Why Two DG Sets with the Same kVA Rating May Need Different RECDs

Picture two generators standing a few metres apart in an industrial estate in Faridabad. Both nameplates say 500 kVA. Both were bought for the same job: carrying a plant through grid outages. A facility manager asks for two quotes for retrofit emission control devices and expects them to match. They do not. One vendor proposes a single device for the first set and two smaller devices for the second. The prices differ, the dimensions differ, and the delivery dates differ.

The manager suspects padding. In most cases, the vendor is right.

The kVA figure on a generator describes what the alternator can deliver to your electrical system. An RECD does not care about the alternator. It sits in the exhaust, and the exhaust is shaped by the engine: how big it is, how it breathes, how many outlets it has, how hard it works and how worn it has become. Two engines that drive the same size of alternator can produce exhaust streams that differ enough to need a different filter volume, a different inlet arrangement or a different number of devices.

This article works through a case file of two imaginary 500 kVA sets, Set A and Set B, and records every difference that affects the RECD in a running ledger. By the end you will know which questions to ask before you compare quotes, and why a quote based on kVA alone should make you cautious.

What the kVA Number Tells You, and What It Hides

A kVA rating is a statement about apparent electrical power at the alternator terminals. In India, diesel generator ratings are usually quoted at a power factor of 0.8, so a 500 kVA set is expected to deliver about 400 kWe of real power. Our explainer on how to read a generator kVA and kW rating covers that conversion.

What the kVA number leaves out is almost everything about the engine:

• The gross mechanical power of the engine at the flywheel
• The engine’s displacement and number of cylinders
• Whether the engine is turbocharged, and how its air is cooled
• The rated speed (almost always 1500 rpm for 50 Hz sets in India, but not always)
• The emission generation the engine was built to
• How many exhaust outlets the engine has, and their diameter
• How much exhaust backpressure the engine maker allows
• The condition of the engine today

Each of these shapes the exhaust gas that the RECD has to handle. That is why the CPCB procedure for RECD compliance, published as the System and Procedure for Emission Compliance Testing of RECDs, deals in engine power rather than alternator kVA. It refers to the gross mechanical power of the engine, and it allows a device family to be approved on the basis of a tested parent model.

Meet the Case File: Set A and Set B

Here are the two sets, built from typical configurations seen in the Indian market. They are composites for illustration and do not describe any particular brand.

Feature	Set A	Set B
Nameplate	500 kVA, 400 kWe, 0.8 PF	500 kVA, 400 kWe, 0.8 PF
Engine layout	Inline six-cylinder	V8
Displacement	About 15 litres	About 22 litres
Aspiration	Turbocharged, air-to-air aftercooled	Twin turbocharged, water-cooled aftercooler
Gross engine output at rated load	About 445 kWm	About 470 kWm
Exhaust outlets	One	Two (one per bank)
Year of manufacture	2019	2009
Emission generation	CPCB II	Earlier generation
Typical running load	60 to 70%	25 to 35%
Location	Acoustic canopy in open yard	Plant room inside a building

On paper, they are twins. In the exhaust, they are cousins at best. Let us work through the differences one by one and keep a ledger of what each one does to the RECD.

Difference 1: The Engine Behind the Alternator

An alternator is not perfectly efficient. A generator set’s engine has to supply the electrical output plus alternator losses plus the power for its own radiator fan and other engine-driven accessories. The engine is usually chosen with a margin above that. Set ratings themselves follow the duty categories of ISO 8528 (standby, prime and continuous), which Cummins explains well in its note on understanding ISO 8528 generator set ratings; the same engine can carry different kVA labels depending on the duty it is rated for.

Set A uses an engine that produces about 445 kWm at rated output. Set B uses a larger engine that produces about 470 kWm. The difference arises from different alternator efficiencies, a heavier mechanical radiator fan on Set B and a manufacturer who chose a more conservative engine rating.

Why does this matter? The amount of fuel burned depends on engine output, and the amount of exhaust gas depends on fuel and air. A higher gross kWm means somewhat more exhaust mass at full load, and it can place the two sets in different approval bands for device families. A device that is approved for engines up to a certain power may not cover the second engine.

Ledger entry: Ask for the engine model and its gross kWm rating, not just the set’s kVA. The engine nameplate is usually on the block or the valve cover. Our guide on which DG nameplate photos to send for an RECD enquiry shows where to look.

Difference 2: How Each Engine Breathes

Set A’s engine is a 15-litre six with one turbocharger and an air-to-air aftercooler. Set B’s engine is a 22-litre V8 with two turbochargers, one per bank, and an aftercooler cooled by engine water.

For the same 400 kWe at the alternator, the larger engine moves more air per minute, because it sweeps more cylinder volume every revolution. It is also working at a lower specific load, so it burns its fuel with more excess air. The result is a larger volume of exhaust at a lower temperature.

Set A, by contrast, works its smaller engine harder. Its exhaust volume is smaller at full load, but hotter.

Two things follow for the RECD:

1. Filter and catalyst volume. RECD designers size the substrate so that exhaust passes through at a sensible velocity. A bigger exhaust volume calls for more frontal area, more substrate or both. A paper on diesel particulate filter sizing approaches reviews how flow, pressure drop and soot loading drive substrate choice.
2. Regeneration temperature. Passive regeneration of a particulate filter relies on hot exhaust. Set A’s hotter exhaust helps. Set B’s cooler exhaust works against it, especially combined with Set B’s light running load.

The topic of aspiration deserves its own page. We cover it in detail in our guide to RECDs for turbocharged and naturally aspirated engines, published alongside this article.

Ledger entry: The same kVA can hide very different exhaust volumes and temperatures. Ask for the OEM exhaust flow and temperature at several loads.

Difference 3: One Outlet or Two

This difference is the one that surprises owners most.

Set A has a single exhaust outlet. One pipe leaves the turbocharger, runs through the silencer and exits through the stack. One RECD in that line will treat the whole exhaust stream.

Set B is a V8 with a turbocharger on each bank. It has two exhaust outlets, and in this installation they run as two separate pipes, each with its own silencer, up through the plant room roof.

There are two broad ways to treat Set B:

• Two devices, one per bank. Each device is sized for half the exhaust flow. This keeps the banks separate, as the engine maker designed them, and often fits better in a crowded plant room. It does mean two sets of sensors and two devices to maintain.
• Merge the banks into one device. A Y-piece joins the two pipes before a single larger RECD. This is possible in some cases, but it needs care. The merger must not create uneven backpressure between banks, which can upset turbocharger balance, and there must be space for a larger device and the merger fitting.

Neither approach is automatically better. The engine maker’s installation guidance and the measured space decide it. What is certain is that a quote for “one 500 kVA RECD” cannot be compared directly with a quote for “two devices for a twin-outlet 500 kVA engine”. They are not the same product.

A related question comes up when two separate generators sit side by side: can they share one device? That is a different problem, covered in our article on whether two diesel generators can share one RECD.

Ledger entry: Count the exhaust outlets on the engine and note their diameter. The answer may change the number of devices.

Difference 4: The Backpressure Budget

Every diesel engine has a maximum exhaust backpressure set by its maker. Above that limit, the engine loses power, runs hotter and smokes more. The figure is given on the engine’s technical data sheet, in kPa, mbar, mm of water or inches of water. For generator engines, it commonly sits somewhere between a few kPa and about 10 kPa, but you must read your own engine’s value.

Think of that limit as a budget. The existing exhaust system already spends part of it: the silencer, the pipe length, every bend, the rain cap and the stack all add resistance. The RECD must fit within whatever is left, including its resistance when the filter is partly loaded with soot, not just when it is new.

Now compare our two sets.

Item	Set A	Set B
OEM maximum backpressure (illustrative)	10 kPa	6.7 kPa
Existing system resistance at full load	About 3 kPa: short run, one residential silencer, open yard	About 4.5 kPa per bank: 18 m run, four bends, critical silencer, roof penetration
Budget left for the RECD	About 7 kPa	About 2.2 kPa per bank

Set A has plenty of room. Set B is tight. To fit an RECD on Set B, the vendor may have to propose a larger device with a lower pressure drop, combine the RECD with silencing to replace the existing silencer, shorten or straighten the pipe run, or use larger pipe diameters. Each of those changes the cost and the design.

Our explainer on how exhaust backpressure affects a DG set describes what happens when the budget is exceeded, and the post on whether an RECD can replace a generator’s exhaust silencer covers the combined option.

Ledger entry: Ask the vendor to show the backpressure calculation for your actual exhaust route, using your engine’s limit. Two quotes that ignore the route are not worth comparing.

Difference 5: Age, Emission Generation and Engine Health

Set A was built in 2019 to the CPCB II generation of emission norms. Set B was built in 2009, before those norms applied to its size, and has run thousands of hours since.

Older engine designs generally produce more particulate matter per unit of energy than newer ones. Wear adds to it. Worn injectors that dribble fuel, tired turbochargers that lag on load steps, worn piston rings that pass lubricating oil into the cylinders: each raises the amount of soot and ash heading for the filter. Our guide on CPCB II, CPCB III and CPCB IV+ norms explains how the generations differ.

For the RECD, higher engine-out soot means the filter fills faster. That calls for more filter capacity, more frequent attention or, more sensibly, fixing the engine first. Lubricating oil consumption also matters because oil leaves ash, which does not burn off and slowly accumulates in the filter.

For Set B, a prudent vendor will ask about oil consumption, smoke at load steps and the date of the last top overhaul before finalising a design. If the engine smokes visibly, fixing it before fitment may be cheaper than buying extra filter capacity. Our article on whether to complete an engine overhaul before fitting an RECD helps with that decision, and the guide to why a generator emits black smoke covers common causes.

Ledger entry: Share the engine’s age, hours and service history. An honest picture of engine health produces a better-sized device.

Difference 6: How Hard Each Set Works

Set A runs at 60 to 70% load for most of its hours, because it carries a production line that does not stop during outages. Set B sits at 25 to 35% for most of its hours, because it was bought for a future expansion that never arrived and now carries only lighting, offices and some pumps.

Load decides exhaust temperature, and exhaust temperature decides how readily the filter burns off its soot during normal running. At 60 to 70% load, Set A will usually keep its filter clean through passive regeneration. At 25 to 35%, Set B’s cooler exhaust may let soot build up faster than it burns, particularly with an older engine producing more of it.

The vendor’s response for Set B might include a catalyst formulation designed to assist regeneration at lower temperatures, a larger filter to extend the time between interventions, a recommendation for periodic loaded runs, or a review of whether the plant’s loads can be rearranged. Each of these shapes the specification.

We look at how to read load records in detail in our companion article on DG load logs and RECD suitability. For background, see understanding generator load factor and duty cycle and whether an RECD suits a DG set that runs only occasionally.

Ledger entry: Provide real load data, not the nameplate rating. Two identical sets with different duties may need different devices.

Difference 7: Where Each Set Lives

Set A sits in an acoustic canopy in an open yard. There is room beside it for a device on a frame, or the existing silencer position on top of the canopy can be adapted. Lifting is easy because a crane can reach it.

Set B sits in a plant room on the ground floor of a building. Its exhaust runs along the ceiling and through the roof. The plant room has a low ceiling, a cable tray along one wall and a doorway too narrow for a large assembled device.

These physical facts affect the RECD as much as any engine figure:

• Device shape. A long, slim device may suit a ceiling run where a short, wide one will not fit.
• Modular build. A device that can be brought in as sections and assembled inside the room may be needed for Set B.
• Mounting. Ceiling-hung devices need structural checks and proper supports. Ground-frame devices need floor space and clear access for inspection.
• Heat. A hot device inside a room adds to the room’s heat load, so ventilation and insulation matter.
• Access for maintenance. Whatever the position, someone must be able to reach the sensors, joints and any service openings.

Our article on whether an RECD can be installed without cutting the existing exhaust pipe and the guide to common RECD installation mistakes cover practical layout issues.

Ledger entry: Arrange a site survey. Drawings and photos help, but measured clearances decide the final design.

The Completed Ledger

Pulling the differences together, here is what the two vendors saw when they quoted for our two “identical” sets.

Factor	Set A	Set B	Effect on the RECD
Engine gross output	445 kWm	470 kWm	Possibly different device approval band
Displacement and aspiration	15 L, single turbo	22 L, twin turbo	Set B has higher exhaust volume, cooler gas
Exhaust outlets	One	Two	Set B may need two devices or a merger
Backpressure budget left	About 7 kPa	About 2.2 kPa per bank	Set B needs a low-resistance design or route changes
Age and emission generation	2019, CPCB II	2009, earlier generation	Set B has higher soot and ash loading
Running load	60 to 70%	25 to 35%	Set B regenerates less readily
Location	Open yard canopy	Indoor plant room	Set B needs a compact or modular build
Likely outcome	One standard device	Two devices or a larger low-drop device, plus engine checks

Seen this way, the two quotes stop looking inconsistent. They are answers to two different engineering problems that happen to share a nameplate number.

What This Means for Pricing

Owners sometimes look up the price of an RECD for their kVA and treat it as the answer. Our article on RECD cost in India gives typical drivers, but it is worth spelling out how the ledger above feeds into a price:

• More substrate costs more. Higher exhaust volume means more filter and catalyst material.
• Two devices cost more than one of the same total capacity, because each needs its own housing, sensors and fittings.
• Low-pressure-drop designs often mean larger housings.
• Indoor and modular builds take more fabrication and installation time.
• Pipe rerouting, supports and structural work add to the installed cost.
• Engine repairs recommended before fitment are a separate cost, though they may save money later.

A quote that comes in well below the others is worth questioning. Ask whether the vendor has accounted for the outlet count, the backpressure route and the duty. Our guide to choosing CPCB approved RECD vendors suggests questions to ask, and the article on how to read an RECD test certificate helps you check that the device offered is approved for your engine’s power.

A Specification Brief That Avoids the kVA Trap

When you request quotes, send each vendor the same brief. It saves time, and it makes the quotes comparable. Here is a template you can copy.

Generator and engine

• Generator make, model and kVA rating
• Engine make and model, from the engine nameplate
• Engine gross output in kWm at rated speed
• Rated speed in rpm
• Displacement and number of cylinders, if known
• Aspiration: naturally aspirated, turbocharged, turbocharged and aftercooled
• Year of manufacture and running hours

Exhaust system

• Number of exhaust outlets on the engine and their diameter
• Existing pipe diameter, length and number of bends to the outlet
• Silencer type (industrial, residential, critical) and its position
• Stack height and outlet arrangement
• OEM maximum exhaust backpressure
• Measured backpressure at load, if available

Operation

• Typical running load in kW or as a percentage of rating
• Typical run length and number of starts per month
• Recent load logs or controller data, if available
• Fuel type and supplier

Engine condition

• Date of last major service or overhaul
• Oil consumption, if tracked
• Any visible smoke under load or on load steps

Site

• Photos of the set, nameplates and exhaust route
• Location: canopy, plant room, rooftop, basement
• Access route for the device and lifting arrangements
• Any space restrictions

If original documents are missing, as often happens with older sets, our guide on how to specify an RECD when original DG set documents are missing explains how vendors fill the gaps.

Situations Where Same-kVA Sets Really Do Get the Same RECD

It would be misleading to suggest that every pair of same-kVA sets needs different devices. In many cases they do not. The match is most likely when:

• Both sets use the same engine model from the same maker
• Both were built in the same emission generation and are in similar condition
• Both have a single outlet of the same diameter
• Both have similar exhaust routes and backpressure margins
• Both run at broadly similar loads

A business that bought a batch of identical sets at the same time, installed them in identical canopies and runs them in rotation will often get the same device for each. That is a good argument for standardising generator purchases across a site, because it simplifies emission control, spares and training later.

Even then, the vendor should still verify each set. Two identical engines can age differently. One may have had a turbocharger replaced, another may smoke because of a faulty injector, and a third may have had its exhaust route altered during a building extension.

Common Misconceptions

“The RECD is sized by kVA, like the dual fuel kit.”

Some vendors list devices by kVA for convenience, and Aceget’s own range is presented by kVA so that owners can find a starting point quickly. Behind that list, every device is matched to engine parameters. The kVA label is a shortcut, not the specification.

“A bigger RECD is always safer.”

A larger device usually has a lower pressure drop and more soot capacity. But oversizing can reduce exhaust velocity and temperature inside the device more than necessary, and it costs more and takes more space. The goal is the right size, not the largest.

“If the device fits the pipe, it fits the engine.”

Pipe diameter is one input among many. A device that bolts neatly into the exhaust line can still overload the backpressure budget or struggle to regenerate on a lightly loaded engine.

Frequently Asked Questions

Why did I get different RECD quotes for two generators with the same kVA?

Because the RECD is matched to the engine and its exhaust, not to the alternator rating. Differences in engine power, displacement, aspiration, number of outlets, backpressure margin, age and running load can all change the device needed.

Is engine kW more important than kVA for RECD selection?

For the RECD, yes. The gross mechanical power of the engine relates directly to fuel burned and exhaust produced. CPCB’s RECD procedure also refers to engine power. The kVA figure is useful only as a starting point.

Can one RECD treat a V-engine with two exhaust outlets?

Sometimes, by merging the two banks before a single larger device. Often two devices, one per bank, is simpler and better suited to the engine’s design. The engine maker’s guidance and the available space decide.

Does an older engine need a larger RECD?

It may, because older and worn engines tend to produce more soot and ash. Fixing smoke problems before fitment can reduce the filter capacity needed and protect the device.

Will a lightly loaded set need a different RECD from a heavily loaded one of the same size?

It might. Light loads mean cooler exhaust and slower regeneration. The vendor may choose a different catalyst, a larger filter or recommend operating changes such as periodic loaded runs.

What information should I send to get comparable RECD quotes?

Engine make, model and kWm, aspiration, outlet count and diameter, exhaust route details, the OEM backpressure limit, typical load, engine condition and site photos. Use the specification brief above.

Look Past the Nameplate

The kVA rating on a generator tells you what it can supply to your building. It does not tell you what comes out of its exhaust. Two sets with identical nameplates can differ in engine power, breathing, outlet count, backpressure margin, condition, duty and location, and each of those differences can change the RECD. Treat the nameplate as the first line of the brief, not the whole brief.

Aceget supplies CPCB type-approved RECDs for DG sets from 25 kVA to 2500 kVA, and every device is matched to the engine after a survey. To get a proposal built on your actual engine and exhaust route, contact the Aceget team or browse the retrofit emission control device range, including the 500 kVA DG set RECD used as the reference size in this article.