How Wrong Oxygen Generator Capacity Damages the Ozone System and the Project
Introduction
In ozone projects, generator capacity (g/h) is usually the first number discussed; the oxygen source is often treated as "we'll put a PSA next to it". Yet the oxygen generator is the fuel of the ozone system, and when its capacity is wrong the damage does not stay in the PSA unit: the ozone generator, the water transfer system, process targets, and the project budget are all affected together.
In this post we look at what wrong oxygen capacity does to the system and the project from both directions (too little and too much), show the correct calculation with numerical examples, and summarise the checks that should be made at project stage. Topics such as feed gas quality, NOx and nitric acid formation, and why medical concentrators are unsuitable are covered in detail in Oxygen Quality Is Ozone Quality; here the subject is capacity and project impact.
In Brief
- Ozone production (g/h) = oxygen flow x ozone concentration. If flow is wrong, either g/h is not met or concentration is distorted.
- Insufficient oxygen: target ozone cannot be produced, the generator heats up, kWh/kg worsens, and a PSA pushed beyond nominal loses purity.
- Excess oxygen: concentration falls, transfer to water weakens, and investment and energy are wasted.
- Project impact: dissolved ozone (ppm) and CT targets are missed, commissioning drags on, performance guarantees become disputed, and an unnecessarily large generator is often requested.
- The correct capacity is set by considering together the generator's flow-concentration curve, the target concentration, the transfer system, and the PSA's purity-flow curve.
The Basic Calculation: What Does Oxygen Flow Determine?
Ozone production (g/h) = Oxygen flow (Nm3/h) x Ozone concentration (g/Nm3)
Ten percent ozone by weight is roughly 143 g/Nm3, and six percent roughly 86 g/Nm3. Producing the same amount of ozone at higher concentration requires less gas; but at high concentration the discharge zone heats more and kWh/kg worsens, so every generator has an optimum operating point. Approximate values:
| Ozone capacity (g/h) | Required O2 @ 10% (L/min) | Required O2 @ 6% (L/min) |
|---|---|---|
| 25 | 2.9 | 4.8 |
| 100 | 11.6 | 19.4 |
| 250 | 29 | 48 |
| 500 | 58 | 97 |
| 1000 | 116 (about 7 Nm3/h) | 194 |
Exact flow should always be taken from the flow-concentration curve of the generator concerned.
Case 1: When Oxygen Capacity Is Insufficient
Numerical example: Suppose a 250 g/h ozone generator is connected to a PSA delivering 20 L/min (1.2 Nm3/h) of oxygen. At 10 percent concentration this flow carries at most about 170 g/h of ozone, roughly 70 percent of capacity. For the remainder the generator either falls short of target or is pushed to a higher concentration.
- On the ozone generator: Gas passes with less flow and at higher concentration; the discharge zone heats up, ozone decomposes faster, and kWh/kg rises. If cooling is insufficient, low-flow or temperature protections trip and the dielectric is subjected to thermal stress, shortening its life.
- On the PSA: A PSA pushed beyond nominal flow loses purity and its nitrogen fraction rises. This creates NOx and nitric acid risk, and the molecular sieve tires faster.
- In continuous operation: Effects accumulate; ozone output quietly drops, maintenance intervals shorten, and unplanned stops increase.
Case 2: When Oxygen Capacity Is More Than Needed
Numerical example: The same 250 g/h generator needs about 29 L/min (roughly 1.75 Nm3/h) of oxygen at 10 percent. If a PSA twice as large, delivering 3.5 Nm3/h, is connected with no flow regulation, the same ozone mass is spread over twice the gas: concentration falls to about 71 g/Nm3, roughly 5 percent by weight.
- Because the gas-phase ozone partial pressure falls, the driving force of transfer to water drops, ozone transfer weakens, and the off-gas share rises.
- Unnecessary investment and energy go into an oversized PSA; surplus gas is vented or creates imbalance on the venturi/diffuser side.
- If the generator's allowed maximum flow is exceeded, internal pressure rises and protection trips occur.
One distinction matters here: in some applications designs where surplus oxygen is deliberately used for a second purpose (for example oxygenation in aquaculture) are intentional. The problem is when the surplus is an unplanned result rather than a design decision.
Impact on the Project: How Does a Technical Error Become a Commercial Problem?
| Area | Insufficient capacity | Excess capacity |
|---|---|---|
| Process target | Dissolved ozone (ppm) and CT target are missed | Low concentration weakens transfer, ppm is delayed |
| Commissioning | Acceptance test fails, adjustment and revision rounds drag on | Tuning and balancing work is needed |
| Investment (CAPEX) | A PSA or generator has to be added later | Unnecessarily large equipment and floor space |
| Operation (OPEX) | High kWh/kg, frequent maintenance, early wear | Unnecessary energy consumption |
| Responsibility | "The generator is weak" dispute; the real cause is overlooked | Warranty and performance commitment become unclear |
The most frequent result is this: the system performs below nameplate, the operator reads it as "the generator is weak" and unnecessarily asks for a bigger generator. Yet the problem is not in the generator but in the balance of feed flow and concentration. We explained why performance should be measured as dissolved ozone rather than g/h in Why ppm, Not g/h?; for the transfer side see Oxygen and Ozone Transfer to Water.
Steps to Determine the Right Capacity
- 1. Requirement: Determine the ozone dose the process needs and the required g/h, taking transfer efficiency into account.
- 2. Concentration: Choose the target concentration at the generator's optimum operating point (flow-concentration curve).
- 3. Flow: Calculate the required oxygen flow from g/h and concentration.
- 4. PSA selection: Choose a PSA that can deliver that flow at the required purity and outlet pressure; operate with a capacity margin above nominal, not at the limit. Allow for factors such as ambient temperature and altitude.
- 5. Buffer and pressure: Choose the buffer tank and outlet pressure according to generator and line back pressure.
- 6. Verification: At commissioning, measure gas-side concentration, dew point, and dissolved ozone in water; define the acceptance criterion in ppm.
What the Specification Must Include
- Model, nominal capacity, and target concentration of the ozone generator to be connected
- Oxygen flow and purity together, and the pressure at which they are guaranteed
- Dew point value and its measurement
- Buffer tank, outlet pressure, and ambient conditions
- Acceptance test method and criterion (dissolved ozone ppm where possible, not g/h)
Frequently Asked Questions
Is a somewhat larger PSA always safe?
A sensible capacity margin is necessary; but if flow is unregulated and excessive, concentration drops. The margin must be considered together with flow control.
My generator is not delivering its nameplate value; is the generator the problem?
First check the feed gas: flow, pressure, purity, dew point, and concentration. In many cases the problem is on the oxygen side.
Can I use a medical concentrator?
We do not recommend it for continuous industrial processes; we explained why in our previous post.
The OCS Ozone Approach
Because we manufacture the oxygen generator, ozone reactor, and transformer in-house, we size PSA capacity, purity, buffer tank, and cooling together around the generator's operating point, and we define the acceptance criterion in terms of dissolved ozone. If you are having performance problems in an existing system, or want to verify oxygen capacity for a new project, get in touch for a free technical assessment.