCl
1000
Shock Dosing 20,000m3/h x 5mg/l = 100kg/h NaOCl
1000
Shock Dosing Daily Frequency 4 Daily
Shock Dosing Duration 15 minutes
Shock Dosing Demand 1 h/day @ 100kg/h = 100kg/day
Continuous Dosing Demand 24-2 h/day = 22 x 20 = 440kg/day
ECP Capacity 540 kg/day ÷ 24 = 22.5kg/h
A capacity specification would normally include a contingency to the productive capacity of the system for any potential increase in demand. Given a design product concentration for that maximum capacity and turn down by DC current variation this will result in the normal operating product concentration being at a lower level than the design value.
Example 2
| Required capacity | 22.5kg/h |
| Contingency | 10% |
| Rated Capacity | 24.75kg/h = 100% |
| Product Concentration | 2000 mg/l |
Electrolyser Seawater Carrier Stream Flowrate
Flow m3/h = NaOClkg/h x 1000
mg/l
= 24.75 x 1000 = 12.375 m3/ h
2000
For the actual demand at 22.5kg/h the product concentration will therefore be:-
mg/l = NaOClkg/h x 1000 = 22.5 x 1000 = 1819mg/l
Flow m3/h 12.375
