Where Should an RECD Be Positioned Along a Long Exhaust Run?

Picture a 30 metre exhaust run. It leaves the turbocharger, drops through a flexible bellows, crosses the generator room, passes through a wall, travels along an external pipe rack and climbs a stack to roof level. Somewhere along those 30 metres, a retrofit emission control device has to go. Put it at metre 2 and it sees the hottest gas the engine can deliver. Put it at metre 28 and the gas arriving at the device may be noticeably cooler, depending on insulation, pipe size and weather.

That difference matters because an RECD’s particulate filter relies on heat to burn off the soot it collects. It also matters for backpressure, condensate, noise, structural support and how a technician reaches the device ten years from now. On short runs, the choice barely arises. On long runs, it is one of the most consequential layout decisions in the project.

This article sets out how to think about position along a run, describes three typical stations where a device can sit, and shows how to combine them with insulation and operating habits to get a layout that works.

Why Position Matters on Long Runs

Four things change as you move the device further from the engine.

Exhaust temperature falls

Hot exhaust loses heat to the pipe wall, and the pipe wall loses heat to the surrounding air. The rate depends on the temperature difference, the pipe’s surface area, wind, rain and, above all, insulation. A bare pipe outdoors on a windy night sheds heat much faster than an insulated pipe inside a warm room.

The soot-burning window narrows

The filter in an RECD traps soot. The catalyst coating, and sometimes an upstream oxidation catalyst, helps that soot oxidise at temperatures lower than it would need on its own. Even so, there is a temperature below which soot accumulates faster than it burns. DieselNet’s overview of diesel particulate filter regeneration explains the chemistry. A device that receives cooler exhaust spends less of its running time above that threshold.

Pressure losses stack up differently

Backpressure at the engine is the sum of resistances along the whole system. Moving the device does not change its own resistance much, but it changes the gas density and velocity it sees, and it changes how many bends and pipe lengths sit upstream and downstream of it. Our explainer on how exhaust backpressure affects a DG set describes why the engine cares about the total.

Access and support conditions change

A device near the engine is usually inside the generator room or canopy zone. A device at the far end may be outdoors on a rack, high on a wall or on a roof. Each brings different structural, weather and access questions.

What Decides How Much the Gas Cools

Before comparing positions, it helps to know what drives the temperature drop along a run. Engineers call this a heat loss calculation, but the main factors are easy to list.

Factor	Effect on cooling	What you can change
Pipe length before the device	Longer pipe means more heat loss	Device position
Pipe diameter	Larger pipes have more surface area but slower gas; net effect depends on the case	Usually fixed by backpressure needs
Insulation	Good insulation cuts heat loss dramatically	Add or upgrade insulation
Surrounding air temperature	Cold air increases loss	Route indoors where possible
Wind and rain on outdoor sections	Increase loss sharply on bare pipe	Weatherproof insulation cladding
Exhaust flow rate	Higher flow at higher load carries heat further	Operating pattern
Engine load	Higher load means hotter exhaust at the turbo outlet	Operating pattern

The last two are worth stressing. A lightly loaded engine produces cooler exhaust to begin with and less of it, so it cools faster along the pipe. Our guide on understanding generator load factor and duty cycle explains how load patterns vary in practice. The general physics of heat moving from a hot surface to air is covered in the heat transfer article.

Three Stations Along the Run

Most long runs offer three broad zones for the device. Think of them as stations on a line.

Station A: Close-coupled, near the engine

The device sits within a few metres of the turbocharger outlet, typically just after the flexible bellows.

What it offers

• The hottest exhaust the system has, giving the best chance of passive soot burning
• A short, simple connection with few joints before the device
• Usually indoors or within the canopy zone, sheltered from weather
• Often the easiest place for routine inspection

What it costs

• Floor or wall space in what is often the most crowded part of the room
• Heat released into the generator room or canopy, which ventilation must remove
• Weight and thermal movement close to the engine, so the bellows and supports must be well designed
• The pipe downstream still carries the full length to the outlet, so total backpressure is unchanged

Station B: Mid-run

The device sits partway along the run, often where the pipe passes through a wall or reaches a pipe rack outside the room.

What it offers

• Frees space in the generator room
• Exhaust is still reasonably hot if the upstream section is insulated
• Often lands at a point with good structural support, such as a wall or rack

What it costs

• Some temperature loss compared with Station A
• Access may need ladders or platforms if the pipe runs high
• Outdoors, weather protection becomes important

Station C: Far end, near the outlet

The device sits near the base of the stack, on the roof, or at the terminal end of a basement riser.

What it offers

• Removes the device from the generator room entirely
• Can combine with stack support structures
• On some sites, the only place with space

What it costs

• The coolest exhaust of the three positions, sometimes significantly so on uninsulated runs
• The longest pipe exposed to condensate before the device, so water management matters more
• Often the hardest place to reach for service, especially on rooftops or high stacks
• Lifting gear may be needed for every filter removal

Comparing the Stations

Consideration	Station A: near engine	Station B: mid-run	Station C: near outlet
Exhaust temperature at device	Highest	Moderate to high	Lowest
Passive regeneration at light load	Best	Fair, better with insulation	Weakest
Space used in the generator room	Most	Some or none	None
Heat added to the room	Most	Some, if indoors	None
Weather exposure	Low	Moderate	High if outdoors
Condensate reaching device	Least	Moderate	Most
Service access	Usually easiest	Varies	Often hardest
Structural demand	Floor or wall in the room	Wall or rack	Roof, stack or riser

The table shows why there is no universal answer. The station that is best for regeneration is often the most awkward for space. The station that frees the room is often the worst for regeneration and service.

Insulation Changes the Map

On a long run, insulation is the tool that most changes the comparison. Insulating the pipe upstream of the device keeps more heat in the gas, so a mid-run or even far-end device can perform closer to a close-coupled one.

Where to insulate

• The whole section between the engine and the device, as a priority, because that is where heat must be kept
• The device itself, to hold temperature through the filter and reduce heat released to the surroundings
• Flanges and joints, with removable jackets so they can still be inspected
• Downstream sections, mainly for personnel protection and to reduce heat in the room, since keeping the gas hot after the device matters less for regeneration

What kind of insulation

Outdoor pipe needs insulation with a weatherproof outer cladding, because wet insulation loses much of its value and can trap moisture against the pipe. Indoors, removable mattress-type jackets are common on flanges and fittings. Insulation must be rated for exhaust temperatures. The general principles of thermal insulation explain why trapped air and low-conductivity materials slow heat loss.

A caution

Insulation keeps heat in the pipe, which also means hotter pipe steel. That is usually fine for exhaust-grade materials, but check that supports, sliding guides and nearby services can tolerate the result. Insulation also makes corrosion under the cladding harder to see, so inspect periodically.

Matching Position to the Running Pattern

The best station depends heavily on how the generator runs. Three common patterns illustrate this.

Pattern 1: Heavy daily use

A set that runs several hours a day at moderate to high load produces hot exhaust in quantity. Even with some cooling along the pipe, a mid-run or far-end device may see enough temperature for passive regeneration. Space and access can drive the decision.

Pattern 2: Standby with short tests

A set that mostly sits idle and runs for weekly no-load or light-load tests produces cool exhaust for short periods. Every degree counts. A close-coupled position, good insulation and periodic loaded runs give the device its best chance. Our articles on choosing an RECD for a generator running at low load and whether an RECD suits a DG set that runs only occasionally cover this pattern in depth.

Pattern 3: Variable and cyclic loads

A set that swings between light and heavy load, or starts and stops several times a day, sees a mix. A close-coupled or insulated mid-run position is usually safer than a far-end one. Our guides on RECDs for generators with rapidly changing loads and RECD selection for DG sets with frequent short start-stop cycles explain the effects.

Load logs make this analysis concrete. If the site has them, use them; our article on using DG load logs to assess RECD suitability shows how.

Water on a Long Run

Long runs collect more condensate than short ones. When a cold engine starts, water vapour in the exhaust condenses on cold pipe walls. Over a long run, that water can be considerable, and it runs to the lowest points.

Rules for keeping water away from the device

• Know the low points. Mark them on the drawing. The device should not sit at the lowest point of the run unless it has a drain.
• Slope the pipe. Horizontal sections should fall gently towards a drain, not towards the device or the engine.
• Drain before the device. A condensate drain just upstream of the device catches water before it reaches the filter.
• Protect the outlet. A rain cap or angled outlet stops rain running down the stack into a far-end device.
• Avoid water traps. A dip in the pipe, such as where it drops under a beam and rises again, collects water that the next start blows into the device.

Our explainer on why a generator emits white smoke notes that water vapour is a normal part of exhaust, which is why condensate management is so important on long systems.

Backpressure on a Long Run

A long run starts with more backpressure than a short one, before any device is added. That limits how much the device can add.

When comparing positions, ask the vendor to calculate total backpressure with the device at each candidate station, at full load. The figures may be close, because the device’s resistance is similar wherever it sits, but layout details around each station can change the total. For example:

• A Station A device that forces two extra bends to fit beside the engine adds those bends’ resistance
• A Station B device that can sit in a straight section adds none
• A Station C device that replaces an existing silencer at the outlet may reduce the total

Engine makers publish a maximum allowable exhaust backpressure. Our guide on using OEM exhaust flow data to specify an RECD explains where to find it, and our companion post on reviewing bends and reducers in a proposed layout shows how individual fittings add up.

Sensors and Monitoring at a Distance

When the device is far from the engine, monitoring becomes more important and more awkward.

• Temperature at the device inlet tells you what the filter actually sees. On a long run, that may differ greatly from the turbo outlet temperature. A sensor here is valuable.
• Backpressure should be measured at the device and, ideally, at the engine outlet as well, so you can tell whether a rise comes from the filter or from somewhere else in the run.
• Display location matters. A far-end device on a roof needs its readings brought down to where staff can see them, or sent to a remote monitoring system.

The RECD maintenance guide explains how readings support service decisions.

A Walkthrough: Commercial Complex in Noida

This is an illustrative case.

The site

• A 1010 kVA set in a ground-floor generator room at the rear of a commercial complex
• Exhaust leaves the canopy, crosses 6 metres of room, passes through the external wall, runs 14 metres along an external pipe rack, then climbs 12 metres up a stack fixed to the building
• About 32 metres of pipe in total, with five bends; the existing pipe was uninsulated outside the room
• The set runs for weekly 30-minute tests at light load and grid outages averaging 20 hours a month, often at 40 to 60 percent load

The candidate stations

Station	Location	Notes
A	Inside the room, after the bellows	Floor space limited; would require moving a cable tray and adding one bend
B	On the pipe rack, 2 metres after the wall	Rack strong enough; clear access from a service road; outdoors
C	At the stack base	Space available; exhaust coolest; condensate from the stack would run back into the device

What the analysis showed

The vendor’s heat loss estimate suggested that, on the uninsulated run, gas reaching Station C at light load would be considerably cooler than at Station A, enough to make passive regeneration unreliable during the weekly tests. Station B, with the upstream section insulated, would see a much smaller drop.

Backpressure was similar at A and B, slightly higher at A because of the extra bend.

The decision

Station B. The team:

1. Insulated the pipe from the canopy outlet to the device with weatherproof cladding
2. Mounted the device on the pipe rack with a fixed saddle at the inlet end and a sliding saddle at the outlet end
3. Added a condensate drain just before the device and sloped the rack section towards it
4. Fitted a rain cap at the stack outlet
5. Placed an inlet temperature sensor and backpressure tapping on the device, with readings sent to a panel inside the generator room
6. Changed the weekly test to include 20 minutes at a meaningful load using the building’s own loads, which the facility team could arrange during low-occupancy hours

Results, illustrative

Inlet temperature at the device during outages stayed within the range the vendor considered suitable for passive regeneration. Backpressure readings rose slowly between services, as expected, without sudden jumps. Large sets in multi-tenant buildings share many of these issues; our guide on RECDs for shopping malls covers coordination. The 1010 kVA DG set RECD page shows the rating used here.

A Station Selection Worksheet

Use this worksheet to record the facts for each candidate position before deciding.

Item	Station A	Station B	Station C
Distance from turbo outlet (m)
Upstream pipe insulated? (Y/N)
Indoors or outdoors
Estimated inlet temperature at typical load
Estimated inlet temperature at lowest regular load
Extra bends needed to fit the device
Total system backpressure at full load
Low point or water trap upstream?
Drain possible just before device?
Structural support available
Service access: standing position and filter withdrawal
Lifting need for filter removal
Heat released into an occupied or enclosed space

Fill in what you know and ask the vendor to supply the temperature and backpressure estimates. Discuss the lowest regular load line first; it often decides the outcome.

Mistakes Seen on Long Runs

Putting the device wherever space is free

Space is important, but a device that never gets hot enough will load with soot and need frequent intervention. Check temperature before accepting a convenient position.

Ignoring the uninsulated section

A device placed mid-run on an uninsulated pipe can perform far worse than the same device after a modest insulation upgrade. Insulation is often cheaper than moving the device.

Placing the device at a low point

Water collects at low points and is blown into the filter on the next start. Keep the device above low points or drain upstream.

Forgetting the stack

Rain running down a stack into a far-end device is a common, avoidable problem. Fit a rain cap.

Leaving readings out of sight

A far-end device with a gauge nobody looks at is effectively unmonitored. Bring readings to where people work.

Our list of common RECD installation mistakes adds errors that apply on any run.

Compliance Note

Device approval is covered by CPCB’s System and Procedure for Emission Compliance Testing of RECDs. The vendor’s installation instructions may include limits on inlet temperature, distance from the engine or maximum pipe length before the device. Follow them, and keep the heat loss estimate, backpressure calculation and drawing with your installation records. Those documents help during inspections; our article on what happens during a pollution control board inspection explains what is typically checked.

Frequently Asked Questions

Should an RECD always be fitted as close to the engine as possible?

Closer usually means hotter exhaust and better regeneration, which is a strong advantage. It is not always practical, though, because of space, heat in the room and support. On long runs, a well-insulated mid-run position is often a good compromise.

How much does exhaust cool along a long pipe?

It depends on length, insulation, pipe size, surrounding air, wind and engine load. Uninsulated outdoor pipes at light load lose heat fastest. Ask the vendor for a heat loss estimate for your specific run.

Can insulation make a far-end position work?

Often it helps a great deal. Insulating the pipe between the engine and the device keeps more heat in the gas. It may not fully match a close-coupled position, especially for very light loads, but it can narrow the gap.

Does device position change backpressure?

The device’s own resistance changes little, but the layout around each position can add or remove bends and straight lengths. Calculate total backpressure for each option.

Is a rooftop or stack-base position a bad idea?

Not always. It suits sets with healthy load and running time better than lightly loaded standby sets. Rain protection, drainage, lifting and access need careful planning.

What if the only space is at the far end?

Insulate the upstream pipe, manage condensate, monitor inlet temperature and consider changing the operating pattern to include regular loaded runs. Discuss device options suited to cooler exhaust with the vendor.

Choose the Station, Then Support It

On a long exhaust run, position is not a minor detail. It sets the temperature the filter works at, the water it has to cope with, the backpressure the engine sees and the effort every service visit will take. Start with the running pattern, estimate temperature at each candidate station, insulate where it counts, keep water away and bring readings to where people can see them. The device will repay that care with fewer surprises over its life.

Aceget models exhaust runs during site surveys, including heat loss and backpressure at each candidate position, and supplies type-approved RECDs for long and complex runs. To plan your device position, reach out to Aceget, or explore ratings on the retrofit emission control device page.