RECD Exhaust Bends and Reducers: Reviewing a Proposed Layout

The drawing arrives as a PDF attached to the vendor’s quotation. It shows the generator, a new retrofit emission control device, and a line of pipe connecting them to the stack. Along that line are small symbols that are easy to skim past: a 90 degree elbow here, a reducer there, another pair of elbows to clear a beam. Each one looks harmless. Together, they decide whether the engine breathes freely or struggles against its own exhaust.

This article is a reviewer’s guide. It explains what bends and reducers do to exhaust flow, why they matter more once an RECD is added, and how to mark up a proposed layout with useful comments before anything is fabricated. It is written for facility engineers, consultants and owners who want to ask the right questions, not for pipe stress specialists.

Why Fittings Deserve Attention After an RECD

Before the retrofit, the exhaust system had a certain backpressure: the resistance the engine pushes against to expel its exhaust. The engine maker sets a maximum. Most original installations sit comfortably below it.

Adding an RECD adds a significant new resistance, because exhaust has to pass through the filter walls. That uses up much of the margin. Every bend and reducer in the new layout then competes for whatever margin is left. A layout that would have been fine before the retrofit can push the total over the limit afterwards.

The consequences of excessive backpressure include higher exhaust temperatures, more fuel used, more smoke and, in the long run, stress on turbochargers and valves. Our explainer on how exhaust backpressure affects a DG set covers this in detail.

There is a second reason fittings matter. An RECD’s filter works best when exhaust spreads evenly across its face. A bend or reducer immediately before the inlet can skew the flow, so part of the filter loads with soot faster than the rest. Our plain-language explainer on what an RECD is and how it works describes the filter structure that makes even flow so important.

A Short Primer on How Fittings Resist Flow

Every pipe resists flow through friction along its walls. Fittings add extra resistance because they force the flow to turn, speed up, slow down or swirl. Engineers call these extra losses minor losses, though on a short exhaust system with several fittings they can be anything but minor. The minor losses in pipe flow article explains the idea.

Two common ways to express a fitting’s resistance are:

• A loss coefficient, a number that multiplies the flow’s velocity pressure to give the pressure lost at that fitting. A higher coefficient means more loss.
• An equivalent length, the length of straight pipe that would cause the same loss. A bend might be described as equivalent to a certain number of pipe diameters of straight pipe.

Published tables, such as the Engineering ToolBox’s list of minor loss coefficients for pipe and tube components, show how widely values vary between fitting types. For an exhaust system, the vendor or engine maker’s calculation method should be used, because exhaust is hot, low-density gas and the details matter.

The key point for a reviewer is this: loss rises with the square of velocity. A fitting in a section where the gas moves fast, because the pipe is small, costs much more than the same fitting in a larger pipe.

The Fittings You Will See on a Drawing

90 degree elbows

The most common fitting. The tightness of the bend matters a great deal.

Elbow type	Description	Relative resistance
Long radius	Bend radius about 1.5 times the pipe diameter	Lower
Short radius	Bend radius about equal to the pipe diameter	Higher
Mitred, single cut	Two straight pieces welded at an angle with no curve	Highest
Mitred, multi-segment	Three or more segments approximating a curve	Between short radius and single mitre
Smooth large radius bend	Radius several times the diameter, often pulled from pipe	Lowest

Single-cut mitres are cheap to fabricate on site and are sometimes drawn as plain right angles. They should be questioned wherever space allows a proper bend.

45 degree elbows

Less resistance than a 90 degree elbow. Two 45 degree elbows with a short straight between them often cost less than one 90 and can sometimes fit where a 90 is drawn.

Offsets and doglegs

A pair of bends used to shift the pipe sideways or up and down, typically to clear a beam, duct or door. Each offset adds two fittings. A layout with three offsets has six bends that may not be needed if the route is rethought.

Reducers and expanders

A reducer joins a larger pipe to a smaller one; an expander, sometimes called an increaser, does the opposite. On RECD layouts, they appear at the device inlet and outlet, where the device flange size differs from the pipe size, and where pipe size changes along the run.

Feature	Concentric	Eccentric
Shape	Centre lines of both ends aligned	One side flat, so the centre line shifts
Common use	Vertical runs; device inlet and outlet	Horizontal runs where drainage or clearance matters
Effect on condensate	In horizontal runs, can form a step where water sits	Flat side at the bottom lets water drain
Effect on flow	Symmetrical	Slightly asymmetrical

The angle of the taper also matters. A gradual taper loses less than an abrupt step. Sudden enlargements, where a small pipe meets a large one with no taper, waste energy as the jet spreads out; the vena contracta effect at sudden contractions does the same in reverse.

Tees and branches

Rare on a single-engine RECD layout, but they appear where a drain connection, bypass or old branch remains. A dead branch can collect condensate and soot. Our article on whether two diesel generators can share one RECD explains why shared manifolds with branches are usually avoided.

The Redline Review: Ten Comments to Look For

Print the drawing, take a red pen and work along the exhaust from the engine outlet to the terminal. At each fitting, ask whether one of these comments applies.

Comment 1: “Is this bend needed?”

Each fitting should have a reason. Ask why it is there. Common answers include clearing a beam, lining up with a wall penetration or turning towards the stack. Sometimes the answer is that the drafter followed the old route without questioning it. If moving the device or rotating it would remove the bend, say so.

Comment 2: “Can this mitre become a long radius bend?”

Single-cut mitres are a common source of avoidable resistance. Wherever space allows, request a long radius or large radius bend instead.

Comment 3: “Can two 90s become two 45s?”

An offset made from two 90 degree elbows can often be made from two 45s with a short straight between them, with less loss and sometimes less space.

Comment 4: “What is the straight length before the device inlet?”

Ask the vendor how much straight pipe they want before the inlet for even flow. Check that the drawing provides it. A bend bolted directly to the device inlet flange is a common problem. If space truly prevents a straight run, ask whether the vendor’s inlet section has a flow distributor designed for close bends.

Comment 5: “Why is the pipe reduced here?”

Reducers often appear because the device flange is smaller than the existing pipe. Ask whether the device can be supplied with a matching flange, or whether the reducer can be made more gradual. A reducer that creates a small-diameter section followed by several bends is expensive in backpressure because of the higher velocity.

Comment 6: “Is this reducer concentric or eccentric, and which way up?”

In a horizontal run, a concentric reducer can create a step where condensate collects. An eccentric reducer with its flat side at the bottom lets water drain. Check the drawing specifies the type and orientation.

Comment 7: “Where does water go at this bend?”

Bends that turn the pipe downward and then up again create a low point. Mark every low point and confirm that a drain is shown or that the bend can be eliminated. Water collecting just before the device is the worst case.

Comment 8: “Is the expander after the device sized and tapered sensibly?”

At the device outlet, the pipe may expand back to a larger size. An abrupt expansion wastes pressure. A gradual taper recovers some of it. The general principle is described in the diffuser article.

Comment 9: “Has the total been calculated with these fittings?”

The vendor’s backpressure figure should include every fitting on the drawing, not a generic allowance. Ask to see the calculation, with each fitting listed, and the total compared with the engine maker’s limit at full load.

Comment 10: “Is the bend supported?”

Bends experience thrust from the gas turning and from thermal growth. A bend at the top of a riser or at a direction change needs a support or guide. Unsupported bends near the engine put load on the flexible bellows and turbocharger.

Where Fittings Hurt Most

Not every bend costs the same. Use this guide to prioritise review comments.

Location	Why it matters	Review priority
Immediately before the device inlet	Skews flow across the filter face	High
In a reduced-diameter section	High velocity makes losses larger	High
Just after the turbocharger	Hottest, fastest gas; also loads the bellows and turbo	High
In a long, already marginal system	Little margin left for extra losses	High
Just after the device outlet	Gas is still hot and fast; expander shape matters	Medium
In a large-diameter section near the terminal	Lower velocity; losses smaller	Lower

Equivalent Length: A Reviewer’s Shortcut

Without running the full calculation yourself, you can compare two layouts roughly by thinking in equivalent lengths. Ask the vendor for the equivalent length they use for each fitting type in their calculation, then add them up for each option along with the straight lengths.

An illustrative comparison might look like this. The figures are placeholders for the method; use the vendor’s values for real decisions.

Element	Layout A (as drawn)	Layout B (revised)
Straight pipe	12 m	13 m
Long radius 90 degree elbows	2	3
Single-cut mitred 90 degree elbows	3	0
45 degree elbows	0	2
Abrupt reducer at device inlet	1	0
Gradual reducer at device inlet	0	1
Total equivalent length, using vendor values	Higher	Lower

Layout B is slightly longer in straight pipe but replaces high-loss fittings with low-loss ones. The result is usually lower total resistance, even though the route looks more complicated on paper.

The Device Itself as a Fitting

The RECD has its own inlet and outlet geometry. Some designs have axial inlets in line with the housing; others have side inlets or outlets at right angles. A side-inlet device effectively includes a bend in its own body. That can save a separate elbow, but it should be counted once, not twice.

Ask the vendor:

• Does the device’s quoted pressure drop include its inlet and outlet cones?
• What inlet configurations are available?
• What straight length, if any, is needed before an axial inlet?
• How does the device’s pressure drop rise as the filter loads with soot and ash?

That last question matters because the clean-filter figure is the best case. Backpressure rises between cleanings. DieselNet’s overview of ash accumulation in particulate filters explains why the residue that cannot be burned keeps building up. The layout should leave margin for a loaded filter, not just a clean one. How quickly the filter loads on your site depends on running pattern, which is why DG load logs are worth sharing with the vendor, and why the cleaning intervals in the RECD maintenance guide should be read alongside the layout.

Vertical Bends and Risers

Where the exhaust turns from horizontal to vertical, three extra points apply.

• Condensate collects at the foot of risers. Provide a drain at the lowest point, ideally with a small pocket below the bend.
• The bend carries the riser’s weight unless the riser is independently supported. Check that the drawing shows a support at or near the base.
• Thermal growth of the riser pushes on the bend. A tall riser grows noticeably when hot. The bend and any horizontal pipe connected to it must accommodate that movement.

A Walkthrough: Printing Plant in Kolkata

This is an illustrative case.

The proposed layout

• A 700 kVA set in an open generator shed beside a printing plant
• The vendor’s drawing placed the RECD on a floor frame 3 m from the engine, connected through the bellows, a short-radius 90 degree elbow, a 1.2 m horizontal section, a single-cut mitre turning up, a concentric reducer to the device’s smaller inlet flange and the device
• Downstream: an abrupt expansion back to the original pipe size, two mitred 90s to clear a roof truss and a vertical stack

The redline comments

No.	Location	Comment
1	Short-radius elbow after bellows	Replace with long radius; space allows it
2	Single-cut mitre before device	Replace with a long radius bend
3	Concentric reducer at inlet	Ask vendor for a device with a matching flange size, or a longer gradual reducer
4	Distance from last bend to device inlet	Only about 300 mm; vendor to confirm inlet tolerance or relocate device to gain straight length
5	Abrupt expansion at outlet	Replace with a tapered expander
6	Two mitred 90s at roof truss	Rotate the device on its frame so the outlet faces the stack line directly, removing both
7	Low point between elbow 1 and mitre	Add drain
8	Backpressure calculation	Recalculate with the revised fittings and a loaded filter

The revised layout

The vendor supplied a device with an inlet flange matching the pipe size, removing the reducer. Rotating the frame and moving it 0.8 m placed the outlet in line with the stack, removing two bends. Long radius bends replaced the mitre and the short-radius elbow. A drain was added at the low point. The recalculated backpressure with a loaded filter sat comfortably within the engine maker’s limit, where the original layout had been close to it.

Printing plants have their own exhaust and ventilation interactions; our guide on RECDs for printing facilities discusses them. The 700 kVA DG set RECD page covers this rating.

When the Site Forces Bends

Some sites cannot avoid fittings. A basement exhaust has to turn up a shaft. A canopy tailpipe has to turn towards a wall. In those cases:

• Use the gentlest bends that fit, even if they cost more to fabricate
• Increase pipe size in the section with the most fittings to cut velocity and losses
• Place fittings in large-diameter, low-velocity sections where possible
• Keep the device inlet straight, moving bends further away from it
• Consider removing the old silencer if the device and the remaining system still meet noise needs; our article on whether an RECD can replace a generator’s exhaust silencer discusses this

What to Request With Every Drawing

When the vendor sends a layout, ask for these alongside it:

1. A schedule listing every fitting by type, size and position
2. The backpressure calculation with each fitting’s contribution shown
3. The engine maker’s maximum backpressure for this engine and the margin remaining
4. The calculation repeated for a filter at its maximum loaded condition before cleaning
5. The required straight length before the device inlet, and confirmation it is provided
6. Drain positions and low points marked
7. Support and guide positions at every bend

Our guide on using OEM exhaust flow data to specify an RECD explains where the engine data comes from, and choosing CPCB approved RECD vendors suggests how to judge the quality of a vendor’s engineering.

Compliance Note

The device approval under CPCB’s System and Procedure for Emission Compliance Testing of RECDs concerns the device’s performance. A layout that pushes backpressure too high can still cause problems that show up at inspection, such as smoke, high exhaust temperature or a device that loads quickly. Keep the reviewed drawing and calculation with the installation records. Our article on how to verify a CPCB tested RECD on a DG set covers the paperwork side.

Frequently Asked Questions

How many bends are acceptable in an RECD exhaust layout?

There is no fixed number. What matters is the total backpressure with all fittings and the device at its loaded condition, compared with the engine maker’s limit. Fewer and gentler bends leave more margin.

Are mitred bends acceptable?

Multi-segment mitres are common and often acceptable. Single-cut mitres cause high losses and should be replaced with proper bends where space allows.

Why does the vendor want straight pipe before the device?

To spread exhaust evenly across the filter face. A bend right before the inlet can concentrate flow on one side, causing uneven soot loading.

Should I use a concentric or eccentric reducer?

Concentric reducers suit vertical runs and many device connections. In horizontal runs, an eccentric reducer with the flat side down helps condensate drain.

Does a bigger pipe always help?

A larger pipe reduces velocity and therefore fitting losses. It also costs more, weighs more and may cool the exhaust more on long runs. The size should come from the backpressure calculation.

Should the calculation use a clean or a loaded filter?

Both. The loaded-filter case is the one that matters for the engine limit, because backpressure rises between cleanings.

Every Symbol Has a Cost

A layout drawing is easy to approve at a glance. The fittings on it are small symbols with large consequences once an RECD has used up most of the system’s margin. Question each bend, prefer gentle curves to sharp corners, keep the device inlet straight, choose reducers for drainage as well as size, and insist that the backpressure calculation lists every fitting with the filter at its dirtiest. Ten minutes with a red pen can spare the engine years of struggle, and it avoids several entries on our list of common RECD installation mistakes.

Aceget provides a full fitting schedule and loaded-filter backpressure calculation with every RECD layout drawing, and reviews third-party layouts on request. To have a proposed layout checked, get in touch with Aceget, or browse the retrofit emission control device range.