EngiMetric Arena / Spot the Error
Review this RO energy calculation: the credit is missing
What you should take away: Recognise that an energy figure issued without the ERD credit is a gross figure, and distinguish a design bound from a predicted operating value.
The situation
A design review receives a specific-energy figure for a seawater RO plant that has an isobaric energy recovery device fitted on the brine stream. The reviewer must decide whether the issued figure can be relied on. Every figure below was produced by the same energy engine used by the RO energy calculator.
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Feed flow | 100 | m³/h | Feed to the reverse-osmosis stage. |
| System recovery | 45 | % | Permeate as a fraction of feed. Sets the brine flow. |
| Feed pressure | 55 | bar | Membrane inlet pressure. |
| Pump efficiency | 85 | % | Assumed pump efficiency. |
| ERD efficiency | 95 | % | Is the energy recovery device fitted, and how efficient? |
Constraints
- Membrane elements are supplied whole. A fraction of an element cannot be ordered.
- Pressure vessels are loaded with a whole number of elements — 6 per vessel here.
- Every vessel in the array is filled to the same element count.
Your answer
Hints
- Start from the brine side. At 45 % recovery, more than half the feed leaves as brine — and that stream leaves the pressure vessel at feed pressure.
- Ask what the engine reports when an ERD efficiency is supplied versus when it is not. Two specific-energy figures, not one.
- The question is not whether an ERD helps, but what the issued number actually represents once you know it was produced without the credit.
Where that comes from
Each limit, checked
Why
- The engine computes a gross specific energy from the feed flow, feed pressure, recovery and pump efficiency alone. At 100 m³/h, 55 bar and 85 % pump efficiency that is about 4.0 kWh/m³.
- It separately computes a brine energy credit from the brine flow, the same pressure and the stated 95 % ERD efficiency — here about 80 kW — and reports the net figure after that credit, about 2.2 kWh/m³.
- A figure issued for a plant with an ERD fitted, but equal to the gross value, therefore omits the credit and **overstates** the electrical energy the motor must supply — by roughly 45 % in this case.
- The practical consequence is that the issued figure is not merely conservative-looking — it is the wrong basis for judging the pump, the motor rating and the operating cost.
- This is also a design bound rather than a prediction. The credit is computed as if the brine expanded to atmospheric, which is an upper bound; a real pressure exchanger recovers the brine pressure drop, so the achieved credit is typically slightly lower and the true figure slightly higher than the net value above.
What this challenge does not establish
- The ERD credit is an upper bound: the engine assumes the brine expands from full feed pressure to atmospheric, so a real pressure exchanger recovers slightly less.
- Feed pressure is taken as given. Concentrate-side pressure and the pressure drop across the ERD are not modelled.
- This assesses the ENERGY figure only. Whether 55 bar is itself correct for this recovery is a separate osmotic-balance question.
- No allowance is made for parasitic ERD power, pump efficiency variation with load, or part-load operation.