500va
to 10kVA
Comms rack and tower UPS models for small sized
applications >
What type of generator? - Gas or Diesel Powered?
It should be considered that gas sets are not
ideal for standby applications as their cold start capabilities are
usually quite poor, typically 40-50% - they are often better as base
load or prime sets running most of the time - this is why they have
a permanent gas supply as this eliminates the need to 'top up' diesel
constantly. Gas generators are basically petrol engines converted
to run on gas, they still use a butterfly valve for speed control
and as such they run on a lean mixture giving low torque and poor
speed regulation
What
output is required? – Calculating the size of the set
It needs to be established if the whole building
is to be supported, in which case electricity bills can provide historical
information useful in obtaining peak load levels or monitoring of
the load to check demands. Future expansion of building facilities
or additional equipment must also be taken into account when sizing
the generator. If loads can be sequentially started, this helps to
reduce the ‘cold start’ demand on the set and could allow
a smaller set to be used thereby reducing capital costs.
If only supporting part of the building load, usually referred to
as essential, it is necessary to determine which loads are most important,
these are typically emergency lighting, UPS, essential air-conditioning,
building lifts and a limited amount of small power. It must also be
considered that many loads of this type have high start up currents
or impose non-linear demands on the generator, so care must be taken
when calculating the size of set needed.
What
type of generator duty cycle? – How often is the set required
to operate? Base load or
Continuously rated – usually required
where there is no power supply authority to provide commercial or
industrial power to your facility, in this instance the generator
may be required to operate continuously for 24 hours a day, 7 days
a week – however, it must be considered that rotating machinery
needs to be serviced every two weeks when operating this type of duty
cycle – so another generator may be needed during these service
periods. Short-term overloads of up to 10% for one hour are usually
permissible
Prime rated –
similar to the above application but usually in an area where the
mains power is more reliable, in this case the set is expected to
carry 75-80% load for several hours but not necessarily continuously.
Standby rated –
usually installed in an area where the mains is generally healthy
but the user cannot tolerate any power loss to certain areas of their
business, typically for process control, computer or communications
related industries. These sets are rarely used for full building support
and usually back up essential loads. A time restriction of a few hundred
hours per year is typical and no overload capacity is available.
Where do
you intend to position the set? – Inside or outside?
Inside the building –
there is often an ideal basement or ground floor room to locate an
open generator, this does not interfere with the fabric of listed
buildings, but is more complex in terms of cooling air inlet and discharge,
engine exhaust and silencers, acoustic noise limitations, fuel deliveries,
vibration and access. All these issues often make open set installations
more expensive and require a great deal of planning for the site services.
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Modular
UPS
30, 60 & 120kVA three phase
scalable comms room UPS range >
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10
to 800kVA Versatile
floor standing modules for IT & Industrial applications >
ServiceAgreements
24-7 callout,
preventative
maintenance,
spare parts
External to the building – this
is usually the most straightforward installation and totally enclosed
weatherproof housings can be both cost effective and environmentally
friendly. Local authorities often need to be consulted to check
acoustic noise restrictions in the area, in particular residential
locations need to be carefully considered. Exhaust emissions are
also critical near buildings and may need special treatment. If
considering a roof top installation, weight and vibration also need
to be carefully checked by specialists. Self-contained sets have
the added benefit of being easily moved to new locations if necessary.
How do you want
the set to work? – Local manual, remote or automatic
Manual start –
a simple electric pushbutton or keyswitch is often used for short
term hire sets or for installations where the site is permanently
manned and the load can suffer a break in supply.
Remote start –
the set can be started by a remote signal from a manual changeover
device.
Automatically –
usually referred to as AMF, auto start upon mains failure, the set
receives a signal from a remote sensing device or phase failure
relay usually located in an automatic changeover panel that switches
from mains to generator either by interlocked Contactors, Moulded
case circuit breakers or Air circuit breakers depending upon the
rating of the set. The return to mains can be automatic or manual
depending upon the site requirements. With automatic starting there
is always a brief delay of several seconds whilst the engine gets
up to speed and ready to accept the load. (If this is unacceptable
for certain types of load it is necessary to install a UPS to ‘bridge
this gap’ and ensure a seamless transition of power)
How do you cool the generator?
Fan cooled radiators are the most common form
of engine cooling; the fans are either engine driven or electrically
powered. Flexible couplings usually provide the link to either a
sound attenuated louvre or ductwork that ensures that hot air is
correctly discharged from the enclosure or plantroom. Occasionally
remote mounted radiators are employed in difficult plantroom installations
where ductwork is not a suitable option.
How do you get rid of exhaust gases?
For plantroom installations, it is normal
to simply discharge the exhaust to atmosphere alongside the hot
air discharge louver, however, this is not always acceptable due
to site limitations or local legislation and it is often necessary
to route the discharge gases through a flue pipe either fixed to
an external building wall or through an internal riser. Both options
need careful planning to keep back pressure to a minimum, typically
by reducing the number of bends and increasing the pipe diameter.
External pipework can be installed using in stainless steel for
a more durable finish although painted mild steel is often acceptable
in view of the cost differential.
Enclosed sets generally employ an internal silencer system consisting
of primary and secondary mufflers with the gases routed out through
the canopy via a weatherproof discharge point often known as a bird’s
beak. Catalytic converters are becoming more common in an effort
to reduce harmful emissions.
How to store fuel and re-fill the
Generator
Most simple enclosed sets incorporate a double
skinned or bunded base tank, built into the main base frame or skids
of the set, these can usually provide adequate running time for
standby applications, typically 8-20 hours. Longer run times or
permanently running sets generally use a separate double skinned
bulk fuel tank, with a fuel transfer system consisting of one or
two pumps and a fuel fill point incorporating overfill alarm, high
and low level detection and various other options such as leak detection,
pipework insulation etc
What about the noise of the generator?
Engines will always create high levels of
noise and the process of reducing this noise is achieved by using
a combination of attenuators, silencers and acoustic enclosures.
Audible noise levels are measured in decibels over a given distance;
site conditions and local legislation usually dictates the level
that can be tolerated. Enclosed sets are typically 75-80dBA at one
metre, higher levels of attenuation can achieve 65dBA or even less,
but the cost and physical size often becomes prohibitive.