RECD Fitment on a DG Set with Multiple Exhaust Outlets

Climb onto the roof of a large generator enclosure at an IT park, a data centre or a big hospital and you will often find not one exhaust stack but two, sometimes four. Each rises from its own silencer, and each traces back to its own turbocharger on one side of a V-shaped engine. To the facility team, the set is one machine with one nameplate. To the exhaust system, it is two or more engines sharing a crankshaft.

When such a set needs a retrofit emission control device, the first question is not which model to buy but how many, and where to put them. One device per outlet? One device fed by a collector that joins the outlets together? A single housing with two inlets? Each choice has consequences for backpressure, balance between the engine’s banks, monitoring, space and approvals.

This guide starts by explaining why sets have multiple outlets and how to identify your layout, then compares the three main fitment architectures side by side, sets out the five things that must stay balanced and closes with a commissioning method and a worked example on a 1750 kVA set.

Why a Generator Has More Than One Exhaust Outlet

Most large DG engines above roughly 750 kVA, and many below, use a V configuration: two banks of cylinders set at an angle on a common crankcase. V12 and V16 engines are common in the 1000 kVA to 2500 kVA range. The Wikipedia article on the V engine describes the layout.

Each bank has its own exhaust manifold. Engine makers usually fit one turbocharger per bank, sometimes two per bank on the largest engines. Each turbocharger has its own outlet. Unless the package designer joins those outlets together, each one leads to its own silencer and stack. The twin-turbo entry explains the common arrangements.

There are other reasons too:

• Package design. Some canopy designers split a single turbocharger outlet into two smaller silencers to fit within the enclosure height.
• Retrofit history. An older set may have had its exhaust modified over the years, with separate stacks added during a relocation.
• Multiple sets in one room. Several generators side by side each have their own outlets. That is a different situation, covered in our article on whether two diesel generators can share one RECD.

Identify Your Layout First

Before talking to a vendor, record which of these layouts you have. Photographs of the engine, turbochargers, silencers and roof outlets make this easy.

Layout	What you see	Typical engines
Single turbo, single outlet	One turbocharger, one pipe, one silencer	Inline engines and smaller V engines
Single turbo, split outlet	One turbocharger feeding a Y-piece and two silencers	Some canopy packages
Twin turbo, twin outlets	One turbocharger per bank, each with its own pipe and silencer	Many V12 and V16 engines
Twin turbo, merged outlet	One turbocharger per bank, joined in a collector before a single silencer	Some open-frame or plant room installations
Quad turbo, two or four outlets	Two turbochargers per bank, joined in pairs or fully separate	The largest engines

The engine maker’s installation manual usually shows the exhaust arrangement and gives exhaust flow data per outlet or for the whole engine. Our article on using OEM exhaust flow data to specify an RECD explains how to read those figures, and the guide on how engine displacement affects RECD selection covers the link between engine size and exhaust volume.

Three Ways to Fit an RECD

Once you know your layout, there are three main architectures.

Architecture A: One RECD per outlet

Each turbocharger outlet keeps its own path, and each path gets its own device. A twin-turbo V16 becomes a set with two RECDs, one for each bank.

Strengths

• Each bank keeps its original exhaust geometry, so the engine breathes as its maker intended
• Each device handles about half the total flow, so devices are smaller and easier to lift and place
• Backpressure and temperature can be monitored per bank, which is a powerful diagnostic tool
• If one device needs service, the other bank’s data still shows engine health

Limitations

• Two devices cost more than one in hardware, supports and sensors
• Space for two devices and two stacks is needed
• The two devices must be matched closely so backpressure stays balanced between banks

Architecture B: Merge the outlets into one RECD

The outlets are joined by a collector or Y-piece downstream of the turbochargers, and a single larger device treats the combined flow.

Strengths

• One device, one set of sensors, one stack
• Hardware cost may be lower for some sizes
• A single device may fit an existing single-stack route

Limitations

• The collector must be designed carefully so that one bank’s exhaust flow does not push back into the other, and so that each turbocharger sees a similar backpressure
• The single device is large and heavy, which complicates lifting, especially on rooftops
• Monitoring shows only the combined condition; a problem on one bank is harder to spot
• Changing the exhaust geometry may need the engine maker’s agreement

Architecture C: A single housing with twin inlets

Some vendors offer a device with two inlet connections feeding a common housing, sometimes with internal separation between the two flows.

Strengths

• One housing and one support structure, but with separate connections for each bank
• Can preserve some separation of the two flows, depending on design

Limitations

• Available only from some vendors and in some sizes
• Internal flow distribution must be designed for the specific engine
• If the two flows mix inside, per-bank monitoring may be limited

The Three Architectures Side by Side

Factor	A: One RECD per outlet	B: Merged into one RECD	C: Twin-inlet single housing
Change to engine exhaust geometry	Minimal	Significant (collector added)	Moderate
Backpressure balance between banks	Depends on matching the two devices	Depends on collector design	Depends on internal design
Per-bank monitoring	Natural	Difficult	Possible with some designs
Device size and weight	Two medium devices	One large device	One large device
Space needed	Two devices, two stacks	One device, one stack	One device, one or two stacks
Hardware cost	Often higher	Often lower	Varies
Diagnostic value	High	Lower	Medium
Engine maker agreement needed	Usually straightforward	Often required	Often required

For most twin-turbo V engines, Architecture A is the most common choice, because it keeps the engine’s exhaust as designed and gives per-bank insight. Architecture B suits some layouts where space allows only one stack, provided the collector is engineered properly. Architecture C is worth considering where a vendor offers a proven design for your engine.

The Five Things That Must Stay Balanced

Whatever architecture you choose, a multi-outlet engine depends on balance between its banks. These are the five points to watch.

1. Backpressure balance

If one bank sees more exhaust restriction than the other, its turbocharger works differently, its cylinders breathe differently and its exhaust temperature rises. Over time, the imbalance can affect turbocharger life and smoke.

What to do: in Architecture A, specify matched devices of the same model and size, with identical pipe lengths, bends and fittings where possible. In Architecture B, insist on a collector designed to equalise backpressure between banks. In every case, measure backpressure at each turbocharger outlet during commissioning. Our explainer on how exhaust backpressure affects a DG set describes the effects of excessive restriction.

2. Soot loading balance

Two devices loaded by two banks will not load at exactly the same rate. A slight difference is normal. A growing difference means one bank is producing more soot, from a dirty injector, a turbocharger problem or a leaking intake on that side.

What to do: monitor each device and compare trends. Agree with the vendor what difference between banks should trigger an investigation. This comparison is one of the main advantages of Architecture A.

3. Temperature balance

Exhaust temperature per bank is one of the best indicators of engine health. Many large engines already show per-cylinder or per-bank exhaust temperatures on their control panel.

What to do: record inlet temperature for each device. A difference that appears suddenly or grows over time points to an engine problem on one bank.

4. Thermal expansion and support

Two hot pipe runs side by side expand at slightly different rates. A merged collector carries two expanding pipes into one rigid joint. Without proper allowance, the stress is transferred to turbocharger outlets, flanges and supports.

What to do: fit a flexible bellows at each turbocharger outlet, support each device independently of the engine, and design collectors with expansion joints on each leg. Our articles on how to prevent generator vibration issues and common RECD installation mistakes cover the support side.

5. Service balance

When one device is cleaned and the other is not, they start from different soot and ash levels. Backpressure becomes unbalanced until both settle again.

What to do: service paired devices together. If a spare filter module is held, hold one for each device or a matched pair.

Approvals and Documentation

RECDs are approved under CPCB’s System and Procedure for Emission Compliance Testing of RECDs, with approvals linked to engine power ranges. A multi-outlet engine raises specific questions that are worth settling in writing:

• For Architecture A, each device treats roughly half the engine’s exhaust. Ask the vendor how their approval documentation covers a twin installation on an engine of your total power, and how the installation should be recorded for your registration or consent
• For Architectures B and C, confirm that the single device’s approval covers the engine’s full power
• For all, keep the drawings showing the exhaust arrangement with the approval documents, so an inspector can see how the engine’s full exhaust is treated

Our guides on how to read an RECD test certificate and how to verify a CPCB tested RECD explain what to look for.

Space, Lifting and Layout

Large sets with multiple outlets usually sit in plant rooms, on rooftops or in large acoustic enclosures, and space is rarely generous.

Plan the route on a drawing

Mark both turbocharger outlets, the existing silencers and stacks, the available floor or roof area and the structural capacity. Sketch each architecture on the same drawing before choosing.

Consider the silencers

Each outlet usually has its own silencer. In some layouts, each RECD can take the place of its bank’s silencer, saving space. Our article on whether an RECD can replace a generator’s exhaust silencer explains when that works and what to check for noise.

Plan the lift

A single large device for Architecture B may need a crane or a structural opening. Two smaller devices for Architecture A may be lifted with simpler equipment. On rooftop installations, this alone can decide the architecture.

Keep service access

Each device needs space for filter removal. Paired devices placed too close together may block each other’s access.

The RECD installation process and the guide on whether an RECD can be installed without cutting the existing exhaust pipe cover practical fitting steps.

Noise, Stacks and Neighbours

Replacing two silencers with two RECDs, or merging two outlets into one, changes how the set sounds and where its exhaust discharges.

• Noise. A particulate filter attenuates some exhaust noise, but its acoustic behaviour differs from a purpose-built silencer. If an RECD replaces a silencer, ask the vendor for expected noise performance, and check against the limits that apply to your set. Our article on noise reduction strategies for diesel generators covers the wider picture.
• Stack height and position. Merging two outlets into one stack may change where the plume rises and how it disperses near windows and air intakes. Check stack height requirements in your consent conditions before changing the number or position of stacks.
• Discharge direction. Two stacks close together can interact, with one plume drawn into the other. Keep outlets far enough apart, or merge them into a single properly sized stack.
• Rain protection. Every outlet needs a rain cap or bent outlet. On a twin installation, one neglected cap lets water into one bank’s device while the other stays dry.

Questions to Put to the Vendor

1. Which architecture do you propose for our layout, and why?
2. What is the predicted backpressure at each turbocharger outlet at full load, with normal soot load?
3. How will you keep backpressure balanced between banks, and what difference do you consider acceptable?
4. Will monitoring record each bank separately, and will it alert on a growing difference?
5. How does your approval documentation cover our engine’s full power in this configuration?
6. What supports, bellows and expansion joints are included for each path?
7. How will the devices be lifted into place, and what structural checks are needed?
8. How long does servicing take for each device, and can they be serviced together?
9. Has the engine maker or dealer reviewed the proposed exhaust arrangement?

Commissioning a Multi-Outlet Installation

Commissioning should prove that both banks are balanced and that total backpressure is within limits.

1. Baseline the engine before fitment. Record exhaust temperature and, if possible, backpressure at each turbocharger outlet at several loads with the old exhaust system.
2. Fit and inspect. Check supports, bellows, sensor positions and insulation on each path.
3. Measure at several loads. Record backpressure and temperature for each bank at 25%, 50%, 75% and, if possible, full load.
4. Compare banks. Agree an acceptable difference with the vendor and the engine maker’s guidance. Investigate any larger gap before handover.
5. Set alarms per device. Each device should have its own backpressure alarm, plus an alert if the difference between banks exceeds the agreed figure.
6. Recheck after the first weeks. Inspect clamps, supports and bellows once the system has been through several heating cycles.

A Worked Example: IT Park in Noida

This illustrative case shows how the choice was made on one set.

An IT park in Noida runs three large sets for backup. The largest is a 1750 kVA set with a V16 engine and two turbochargers, one per bank. Each turbocharger leads to its own residential-grade silencer and stack on the plant room roof. The roof has limited structural capacity and no crane access except by a large mobile crane from the street, which needs traffic permission.

Comparing the options

Factor	Architecture A	Architecture B
Exhaust geometry	Unchanged	New collector needed above the engine
Lifting	Two medium devices, liftable with a roof-mounted gantry	One large device, needs mobile crane
Roof loading	Spread over two existing stack positions	Concentrated at one point; structural check needed
Monitoring	Per bank	Combined only
Engine maker view	No objection	Asked for collector design review
Cost	Higher hardware cost	Lower hardware cost, higher lifting and structural cost

The facility team chose Architecture A, with each RECD replacing its bank’s existing silencer.

What commissioning revealed

At 75% load, the left bank’s device inlet ran about 35°C hotter than the right bank’s, and its backpressure trend rose slightly faster in the first fortnight. The vendor and the engine service provider traced the difference to a partly blocked charge air cooler passage and a worn injector on the left bank. After repairs, the two banks ran within a few degrees of each other.

Without per-bank monitoring, the imbalance would have stayed hidden, and the left bank’s device would have loaded faster for months.

Ongoing routine

• Weekly comparison of the two backpressure trends
• Alert if the difference between banks exceeds the agreed threshold
• Both devices serviced together
• Engine per-bank exhaust temperatures reviewed alongside RECD data at each service

IT parks and data centres share many traits, including frequent testing of large standby sets; our guide on RECDs for data centres covers testing and redundancy.

Misconceptions About Multi-Outlet Sets

“Two outlets means the engine needs twice the RECD capacity.”

The total exhaust flow is the same whether it leaves through one outlet or two. What changes is how that flow is divided. Two devices each handle about half; one merged device handles the whole.

“Any Y-piece will do for merging the outlets.”

A poorly designed collector can push one bank’s exhaust against the other, unbalance the turbochargers and create stress at the joint. Merging needs an engineered collector and, ideally, the engine maker’s review.

“If the total backpressure is fine, the banks must be fine.”

An average can hide a problem. One bank may sit near its limit while the other is comfortable. Measure each outlet.

“One device per bank doubles the maintenance.”

Two devices mean two filters to service, but they are serviced together and each is smaller and easier to handle. The per-bank data often saves more effort in engine troubleshooting than the extra service time costs.

Common Questions About Multi-Outlet Sets

My DG set has two exhaust outlets. Do I need two RECDs?

Not necessarily, but it is the most common approach for twin-turbo V engines. Two devices keep the exhaust geometry unchanged and allow per-bank monitoring. A merged single device is possible with a properly designed collector and the engine maker’s agreement.

Can I join both exhaust outlets and fit one larger RECD?

Yes, in some layouts. The collector must keep backpressure balanced between banks and allow for thermal expansion. Check with the engine maker before changing the exhaust arrangement.

Why does one bank’s RECD load faster than the other?

Usually because that bank is producing more soot, from injector, turbocharger or air supply problems. A growing difference between banks is a useful early warning of engine faults.

Should both RECDs be cleaned at the same time?

Yes. Servicing paired devices together keeps backpressure balanced between the banks.

How is approval handled when two devices treat one engine?

Ask the vendor to explain in writing how their approval documentation covers a twin installation on your engine’s total power, and how it should be recorded with your registration or consent.

Does a multi-outlet set need a different backpressure limit?

The engine maker’s limit usually applies at each turbocharger outlet. Measure and keep backpressure within the limit at each outlet, not only on average.

Two Banks, One Balanced System

A generator with more than one exhaust outlet is really several exhaust systems feeding one engine, and an RECD installation has to respect that. Identify your layout, choose the architecture that suits your space, lifting and monitoring needs, keep backpressure, soot, temperature, expansion and service balanced between banks, and settle the approval documentation in writing. Do that, and the extra outlet becomes an advantage: a second set of eyes on the engine’s health.

Aceget designs multi-outlet RECD installations for large V-engine sets and supplies CPCB type-approved devices for engines up to 2500 kVA. For the size in this article’s example, see the 1750 kVA DG set RECD; larger engines are covered on the 2500 kVA DG set RECD page. To review a twin or quad outlet layout, send your drawings to Aceget, or browse the full retrofit emission control device range.