RECDs for Generators with Rapidly Changing Loads: A Selection Guide

Stand next to a DG set at a stone crushing unit and listen. The engine note rises as the crusher bites into a rock, falls as it clears, rises again. Watch the exhaust and you may see a faint grey puff each time the governor reaches for more fuel. A trend chart from the controller would look like a mountain range.

Most writing about emission control devices assumes a calm engine running at a steady load. Plenty of Indian generators do not work that way. They drive lifts, welding sets, injection moulding machines, refrigeration compressors, tower cranes and imaging equipment, all of which swing the load up and down in seconds.

This guide is for those sets. It begins by sorting fluctuating loads into recognisable signatures, then looks at the four stresses that fast-changing loads place on an RECD, the device and installation features that answer each stress, the changes on the electrical side that calm the load, and the clauses worth writing into a specification.

Load Signatures: Recognising Your Pattern

“Fluctuating load” covers several quite different behaviours. Identifying yours makes the conversation with a vendor much more productive.

Signature	Typical sources	What the trend looks like	Main concern for the RECD
Large single steps	Big induction motors started direct-on-line, building transfer after grid failure	Occasional sharp jumps from low to high load	Soot burst on each step, backpressure spike
Repeating cycles	Injection moulding, presses, batch mixers	Regular rise and fall every few seconds to minutes	Constant thermal cycling, repeated soot puffs
Random surges	Stone crushers, wood chippers, welding sets	Irregular spikes with no fixed period	Combination of soot bursts and vibration
Stop-go blocks	Lifts and escalators, tower cranes, hoists	Clusters of short heavy pulls with idle gaps	Rapid temperature swings at low average load
Compressor cycling	Cold storage, chillers, air compressors	Medium steps on and off as thermostats or pressure switches act	Moderate cycling, often at night with low base load
Pulse loads	CT and MRI scanners, X-ray, large UPS recharge after transfer	Brief, very sharp peaks on top of a steady base	Momentary flow and pressure peaks

Many sites combine two or more. A hospital may have a steady base load with imaging pulses on top. A cold store may combine compressor cycling with a lift and a few motors. Our sector guides for cold storage refrigeration load patterns and food processing units describe two common mixtures.

How Engines Respond to Sudden Load

When load jumps, engine speed dips for a moment. The governor responds by adding fuel. On a turbocharged engine, the extra air needed to burn that fuel cleanly arrives a little later, because the turbocharger has to spin up on exhaust energy that has not yet increased. For that short interval the engine is running rich, and it smokes.

Generator performance under load changes is described by ISO 8528-5, which defines performance classes from G1 to G4 based on how far voltage and frequency may deviate and how quickly they must recover. A set meant to carry sharp loads is often specified to a tighter class. Kohler’s white paper on ISO 8528-5 and generator transient performance explains why engines with high output per litre tend to accept smaller first load steps and how makers express load acceptance.

What matters here is the exhaust side of that story. Each step produces a pulse of extra soot, a change in exhaust temperature and a change in exhaust flow. A set that sees a few steps a day barely notices. A set that sees several a minute lives with them permanently.

Stress 1: Soot Arrives in Bursts

Under steady load, soot enters the filter at a fairly constant rate. Under fluctuating load, a large share of it may arrive in short bursts at each step.

Why this matters:

• Faster loading. A set with frequent transient smoke may load its filter noticeably faster than a steady set with the same average load and hours.
• Uneven deposits. Bursts can deposit soot unevenly across the inlet face, particularly if flow distribution into the device is poor.
• Masked engine problems. A worn turbocharger, a dirty air filter or tired injectors make transient smoke much worse, and the filter quietly absorbs the evidence.

What helps: an engine in good condition, a filter with adequate soot capacity for the measured smoke rate, and an inlet design that spreads flow evenly across the substrate. Before fitment, check the engine on a few deliberate load steps. Heavy black smoke on each step points to the engine, not the duty. Our guide to why a generator emits black smoke lists the usual causes, and the generator air filter maintenance guide covers one of the most common.

Stress 2: Temperature Never Settles

Exhaust temperature follows load, with a short lag. On a set with large, frequent swings, the device inlet may move up and down by a hundred degrees or more many times an hour.

This has two effects.

Mechanical stress on the substrate

Ceramic filter substrates expand and contract with temperature. Rapid, repeated changes create temperature differences across the substrate, and those differences create stress. Substrate materials respond differently. Cordierite has very low thermal expansion, which helps it tolerate gradual changes, but its relatively low thermal conductivity means hot spots can form during heavy soot burn-off. Silicon carbide conducts heat much better and withstands higher peak temperatures, and is often built in bonded segments to manage its higher expansion. Each has its place, and the device’s housing, mounting mat and canning design matter as much as the material. Ask the vendor how their design handles repeated thermal cycling.

Regeneration that comes and goes

If the average exhaust temperature hovers around the catalyst’s working range, regeneration may switch on during peaks and off during troughs. That can be perfectly adequate over a working day. The risk comes when a long stretch of low base load with occasional spikes gives the filter peaks too short to burn much soot. Look at the time-weighted temperature, not only the peaks.

What helps: a device and mounting designed for thermal cycling, adequate insulation upstream to smooth temperature swings, and real temperature data from the site.

Stress 3: Flow and Pressure Pulses

Exhaust flow rises with load. When load jumps, flow jumps with it, and so does pressure drop across the filter. On a set whose existing exhaust system already uses much of the engine maker’s backpressure allowance, these peaks can push the total close to the limit.

Pressure pulses also affect monitoring. A backpressure sensor sampling once a second on a crusher set will see a reading that jumps around constantly. If alarm thresholds are set on raw readings, nuisance alarms follow. If they are set too loosely, real problems go unnoticed.

What helps:

• Size the device and exhaust route for peak flow, not average flow, with soot load included.
• Ask the vendor how the monitoring system handles noisy signals: averaging over a defined window, comparing readings at similar load, or using a load-corrected trend.
• Use alarm delays so that a brief peak does not trigger an alert, while a sustained high reading does.

Our explainer on how exhaust backpressure affects a DG set covers measurement and limits.

Stress 4: Vibration and Movement

Load swings change engine torque reaction, and the whole set rocks slightly on its mounts each time. Machinery such as crushers and presses adds ground-borne vibration. Over months, that movement works on every joint in the exhaust line.

Typical weak points:

• Rigid connections between the engine outlet and the device, with no flexible section to absorb movement
• Device supports that are too light or fixed to a structure that moves differently from the engine
• Sensor tappings that loosen or crack
• Clamps and gaskets that relax and begin to leak

What helps: a properly specified flexible bellows between engine and exhaust system, supports that carry the device’s weight independently of the engine, vibration-isolated mounts where needed, and periodic checks of clamps and fittings. Our article on how to prevent generator vibration issues covers the generator side, and common RECD installation mistakes covers several support and alignment errors.

The Four Stresses Side by Side

Stress	What causes it	Device and installation response	Site response
Soot bursts	Rich running during load steps	Adequate soot capacity, even inlet flow distribution	Engine tuning, smaller load steps
Thermal cycling	Exhaust temperature tracking load	Substrate and canning designed for cycling, upstream insulation	Smoother load profile
Flow and pressure pulses	Exhaust flow tracking load	Sizing for peak flow, filtered monitoring, alarm delays	Reduce peak steps
Vibration	Torque reaction and machinery	Flexible bellows, independent supports, isolation	Check engine mounts and foundations

Calming the Load at the Source

The best way to protect both the engine and the RECD is to make the load less abrupt. These measures are electrical rather than emission-related, but they pay off on the exhaust side.

Soft starters and variable frequency drives

A large induction motor started direct-on-line can draw several times its running current for a few seconds. A motor soft starter limits that inrush; a variable frequency drive can bring the motor up to speed gently and also control it in operation. Both reduce the size of the step the generator sees. VFDs introduce harmonics, so the generator and alternator need to be suitable, but on sets with large motors the benefit is usually clear.

Staggered starting

After a grid failure, the whole building lands on the generator at once unless something prevents it. Sequencing loads so that large motors start one after another, a few seconds apart, replaces one large step with several smaller ones. Many building management systems and motor control centres can do this.

Load-sharing across sets

On multi-set plants, sharing a fluctuating load across synchronised sets reduces the swing each engine sees, as a percentage of its rating.

Flywheel, battery or UPS buffering

For some pulse loads, such as imaging equipment, a buffer can absorb the peak and draw a smoother current from the generator. This is a specialised design choice and needs input from the equipment maker.

Check the generator’s load acceptance

If the set is regularly asked to accept steps larger than it was designed for, the problem is not only emissions. Our guides on how to diagnose voltage fluctuations in your generator and generator frequency problems help identify when the set is struggling.

A Worked Example: The Bawana Moulding Unit

This illustrative case is built to show the method rather than describe a specific site.

A 400 kVA set (about 320 kWe) supplies a plastics injection moulding unit in an industrial area of Delhi during outages. Six moulding machines run on a cycle of roughly 30 to 45 seconds each. A 55 kW air compressor starts and stops on pressure. A 37 kW chiller compressor cycles on temperature.

What the controller trend showed

• Base load with machines idle: about 60 kW
• Load swinging between roughly 110 and 240 kW during production, with peaks several times a minute
• Air compressor starts direct-on-line, causing a step of about 55 to 60 kW plus inrush, several times an hour
• Average load during production: about 52% of rating
• Visible grey puff on each compressor start

What the review found

• The set had a healthy average load, which was good news for regeneration.
• The repeated compressor starts were the largest steps and the main source of visible smoke.
• The existing exhaust used about half of the engine maker’s backpressure allowance at full load.
• The exhaust pipe ran rigidly from the engine outlet with a short, worn flexible section.

What was agreed

1. Soft starter fitted to the air compressor, reducing its step to a gentle ramp.
2. Engine service including injector check and air filter replacement; transient smoke on a test step reduced noticeably.
3. Device sized for peak flow with soot load included, with an insulated pipe from engine to device.
4. New flexible bellows of suitable length and rating, and an independent support frame for the device.
5. Monitoring set to record backpressure as a one-minute rolling average, with alarms requiring a sustained reading above threshold for several minutes.

Outcome, illustrative

After three months, the backpressure trend at comparable load was stable, with no nuisance alarms after the first week’s tuning. The facility team noted fewer visible puffs at the stack, mainly thanks to the soft starter.

How Fluctuating Load Changes the Specification

When you send an enquiry, describe the load in enough detail for the vendor to design for it. Here are clauses and data points worth including.

Load description

• Base load, typical peak and maximum peak, in kW
• Size and starting method of the largest motors
• How often large steps occur: per minute, per hour or per day
• Nature of cyclic loads and their period
• A controller trend export or meter data at one-second or one-minute resolution if available

Device requirements

• Designed for frequent thermal cycling, with the vendor to state substrate type and mounting approach
• Sized for peak exhaust flow and backpressure with normal soot load, within the engine maker’s limit
• Even flow distribution at the device inlet
• Soot capacity appropriate to the measured transient smoke

Installation requirements

• Flexible bellows between engine and exhaust, rated for temperature and movement
• Independent device supports that do not load the engine outlet
• Insulation from engine outlet to device inlet
• Sensor tappings designed to resist vibration

Monitoring requirements

• Continuous backpressure and inlet temperature recording
• Filtering or averaging method stated by the vendor
• Alarm delays and thresholds agreed at commissioning

Approval

• Confirmation that the device model is covered by a valid approval for the engine’s power under the CPCB procedure for RECD emission compliance testing

Our guide to choosing CPCB approved RECD vendors suggests further questions, and DG nameplate photos for an RECD enquiry covers the documentation side.

Commissioning a Set with Fluctuating Load

Commissioning should test the device under the conditions it will actually meet, not only at a steady load.

1. Baseline at steady load. Record backpressure and inlet temperature at a stable load for comparison.
2. Deliberate steps. With the vendor present, apply the site’s largest normal load steps and watch backpressure and smoke.
3. Production run. Record at least an hour of normal fluctuating operation and review the trend.
4. Alarm tuning. Adjust alarm delays and thresholds to avoid nuisance alarms without hiding real ones.
5. Mechanical check. After the first few weeks, recheck clamps, supports, bellows and sensor fittings.

The RECD installation process places these steps in the wider commissioning sequence.

Maintenance Rhythm for Fluctuating Duty

• Weekly: glance at the backpressure trend for any upward drift at comparable load.
• Monthly: check exhaust joints, clamps, bellows and sensor fittings for leaks or looseness. Look at the stack during a few load steps for any increase in smoke.
• Quarterly: review the monitoring data with the vendor; check engine air filter, turbocharger and injectors.
• As needed: cleaning or regeneration support according to the trend.

The RECD maintenance guide covers routine care in more detail.

Reading a Controller Trend Without Software Skills

Many modern DG controllers can export a load trend as a spreadsheet. You do not need specialist tools to get useful answers from it.

1. Choose a representative day. Pick a normal production day during an outage, not a test run.
2. Plot kW against time. A simple line chart in any spreadsheet shows the signature at a glance: steps, cycles, surges or pulses.
3. Note the base. The lowest level the load returns to between peaks tells you how cool the exhaust gets in the troughs.
4. Note the peaks. The highest sustained level, ignoring single-sample spikes, tells you the flow the device must pass.
5. Count the big steps. Count changes larger than about 20% of rating within a few seconds. That number per hour is a useful single figure for the vendor.
6. Check the time-weighted average. Your spreadsheet’s average of the kW column, over a period of steady sampling, gives the figure that best reflects regeneration conditions.

If the controller samples only once a minute, short steps may be hidden. In that case, a clamp-on power logger set to one-second intervals for a few days will reveal what the controller misses.

Myths About Fluctuating Loads

“If the average load is healthy, the swings do not matter.”

The average speaks to regeneration. It says nothing about soot bursts, peak backpressure or vibration. A set averaging 55% with violent swings needs different attention from a set holding a steady 55%.

“Bigger devices handle swings better.”

A larger device lowers peak pressure drop, which helps. It also adds thermal mass and takes longer to heat during troughs. Size for the measured peak flow and the measured soot rate, not for comfort.

“Transient smoke is normal, so it can be ignored.”

Some smoke on large steps is normal for many engines. Heavy smoke on every step usually points to an engine or air supply problem, and the filter pays for it.

Questions Readers Often Ask

Will rapidly changing loads damage an RECD?

Not if the device and installation are designed for them. Frequent thermal cycling, pressure peaks and vibration need to be considered in the substrate and mounting design, sizing, supports and monitoring.

Why does my generator puff smoke every time a motor starts?

The governor adds fuel immediately, but the turbocharger needs a moment to provide the extra air. The engine runs rich for that interval. Large direct-on-line motor starts make it worse. A soft starter or VFD reduces the step.

Should the RECD be sized for average or peak load?

For backpressure, size for peak flow with normal soot load included. For regeneration, look at the time-weighted temperature over typical operation.

Can monitoring alarms cope with fluctuating backpressure?

Yes, if the system averages readings over a sensible window and uses alarm delays. Agree the method and thresholds at commissioning.

Do soft starters help the RECD?

Indirectly, yes. Smaller load steps mean less transient smoke, smaller temperature swings and lower pressure peaks.

What data should I give the vendor?

Base and peak loads, the size and starting method of large motors, how often large steps occur, and a controller or meter trend at high resolution if you have one.

Steady Thinking for an Unsteady Load

Generators that drive crushers, presses, lifts and compressors live with a load that never sits still. That makes life harder for an RECD, but not impossible. Recognise your load signature, address the four stresses of soot bursts, thermal cycling, pressure pulses and vibration, calm the worst steps at the source, and write those needs into the specification. A device chosen this way will keep working long after a device chosen from a kVA table would have started raising alarms.

Aceget surveys load behaviour as part of every RECD proposal and supplies CPCB type-approved devices for generators from 25 kVA to 2500 kVA. For the size in this article’s example, see the 400 kVA DG set RECD. To discuss a set with a demanding load, reach the Aceget team or explore the full retrofit emission control device line-up.