FMJ.CO.UK SUSTAINABILITY FOCUS
MAY 2022 57
If the Return T drops, then the cooling
demand is lower than design, so the Flow
T can be allowed to increase, which makes
the chiller run more e iciently.
Delta T Flow Temp
>5°C 6°C
>4°C 7°C
>3°C 8°C
FREE COOLING
When the external ambient temperature
is cold enough, the first stage of cooling
utilises free cooling, where external air
is introduced into the building and the
return air extracted to outside. Hence,
warm internal air is replaced with
cool external air. When the external
ambient temperature is cold enough the
mechanical cooling will not be required.
Free cooling requires a 3 damper system
and a mixing box that allows external air
to mix with return air, creating the correct
supply air temperature.
Another way to use free cooling is to
use cool external ambient air to precool
the (chilled) water returning to a
chiller, thereby reducing the load on the
compressors (and the energy consumed).
When the ambient temperature falls
1°C below the temperature of the water
returning to the chiller, for example the
water return temperature is 15°C and
the ambient is 14°C, free cooling can be
activated. A free cooling system operates
by the return water being automatically
diverted through the free (dry air) cooler
before going to the chiller.
Free cooling is particularly e ective in
the UK and can bring significant energy
savings, as the ambient temperature is
below 15°C for almost 75 per cent of the
year.
Typically, with an ambient temperature
between 3°C and 5°C below the set
point temperature, the compressors will
completely switch o and the only energy
consumed will be by the small fans on
the free cooler. Free coolers can easily
be retrofitted to existing chilled water
systems.
ANTI-CYCLING
Where heating and cooling systems are
operating very close to their set points,
they will tend to operate in short bursts
as the set point is continuously met. This
switching on and o is very ine icient, and
hence the heating and cooling plant is run
for minimum periods. This tends to cause
the set points to be exceeded and reduces
the on/ o times.
All heating and cooling equipment in one
area should be grouped and controlled
together, to prevent them fighting each
other.
CALIBRATION
It is very important to ensure that the
sensors of the BEMS are calibrated
regularly, as it controls according to its
numerous sensor readings. For instance, if
the external ambient temperature sensor
reads 5° too low (19°C instead of 24°C) the
free cooling strategy will draw in ambient
air at 24°C, thinking it is 19°C. If the extract
air was 23°C, it would be more economical
to utilise this when cooling rather than the
fresh air. In this instance, the free cooling
would actually require greater mechanical
cooling.
BUILDING ANALYTICS
In addition to writing energy e icient
strategies in the BEMS, a data analytics
package can also be added to detect
ine iciencies with the system. Having
remote access to the BEMS allows you to
analyse what the system and plant are
doing.
Building Analytics provides a smarter,
data driven approach to maintaining
buildings. The platform collects and
amalgamates large amounts of data
from disparate sources and carries out
autonomous data analysis, providing
stakeholders with real-time alerts and
reports on the problems and ine iciencies
that exist. The platform is capable of
carrying out multiple simultaneous
calculations (Rules) in real-time,
referencing thousands of points and
historic data, reporting on root cause
issues.
These are then displayed on a timeline,
allowing you to pinpoint when these
issues occur, for how long and how
frequent. You can quickly identify
whether a building requires additional
maintenance or has comfort issues, track
issues from identification through to close
out, track the performance of connected
assets over time and identify areas for
improvement. This enables you to improve
asset performance, reduce downtime and
increase operational energy savings.
Reducing energy usage and realising cost
savings are hot topics for most businesses
today and converting your building BMS
into a BEMS can quickly provide significant
paybacks. An energy and sustainability
expert will guide you through the changes
required to maximise heating and cooling
e iciency, whilst ensuring that plant and
equipment are maintained in the best
condition.
CONVERSION
building is cool enough. It is important to
avoid simultaneous heating and cooling.
Plant should be sized so that units can
come on incrementally. We do not want the
total heating/ cooling capacity to operate
in one hit, when only a proportion would
su ice. For instance, when cooling is called
for, the first stage will operate for a set
period. If this can maintain the set point
temperature, then the second stage is not
required. However, if the temperature is still
too high the second stage will be brought in,
followed by the third, fourth etc, as required.
UTILISING THE EXTERNAL CONDITIONS
The di erence between the heating and
cooling set points is known as the deadband,
which is the temperature range where
the system remains idle. Only when the
heating or cooling set points are exceeded
will the system reactivate. For instance, if the
heating set point is 19°C and the cooling set
point 23°C, then heating will only operate
below 19°C and cooling above 23°C. The
system will not operate between 19°C to
23°C.
The heating set point is normally fixed,
but the higher the external ambient
temperature, the higher the cooling set
point can be, and hence the wider the deadband.
For instance, if the external ambient
temperature is 30 °C then the cooling set
point can be increased to 24 °C (and the
dead-band increases from 4 to 5 °C).
When the external ambient temperature
exceeds 16°C, buildings generally do not
require any heating. Hence, the heating
system should be locked out, providing it
doesn’t also heat the hot water.
When the external ambient temperature
drops below 13°C, o ice buildings will
generally not require any mechanical
cooling. Free cooling should su ice, and the
mechanical cooling system should be locked
out, providing local hot spots are still cooled.
COMPENSATED FLOW TEMPERATURE
The flow temperature of a LPHW heating
system (and cooling systems) can be varied
with external ambient temperature. As the
external ambient temperature increases
so the flow temperature decreases, as the
heating requirement is reduced.
The flow temperature of a chilled water
system can be varied with the di erence in
flow and return water temperature (Delta T).
As the return water temperature decreases
so the flow temperature can be increased (as
the cooling requirement is reduced).
Design Flow temperature: 6°C
Designed Delta T: 6°C
Hence, Return T: 12°C