Open frame range

Open Frame Diesel Generators

An open frame, or open skid, diesel generator is the bare machine: engine, alternator, radiator and control panel mounted on a steel base frame, with no enclosure around them. It is the cheapest and most accessible form of the same set, and it is almost never installed outdoors. The room it stands in does the work that a canopy or a container would otherwise do.

This page is about that room: airflow, heat rejection, foundations, exhaust and the transient behaviour that decides whether the set will accept your loads.

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Contents

What an open skid actually includes

Typical open skid build. Everything in the enclosure column is replaced by the plant room when you buy an open set.
Item Open skid What the enclosure version adds
Base frame Welded steel skid with integrated bunded fuel tank, forklift pockets, lifting eyes Same skid, then panels over it
Engine and alternator Identical to any other form of the same rating Nothing electrical changes
Radiator Belt or gear driven fan, push or draw configuration, ambient rating per project A larger core and a slower fan, because the canopy restricts airflow
Exhaust Bare manifold and turbo outlet, or a loose silencer supplied for site fitting Silencer mounted inside the enclosure
Control panel Panel mounted on the skid or supplied loose for remote mounting Panel inside the enclosure, usually behind a door
Batteries and charger Mounted on the skid, open to the room Mounted inside, protected from dust and the weather
Cabling Site cabling taken directly to the alternator terminal box or breaker Cable gland plate and internal tray
Weather protection None. The room is the weather protection Canopy or container skin, seals and louvres
Noise 78-96 dB(A) at 7 m, and often 95-105 dB(A) at 1 m 62-78 dB(A) at 7 m depending on the build

Air and heat

Sizing the plant room ventilation

An open set rejects its heat into the room and then out through louvres, or it is ducted to the outside. Get this wrong and the set derates itself in summer, which is the single most common installation fault on open machines in hot climates.

Budget figures for plant room design. Engine and alternator datasheets govern; the numbers here are for a first-pass sizing before drawings are produced.
Quantity Typical figure How to use it
Radiator airflow About 0.02-0.03 m3/s per kW of engine output, for roughly a 10 K air temperature rise Sets the discharge louvre area and the duct cross-section
Combustion air About 0.10-0.15 m3/s per 100 kW of engine output Must be supplied independently of the radiator air path
Heat to the room About 5-10% of the rated kW when the radiator is ducted out Drives room ventilation when the radiator is not ducted
Louvre free area 1.5-2.0 times the radiator core face area, on both intake and discharge Manufacturers quote free area, not the frame size, so read the data sheet
Air velocity at louvres 2.5-5 m/s Above this the louvre itself becomes the noise source
Room temperature rise Keep below 10 K above outside ambient Above that, apply the engine and alternator derating curves

Air in at one end, air out at the other. A plant room that takes its intake and its discharge from the same wall makes the set breathe its own hot air, and no amount of ducting downstream will recover the lost rating.

Installation

What the site has to provide

Site items that belong in the enquiry, because they change the set and the price.
Item Typical requirement Why it is asked early
Foundation Reinforced concrete plinth, commonly about the set weight for an isolated plinth and 1.5-2 times the set weight for a rigid inertia block Decides whether anti-vibration mounts or a heavy block is used
Anti-vibration mounting Rubber pads for most sites; spring mounts where the structure is light or the machine is on an upper floor Prevents the building transmitting structure-borne noise
Radiator ducting Flexible duct or a canvas sleeve from the radiator to the discharge louvre Short-circuits the air path if it is left open
Exhaust route Insulated pipe, expansion bellows, silencer, back pressure within the engine maker limit, discharge above roof level Back pressure above the declared limit costs power and raises exhaust temperature
Silencer grade Industrial for a remote plant room; residential or critical grade for a plant room near occupied space Exhaust is usually the dominant source once the room is lined
Fuel system Base tank, day tank or bulk tank with transfer pump, level alarms and bunding Autonomy is a fuel storage decision, not a generator decision
Cable route Breaker and cable sized for the set output current plus the standby rating, with a gland plate entry Undersized cabling fails inspection and derates the installation
Room acoustics Absorbent lining and acoustic doors and louvres where a boundary limit applies An untreated concrete room can add 3 dB or more through reflections

Transient behaviour

Will the set take the load in one step?

Steady-state ratings say nothing about what happens in the two seconds after a changeover. Motors, chillers and transformers draw several times their rated current when they start, and the question is whether the set accepts that step without stalling, or dips enough to drop the load.

What to ask for, and what each answer tells you.
Item What to request Why
Performance class The ISO 8528-5 class the set is declared to, normally G2 for commercial sets The class fixes the permitted transient voltage and frequency deviation and the recovery time
Load acceptance curve The permitted load step at 25, 50, 75 and 100% of rating, cold and hot This is the curve that decides whether a single-step changeover is possible
Largest motor start The kW, starting method and locked rotor current of the largest motor on the essential bus Soft start or star-delta can turn an impossible step into a comfortable one
Voltage dip that the load tolerates What the UPS, chillers and drives will accept before they trip The generator is only half of the transient problem; the load is the other half
Staging or load shedding A staged changeover sequence, or a shed group that is dropped on generator Staging is the cheapest way to keep a smaller set
Alternator excitation Permanent magnet excitation or an oversized exciter where the step is severe Faster voltage recovery under motor starting

Choosing

Open frame or silent canopy

The two forms side by side, for the same duty rating.
Question Open frame set Silent canopy set
Where does it stand? Inside a plant room or a purpose-built acoustic house Outdoors, on a roof, beside an occupied building
Noise at 7 m 78-96 dB(A) from the machine alone; the room decides the final figure 62-76 dB(A) from the machine itself
Building cost The room, the louvres, the ducting and any acoustic treatment No building, but a larger clear footprint outdoors
Set cost Lowest of the forms Higher, by the cost of the enclosure
Service access All four sides open; major work is easy Through doors; some tasks need panels removed
Weather and dust Depends entirely on the room, filters and louvres Protected at the factory, to a stated IP rating
Relocation Hard: the room stays behind Straightforward, with a crane
Typical use Factories, hospitals, basements, plant rooms replacing an existing set Urban sites, hotels, hospitals, telecom, rental fleets

If the room already exists, an open set is usually the better answer: cheaper to buy, easier to service, and the acoustics can be dealt with in the room. If there is no room and no space to build one, the canopy set is the answer, and the enclosure is what you are paying for.

Questions

Questions we are asked about open frame sets

Can an open frame set be installed outdoors?

Only with a weatherproof cover, and then it is no longer really an open set. Rain into the alternator terminal box and dust into the windings are the two failure modes, and both are avoidable by buying a canopy in the first place. On a temporary site a simple roof or a shipping container with the doors removed is common, provided the airflow is designed.

How much ventilation does the plant room need?

Start from the radiator airflow: roughly 0.02-0.03 m3/s per kW of engine output for a 10 K rise, which for a 400 kW machine is in the region of 8-12 m3/s. The intake and discharge louvres together need free area of about 1.5-2 times the radiator core face, and the combustion air has to be supplied separately from that path.

Does the set need a heavy concrete foundation?

Not always. With good anti-vibration mounts, a level reinforced plinth of roughly the set weight is usually adequate, and it is the correct answer where the structure below is stiff. A heavy inertia block — commonly 1.5-2 times the set weight — is used where the structure is light, where the machine is on an upper floor, or where structure-borne noise is the concern.

What silencer does an open set need?

In a remote plant room, an industrial grade silencer is normally enough. If the room is near occupied space, a residential grade silencer plus absorbent lining in the room is the usual combination. The exhaust must also stay within the engine manufacturer’s back pressure limit, which is why the silencer is selected with the pipe route rather than after it.

How much derating applies in hot climates?

Both the engine and the alternator are quoted for a stated ambient, normally 40 °C, and both are derated above it. The derating curve is declared by the manufacturer and is expressed per step — commonly per 5 K — so the calculation should use the site’s summer design temperature, not the annual average. In a plant room, add the room temperature rise to the outside ambient before applying the curve.

Related

Other form factors and ranges

Replacing a set in an existing plant room?

Send us the room dimensions, the louvre sizes, the exhaust route and the essential load list. That is usually enough to confirm whether the new set will breathe.

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