- October 5, 2026
- Posted by: Aceget
- Category: RECD & Emission Control Devices
Every expansion plan has a power line in it somewhere. A factory adds a second production hall in three years. A hospital plans a new wing. An office park leases its empty floors. A cold store adds chambers. Each time, the generator that looks comfortably oversized today will carry more tomorrow, and sometimes it will be joined or replaced by a bigger one.
If that generator needs a retrofit emission control device now, the specification has to look in two directions at once. It must suit today’s load, which may be light, and tomorrow’s, which may be much heavier. Get it wrong one way and the filter struggles to regenerate in the early years. Get it wrong the other way and the device becomes a bottleneck, or must be replaced, when the expansion arrives.
This guide is built around four growth stories, because the right specification depends on which story you are living. It then sets out the two numbers that pull against each other, a growth worksheet to fill in before you ask for quotes, the clauses that keep options open and a phase-by-phase worked example.
The Four Growth Stories
Not all load growth is the same. Before writing a word of specification, decide which of these describes your plan.
| Story | What happens | Effect on the generator | Effect on the RECD |
|---|---|---|---|
| 1. Growth within rating | Load rises but stays within the existing set’s rating | Same engine, higher average load | Same device; regeneration improves, service cadence changes |
| 2. Replacement with a larger set | Existing set swapped for a bigger one | New engine, possibly in a different power band | Device may need to change or be transferred |
| 3. Adding a parallel set | A second set is added and synchronised with the first | Two engines sharing load | Each set needs its own compliance route |
| 4. Uncertain growth | Expansion is possible but undecided | Unknown | Specification must keep options open without overspending |
Most projects contain a little of each. A factory might grow within its set’s rating for two years, then add a second set. A hospital might be certain about one wing and unsure about another. Write down which story applies to each phase.
The Two Numbers That Pull Apart
An RECD specification rests on two physical requirements, and load growth affects them in opposite directions.
Number 1: Backpressure at the highest flow
The device must pass the engine’s exhaust without pushing total backpressure above the engine maker’s limit. The worst case is the highest exhaust flow, which occurs at the highest load. That is a ceiling.
Number 2: Exhaust temperature for regeneration
The particulate filter cleans itself best when the exhaust is hot, which happens at moderate to high load. At light load, exhaust is cool and regeneration slows. That is a floor.
Why growth stretches the gap
Today, the set may run at 25% load: cool exhaust, slow regeneration. In five years, it may run at 75%: hot exhaust, high flow. A device sized only for today might struggle with tomorrow’s flow. A device sized generously for tomorrow might take longer to heat up today and hold more thermal mass than the light early load can warm.
The answer is not to choose one number and ignore the other. It is to size for the ceiling and manage the floor.
Rule One: Size for the Set, Not for Today’s Load
For growth within rating, there is a simple principle that removes most of the risk. Size the RECD for the engine’s rated exhaust flow, not for the load the set happens to carry today.
Responsible vendors usually do this anyway, because an engine can always be asked for its full output during an outage, whatever its average load. If a quotation is based on current load rather than rated output, ask why. A device sized for 30% load could push backpressure beyond the limit when the expansion arrives, or even during a single heavy outage today.
Exhaust flow at rated output comes from the engine maker’s data sheet. Our article on using OEM exhaust flow data to specify an RECD explains where to find it, and why same kVA DG sets can need different RECDs shows why kVA alone is not enough.
Engine ratings themselves follow the categories in ISO 8528-1. If the set may be run up to its standby rating during a long outage after expansion, that higher figure is the one to size against. Our guide on whether prime-power and standby DG sets can use the same RECD specification explains the difference.
Rule Two: Manage the Light-Load Years
With the device sized for rated flow, the early years at light load need active management rather than a smaller device.
- Regular loaded runs. Schedule runs at 50% load or more for 30 to 60 minutes, using building load or a load bank, so the filter regenerates.
- Placement and insulation. Mount the device close to the engine outlet and insulate the pipe, so the light-load exhaust arrives as hot as possible.
- A catalyst suited to lower temperatures. Ask the vendor how their catalyst performs at the exhaust temperatures your set produces today.
- Monitoring with a clear trend. Watch backpressure at a fixed load. A rising trend that does not clear after a loaded run calls for more frequent exercise runs.
Our guide on choosing an RECD for a generator running at low load covers these measures in depth.
The Growth Worksheet
Fill in this worksheet before requesting quotations. It turns a vague expansion plan into numbers a vendor can design with.
| Item | Today | After phase 2 | After phase 3 |
|---|---|---|---|
| Expected year | |||
| Connected load (kW) | |||
| Expected peak load on DG (kW) | |||
| Expected average load on DG while running (kW) | |||
| Annual DG running hours | |||
| Number of sets and ratings | |||
| Engine power of each set (kWm) | |||
| Duty: standby or prime | |||
| Exhaust route changes | |||
| Certainty of this phase (high, medium, low) |
Our guides on load calculation basics before buying a generator, generator sizing for factories and how to size a generator for a commercial building help estimate the figures. Existing controller data is the best starting point for the “today” column; see using DG load logs to assess RECD suitability.
What Each Growth Story Means for the Specification
Story 1: Growth within rating
The device stays the same. What changes is how it lives.
- Regeneration improves as average load rises. The light-load measures above can be relaxed over time, guided by monitoring data.
- Annual hours often rise as the site depends more on the set. Ash from lubricating oil builds with hours, so cleaning intervals shorten.
- Service plan clauses should be stated in running hours as well as calendar time, so the plan adjusts automatically.
- Warranty terms should be clear about hours as well as years.
Story 2: Replacement with a larger set
Here the device’s future depends on the new engine.
- Approval range. RECDs are approved under CPCB’s System and Procedure for Emission Compliance Testing of RECDs for engine power ranges. If the new engine falls outside the range covered by your device, the device may not be usable on it.
- Flow and backpressure. A larger engine produces more exhaust. Even within the same approval range, the existing device may not have the capacity.
- Transfer terms. Ask the vendor at the outset what happens if the set is replaced: can the device be transferred, upgraded or taken back against a new one? Our article on whether an RECD can be transferred to another diesel generator explains the conditions.
- Regulatory band. In NCR, a larger engine may move into a different power band under CAQM Direction No. 76, which changes the available compliance routes. A set crossing 800 kW, for example, moves into the band where an RECD is the route.
If replacement is likely within a few years, compare the cost of retrofitting the current set with buying the future set now. Our comparison of RECD vs a new CPCB IV+ genset helps frame that.
Story 3: Adding a parallel set
When a second set joins the first, each engine needs its own compliance route.
- Separate devices are the normal approach. Each engine’s exhaust is treated by its own RECD sized for that engine.
- Shared devices are sometimes proposed when two sets have a common exhaust. Our article on whether two diesel generators can share one RECD explains why this is usually not advisable.
- Load sharing affects regeneration. Two sets sharing a load evenly may each run at a lighter load than one set alone. Load-demand control that runs one set at higher load and brings in the second only when needed is often better for both engines and filters.
- Space and exhaust routing for the second set’s RECD should be reserved now, even if the set comes later.
Story 4: Uncertain growth
When the future is genuinely unclear, keep options open at low cost.
- Size the device for the current set’s rated output (Rule One)
- Reserve space and structural capacity for a possible larger device or a second set
- Choose pipe sizes and supports that will not need replacing if a larger engine arrives
- Agree transfer or upgrade terms in the contract
- Avoid buying a much larger device “just in case,” which brings light-load penalties now for a benefit that may never come
Future-Proofing Clauses for the Specification
Add these clauses to an RECD specification when growth is expected.
| Clause | Purpose |
|---|---|
| Device sized for the engine’s rated exhaust flow, at the highest rating the set may run to | Prevents a backpressure bottleneck when load rises |
| Total backpressure within the engine maker’s limit at rated output, with normal soot load, and with stated margin | Keeps the engine safe at future peak load |
| Vendor to state performance at current typical load, including expected regeneration behaviour | Addresses the light-load early years |
| Service plan in running hours and calendar time, revisable by monitoring data | Adjusts automatically as hours rise |
| Warranty stated in years and running hours | Clarifies cover as usage grows |
| Mounting frame and space sized for a possible larger device or second set | Avoids structural rework later |
| Exhaust pipe sized for the largest engine likely to be installed on the same route | Avoids replacing pipework later |
| Monitoring system capable of adding a second set’s sensors | Supports Story 3 |
| Transfer, upgrade or buy-back terms if the set is replaced | Protects the investment in Story 2 |
| Confirmation of the approval range covering current and likely future engine power | Flags early whether the device can follow a larger engine |
Our guide on choosing CPCB approved RECD vendors suggests how to compare vendors on these points.
How the Maintenance Calendar Should Evolve
As load and hours grow, the RECD routine should change with them. A plan written for the light-load years will be wrong for the busy ones, and the reverse is also true.
| Stage | Typical condition | Maintenance emphasis |
|---|---|---|
| Early years, light load | Cool exhaust, slow regeneration, low hours | Scheduled loaded runs, close watch on soot trend, insulation checks |
| Transition, load rising | Regeneration improving, hours climbing | Review exercise run frequency; reset alarm thresholds at the new typical load |
| Mature, high load | Hot exhaust, routine regeneration, high hours | Ash cleaning by hours, engine oil consumption tracking, sensor calibration |
| After adding a parallel set | Two engines sharing load | Compare both trends; check load-sharing settings keep each set loaded enough |
Two practical habits help through every stage. First, always record backpressure at the same reference load, so readings from different years can be compared. Second, revisit alarm thresholds whenever the typical load changes substantially, because a threshold set during light-load running may cause nuisance alarms, or miss real problems, at a much heavier load.
Budgeting Across the Phases
Finance teams often ask for a single number for the RECD. With growth expected, a phased budget is more honest.
- Phase 1 capital: device sized for rated output, installation, monitoring, any space and support reservation for future equipment
- Phase 1 running costs: load bank hire or purchase for exercise runs if building load cannot provide them, routine inspections
- Phase 2 running costs: higher service frequency as hours rise, earlier ash cleaning, possibly a spare filter module if downtime becomes costly
- Phase 3 capital: a second device for an added set, or the cost difference of upgrading if the set is replaced, offset by any agreed transfer or buy-back value
- Contingency: structural or exhaust route changes if the expansion layout differs from today’s plan
Setting these out early avoids the common surprise of a second-phase budget that forgot the emission control. Our guide on RECD cost in India helps with device pricing, and can an RECD be transferred to another diesel generator explains the factors that decide transfer value.
Common Mistakes When Planning for Growth
Sizing the device for today’s average load
The most common error. The device may restrict the engine at full output, even before any expansion. Always size for rated flow.
Buying a much larger device for a distant future
A device far larger than the current engine needs adds cost, thermal mass and slower warm-up, all for a set that may be replaced before the extra capacity is used.
Forgetting the hours
Load growth usually brings more running hours. Ash cleaning intervals measured in years at today’s hours may shrink considerably.
Ignoring the approval range
A device that cannot legally follow a larger engine is a sunk cost when replacement comes. Check the range at purchase.
Leaving no space
Plant rooms fill up. If a second set or a larger device will be needed, reserve the space and the exhaust route on the drawings now.
A Worked Example: Phased Factory in Sonipat
This illustrative case follows a factory through three phases.
A packaging materials manufacturer in an industrial estate in Sonipat runs a 700 kVA set (about 560 kWe). Phase 1 is operating. Phase 2, a second production hall, is planned in two years. Phase 3, a further expansion, is possible in five to six years but not yet approved.
The worksheet, summarised
| Item | Today (phase 1) | Phase 2 | Phase 3 |
|---|---|---|---|
| Expected peak load on DG | 240 kW | 450 kW | 820 kW |
| Average load while running | 160 kW (about 29%) | 330 kW (about 59%) | 600 kW across two sets |
| Annual running hours | 400 | 900 | 1,200 per set |
| Sets | 1 x 700 kVA | 1 x 700 kVA | 2 x 700 kVA, synchronised |
| Certainty | Actual | High | Medium |
Identifying the stories
- Phase 1 to 2: Story 1, growth within rating
- Phase 2 to 3: Story 3, adding a parallel set
- Phase 3 overall: Story 4, uncertain
What was specified
- Device sized for the engine’s rated exhaust flow, with backpressure at full output within the engine maker’s limit and a stated margin
- Light-load measures for phase 1: device mounted close to the engine outlet with insulated pipework; a monthly 45-minute loaded run using a hired load bank until phase 2 load arrives
- Service plan in running hours, reviewed annually against monitoring data
- Space and supports reserved in the DG yard for a second 700 kVA set, with a second exhaust route and RECD position marked on the drawings
- Monitoring system specified with spare inputs for a second set
- Synchronising panel planned for phase 3 with load-demand control set to run one set at higher load and bring in the second only above about 75% of one set’s rating
- Contract clause covering the second set’s RECD at agreed terms if ordered within a stated period
How it played out, illustrative
In phase 1, monitoring showed a slow backpressure rise between monthly loaded runs, falling back after each. When phase 2 load arrived, the average load roughly doubled, regeneration became routine and the load bank was no longer needed. The service plan’s hour-based clause brought the first ash inspection forward as hours climbed. Phase 3 remains under discussion, but the space, route and monitoring capacity are ready.
Aceget’s 700 kVA DG set RECD page shows the device class used in this example. Construction-phase power, which often precedes such expansions, has its own challenges; see RECDs for construction projects.
Ordering Ahead of an Expansion
Sometimes the generator for a future phase is bought before the building is ready, and the RECD with it. Our article on whether you can order an RECD before your new facility is ready covers storage, installation timing and documentation. The RECD installation timeline helps align the device with construction milestones.
Questions Planners Ask About Growth and RECDs
Should I size an RECD for my current load or my future load?
Size it for the engine’s rated exhaust flow. That covers any load growth within the set’s rating. Manage the light-load early years with loaded runs, good placement and monitoring.
What if I replace my generator with a larger one later?
The existing RECD may not suit the new engine if it falls outside the device’s approval range or exceeds its flow capacity. Agree transfer or upgrade terms with the vendor at purchase.
Can two generators share one RECD after expansion?
It is usually not advisable. Each engine should have its own device sized for its exhaust. Shared arrangements raise backpressure, flow balance and compliance questions.
Will more load make my RECD work better?
Often yes, because higher load means hotter exhaust and easier regeneration. Higher hours also mean faster ash build-up, so service intervals shorten.
Is it worth buying a larger RECD now for future use?
Rarely. A device much larger than the current engine needs adds cost and slows warm-up at light load. Reserve space and agree upgrade terms instead.
What information should I give the vendor?
A completed growth worksheet with today’s and future loads, hours, set ratings, engine power, duty type and the certainty of each phase, plus engine nameplate photos and exhaust route drawings.
Plan for the Plant You Will Have
An RECD bought for an expanding site should not be a snapshot of today. Size it for the engine’s rated output, manage the light early years with exercise runs and good placement, write service terms that grow with the hours, and reserve the space, routes and contract terms the next phase will need. Then, when the new hall opens or the new wing fills, the emission control will already be ready for it.
Aceget plans RECD specifications around expansion roadmaps and supplies CPCB type-approved devices for engines from 25 kVA to 2500 kVA, including larger ratings such as the 1000 kVA DG set RECD for sites that outgrow their first set. To build a phase-by-phase plan, share your expansion details with Aceget, or review the full retrofit emission control device range.