Can an RECD Replace a Generator’s Exhaust Silencer?

If you have just learned that your generator needs a retrofit emission control device, one question tends to surface early. The set already has a silencer, the RECD is a large steel canister that also sits on the exhaust, so surely one of them is redundant? Some owners even hope the new device can take over the silencer’s job and save a fitting.

The short answer is no. A silencer and an RECD are built to solve two different problems, they are tested against two different yardsticks, and regulators look at them through two different lenses. One handles sound energy. The other handles particulate matter and gases. Neither can stand in for the other, and taking off the silencer after fitting an RECD would trade a pollution problem for a noise problem.

This guide explains that in an unusual way. Instead of listing features, it writes a job posting for each device, then puts them through a cross-training test, a seating plan, a performance review and a symptom desk. By the end you should know exactly what each one is responsible for, where each belongs on the pipe, and what to ask a vendor before anything is welded on.

The Quick Verdict Before the Detail

Here is the whole answer in a few lines, so you can stop reading here if that is all you need.

• A silencer reduces exhaust noise. It does not, by design, reduce particulate matter or harmful gases.
• An RECD reduces pollutants, mainly particulate matter, and depending on the design, some gases. It is not rated as a noise device.
• Both create some flow restriction, so both count against the same back pressure limit set by the engine maker.
• Both belong on the line. Their order and spacing are decided by the RECD supplier, and the site survey is where that is settled.

The rest of this article earns that verdict with evidence and practical detail.

Two Vacancies on One Exhaust Line

Picture the exhaust pipe as a small workplace. Hot gas leaves the engine in violent pulses, and two roles need filling before that gas is allowed out into the neighbourhood. One role is about what the ear hears. The other is about what the lungs receive.

Every generator that meets Indian norms has, in effect, both roles filled. The noise role is usually held by a silencer, sometimes together with an acoustic enclosure. The pollution role, for older sets covered by the retrofit requirement, is held by an RECD. Nobody is doing a second person’s work; the two employees simply sit in the same corridor and are often confused for each other.

To make the differences concrete, here are the two job postings as they might be written by an exhaust-line HR department.

Job Posting One: The Silencer

Role title: Exhaust noise handler

Reports to: The acoustic compliance target for the site

Main responsibility: Take the sharp, pulsing pressure waves that come out of the engine and turn them into a much quieter outflow.

How the silencer does its job

An engine exhaust is not a smooth stream. Each firing stroke sends a burst of pressure down the pipe, and the sequence of bursts is what you hear as a loud, low, thumping roar. A silencer breaks that up. According to the engine silencer overview published by E.I. Williams, the most widely used designs are the reactive or chamber type, which use changes in cross-section area to reflect sound energy back towards the source, plus straight-through resonator designs where the gas flows through a ported tube surrounded by a resonating volume. Some designs add sound-absorbing packing as well.

None of these mechanisms filter or convert anything. The gas goes in, the pulsation is smoothed, and essentially the same chemical mixture comes out. That same overview presents silencers purely as noise-control devices, with no claim about emission reduction.

What the silencer is measured on

Noise performance is stated as insertion loss, the drop in sound level that the device provides. In India, the rules for new diesel generator sets up to 1000 kVA set a maximum sound pressure level of 75 dB(A) at one metre from the enclosure surface, and call for acoustic treatment that delivers a minimum insertion loss of 25 dB(A). These figures appear in documents such as this National Green Tribunal record quoting the DG set rules. A practical explainer on how insertion loss is tested, by comparing sound levels with and without the treatment, is available from Perfect Pollucon.

What the silencer is not hired to do

It does not capture soot. It does not oxidise carbon monoxide or unburned hydrocarbons. It carries no particulate test certificate. If a supplier ever shows you a silencer with an emission claim, ask for the laboratory report, because that would be unusual.

Job Posting Two: The RECD

Role title: Exhaust pollutant handler

Reports to: The State Pollution Control Board and the type approval on the certificate

Main responsibility: Reduce the mass of particulate matter, and depending on design, some harmful gases, that leave the exhaust stack.

How the RECD does its job

The details vary by design, and our explainer on what an RECD is and how it works walks through them. In broad terms, catalyst-and-filter designs oxidise gases such as carbon monoxide and hydrocarbons on a catalyst and hold particulate matter on a trap that is cleared by regeneration, as described on the Aceget RECD product page. Not every RECD on the market uses a filter. Cummins India’s launch release for its own device describes a filterless design based on electrostatic precipitation, with more than 70 percent particulate capture and minimal back pressure, according to the Cummins India announcement.

What the RECD is measured on

An RECD is judged by its emission performance, not its sound. Its paperwork is a test certificate from an approved laboratory, matched to a particular engine power band. Our guide to reading an RECD test certificate shows which lines to check.

What the RECD is not hired to do

It is not sold or certified as an acoustic device. Kirloskar’s RECD page says its unit keeps noise within permissible limits, according to Kirloskar’s RECD product page, which is a statement that the device does not make the site noisier. It is not a claim that the device replaces the silencer. Those are very different sentences, and mixing them up is where the “one canister does both” idea starts.

Where the Two CVs Overlap (and Why the Myth Survives)

If the jobs are so different, why do owners keep asking? Because the two devices share several surface traits, and surface traits are what you see when you walk past an exhaust line.

Trait	Silencer	RECD	Why it fools people
Sits in the exhaust path	Yes	Yes	Both are large steel bodies on the pipe
Adds flow restriction	Some	Some	Both show up on a back pressure gauge
Runs hot	Yes	Yes	Both are lagged or guarded for safety
Can be damaged by oil and soot	Yes	Yes	Both suffer from a poorly tuned engine
Has a shell that looks like a drum	Often	Often	Visual similarity
Made of stainless or coated steel	Often	Often	Same materials, different purpose

Notice that every overlap is about physical presence and shared exposure. None of them is about function. Two employees can wear the same uniform and still hold different jobs.

The Cross-Training Test: Could One Cover for the Other?

Let us put the idea to the test in both directions, the way an audit would.

Test 1: Can the RECD cover the silencer’s shift?

Some owners notice that a generator sounds slightly softer after an RECD is fitted and conclude that the noise job has been taken over. There are three problems with that conclusion.

First, the effect is accidental, not designed. Any large volume in the exhaust path can soften some frequencies. A canister with internal catalysts or passages is bound to alter the sound a little. But a design that is tuned for pollutant removal and one that is tuned for insertion loss are two engineering exercises. An incidental softening tells you nothing about whether the site would meet its acoustic limit without the silencer.

Second, nobody has tested it as one. The noise rules quoted above are written around treatment that has a stated insertion loss. An RECD certificate has no such number. If your site is ever asked to demonstrate noise compliance, you cannot point to an RECD paper and call it a silencer paper.

Third, the RECD may not be built to face raw exhaust pulsation. The Cummins release describes its RECD as installed after the DG exhaust muffler or silencer. That positioning suggests a design that expects the pulses to have already been smoothed. If a device is designed to sit downstream of the silencer, removing the silencer puts it into a condition its design did not assume. Whether a particular unit tolerates that is a question for its maker in writing, and it is not a risk worth taking to save one component.

Test 2: Can the silencer cover the RECD’s shift?

This direction is easier to dismiss. A silencer has no catalyst, no collection surface and no test data for emissions. If your generator is in the group that must be retrofitted, a silencer of any quality will not satisfy that requirement. Our explainer on why an RECD is mandatory in India covers the compliance background.

Test 3: What if the silencer is removed after the RECD is fitted?

Do not do this. Beyond the noise consequences, three things can go wrong at once. The site may exceed its acoustic limit and attract a complaint. The RECD may see gas conditions its supplier never planned for. And your RECD supplier or engine maker may point to the modification when discussing any later fault. If you ever wonder whether a change is safe, put the question to your supplier in writing first.

The Seating Chart: Where Each Device Sits on the Line

Placement is not a matter of taste. A typical exhaust line runs like this, from engine to sky.

1. Engine exhaust manifold and, on turbocharged sets, the turbocharger outlet.
2. A flexible bellows to absorb engine vibration.
3. The silencer, sometimes inside an acoustic canopy.
4. The RECD, mounted after the silencer according to the Cummins description above.
5. The remaining pipe and stack that carry the gas above roof level.

Two practical points sit inside that sequence. First, the exhaust temperature at the RECD inlet is a design input for any catalyst or regeneration process, so vendors specify where the device may be mounted and how much pipe may sit between the silencer and the RECD. Second, the RECD needs room around it for safe access, support and service, and a heavy body should never hang from a light pipe run. Those details are settled during the site survey, and our RECD installation step-by-step guide explains how the survey unfolds. If you are curious about where installers often slip up, the list in common RECD installation mistakes to avoid is worth a read before the crew arrives.

A note on canopy sets

Many modern sets have a silencer built into an acoustic canopy, sometimes with a second stage inside the enclosure. That is a good reason to read our comparison of open-frame and enclosed canopy generators and the explainer on silent and soundproof diesel generators. On a canopy set, the RECD usually sits outside or above the canopy, and the pipe that carries gas through the canopy wall needs to be planned so the acoustic seal is not broken. A hole cut carelessly can undo part of the very insertion loss the canopy was built to give.

The Shared Budget: Back Pressure

Silencers and RECDs may have different jobs, but they share one paycheque: the back pressure limit published by the engine maker. Every component in the exhaust path takes a slice, and the sum has to stay inside the allowed total. If you add an RECD to a line that already has a restrictive or clogged silencer, the combined resistance can exceed the limit even when each part looks acceptable alone.

That is why a site survey should record the existing silencer’s size and condition, and why the RECD supplier should confirm the added restriction in kilopascals or a similar unit, compared against the engine’s data sheet. Our detailed piece on how exhaust back pressure affects a DG set explains what excess restriction does to fuel use, temperature and wear, and the post on whether an RECD changes usable power output shows how to check before and after.

A useful side effect of this shared budget is a small savings opportunity. Some older sets carry a silencer that is oversized or, more often, one that is corroded, half-collapsed inside, or packed with carbon. Replacing a badly worn silencer during the retrofit can free up back pressure headroom that the RECD then uses. That is a legitimate reason to change a silencer, but the purpose is to protect the exhaust budget, not to swap one device for the other.

The Performance Review: How You Check Each Employee

You would not judge a chef by how well they clean the floor. The same logic applies here: each device has its own appraisal.

Question	Silencer	RECD
What does it deliver?	Reduction in noise level	Reduction in emissions, mainly particulate matter
Unit of measure	dB(A) insertion loss and ambient noise	Percent or mass reduction on a lab test
Who tests it?	Acoustic test at site or by the maker	Approved emission laboratory
What document proves it?	Acoustic test report or maker data	RECD test certificate and type approval
Day-to-day check	Listen and measure at the boundary	Back pressure gauge, inspection, servicing
Typical ageing sign	Rising noise, rust, rattle	Rising back pressure, visible smoke, heat
Where it fails when neglected	Noise complaint	Compliance failure and engine stress

If a vendor tries to answer a noise question with an emission certificate, or an emission question with a noise report, that is your signal to slow down.

The Symptom Desk: Who Is Responsible for This Problem?

After an RECD goes on, minor changes are normal. What matters is telling apart which device is speaking. Use the table as a first-pass triage, then bring the findings to your supplier or service engineer.

What you notice	Most likely area	First step
Exhaust is louder than before the RECD	Silencer condition, loose joint, gasket leak, or a change in pipe layout	Check joints for leaks, inspect silencer, compare with the earlier noise reading
A rattling or ringing sound from the pipe	Loose internal baffles or mounting	Stop the set, inspect supports and the silencer shell
More smoke than before	Engine tuning, fuel, or a clogged trap	Read our note on reducing smoke from DG sets, then check back pressure
Engine runs hotter or loses power	Total back pressure over the limit	Read the gauge, compare with the data sheet
Water dripping from the exhaust	Condensation on cold starts, rain ingress if no rain cap	Check drain points and the rain cap at the stack
Oil stains or wetness around the RECD	Oil carry-over from the engine	Check engine oil consumption and turbo seals
Burning smell during the first hours	Coatings curing on new metal	Usually fades, confirm with the vendor

A word about the oil row. Oil that escapes past the piston rings ends up as ash in any filter downstream. DieselNet notes that lubricant additives are generally the largest source of ash in a particulate filter, and that ash does not burn off during regeneration, as explained on its page about ash accumulation in diesel particulate filters. Keeping your engine oil consumption in check therefore protects the RECD, not only the engine. The RECD maintenance guide covers this in more depth.

Special Cases Worth Discussing

The “combined” unit

Some suppliers offer housings that integrate a silencing section and an emission section in one shell. That is a fair design choice, but read the paperwork carefully. Ask for two separate statements: the emission test certificate for the emission part, and an acoustic performance statement for the silencing part. A single line such as “silencer cum RECD” is not enough evidence. The person buying the device should know which line of the paperwork covers which function.

Older sets with no silencer at all

Some very old or heavily modified sets run with a cut-down silencer, a straight pipe or a home-made muffler. Before an RECD goes on, that is the moment to restore a proper silencer, because the noise problem existed before and will remain afterwards. Adding an RECD to a straight pipe would give you cleaner but still very loud exhaust.

Sites with strict neighbours

Hospitals, residential societies and office parks often measure noise at their boundary. If you are such a site, ask your RECD supplier for the pre-installation and post-installation acoustic readings in writing, along with the position of the measuring point. Our broader article on noise reduction strategies for diesel generators lists other treatments, such as enclosure lining, baffled air paths and vibration isolation.

Sites where the questions are stacking up

An owner who has asked whether an RECD can replace a silencer will often ask three more. Whether a cleaner-looking exhaust is proof of compliance is answered in our post on whether a smokeless DG set still needs particulate measurement. Whether a taller stack removes the need for the device is taken up in Is increasing chimney height an alternative to an RECD?. And whether the marketing line about “no maintenance” is believable is the subject of what a maintenance-free RECD claim actually means.

Questions to Put to an RECD Supplier About the Silencer

Bring these to the site survey. They take ten minutes and save weeks of argument later.

1. Where on the exhaust line should the RECD be mounted relative to the existing silencer, and what is the maximum pipe length between them?
2. Does your device tolerate raw, un-silenced exhaust pulsation, or is it designed for a smoothed flow?
3. What is the added back pressure of your device at my engine’s full-load exhaust flow, and how does it add to my current silencer and pipe restriction?
4. Should the existing silencer be inspected, cleaned or replaced as part of the job?
5. Will you record noise readings at an agreed point before and after installation?
6. If the RECD includes a silencing section, can you provide a separate acoustic statement for it?
7. What supports and flexible joints are included, so the weight of the device does not load the silencer or the turbocharger?
8. What does your maintenance schedule assume about engine oil consumption?
9. If a dispute arises about noise, back pressure or emissions, who takes the first measurement and with which instrument?

A supplier who answers these with specifics is one you can work with. A supplier who says “one device does everything” should be asked to put that in writing.

What to Do This Week

If you are planning an RECD retrofit, here is a short, practical sequence.

1. Walk the exhaust line. Note the silencer’s size, make, position and condition. Photograph it, including any rust, dents or leaks.
2. Record a baseline noise reading. A phone app is not a compliance instrument, but a basic reading at the same spot before and after gives you a useful comparison. For formal compliance, use a calibrated meter.
3. Find your engine’s back pressure limit. It is on the engine data sheet or the maker’s service documentation.
4. Request the RECD supplier’s placement drawing. It should show the silencer, the RECD, the supports and the stack.
5. Ask for the pre and post installation checks in the quotation. These should include a back pressure reading and a visual inspection of joints and mountings.
6. Keep the old silencer paperwork. If a noise complaint ever arises, you will want the history.

When you are ready to discuss a specific set, share the engine details and a photo of the exhaust arrangement through the Aceget contact page, and the team can advise on mounting position and sizing for your kVA rating.

Quick Answers

Can I remove my silencer after installing an RECD? No. The silencer is your noise device, and the RECD is not certified to replace it. Removing it could breach noise limits and put the RECD into gas conditions it may not have been designed for.

Does an RECD make the generator quieter? It may soften some frequencies as a side effect, but it is not rated for noise reduction. If noise matters, rely on a properly rated silencer and, where applicable, an acoustic enclosure.

Will the RECD need a different silencer? Not usually, but the existing silencer must be in good condition and must not push total back pressure beyond the engine limit. Your supplier should confirm this at the survey.

Is there any silencer that also acts as an RECD? A silencer alone does not carry emission approval. Combined housings exist, but each function must be documented separately.

The Bottom Line

A silencer and an RECD hold two different jobs on the same corridor. One quiets the pulses, the other cleans the gas, and both must live within the same back pressure budget. Trying to make one do the work of the other leads to a noisy site, a non-compliant one, or both. The sensible path is to keep the silencer in good condition, fit the RECD in the position the supplier specifies, measure before and after, and keep the records where an inspector can find them.