Cold Link Africa January / February 2020 | Page 37

CONTRIBUTORS
INCORPORATING COLD CHAIN
When it comes to COP, I say “don’t tell me,
show me!”, because hundreds of scientific
in-field test data using ClimaCheck shows
that a tiny fraction of systems operate at
the exact conditions used in the factory.
The data also shows how insufficient
maintenance, sub-standard installations,
poor control set-up, lack of or incorrect
commissioning procedures, amongst others
destroys efficiency as well as reliability.
So, next time someone tells you the
COP of their system is six (or any other
number), ask them: 1. “When was it six?”;
and 2. “How did you measure that?”.
‘HIGH-EFFICIENCY’?
COMPARED TO WHAT?
It is extremely common to hear the
phrase ‘high-efficiency’ in discussions in
the HVACR industry. You will be hard-
pressed to observe any discussion that
does not contain this phrase. In today’s
market, is it even possible to buy a ‘low-
efficiency’ or for that matter, ‘standard-
efficiency’ system? If this is the case, why
do we use this phrase so frequently?
By definition, high-efficiency is a
relative term, so when a manufacturer
or supplier quotes ‘high-efficiency’
what are they comparing it to? A
cynical person might say that they are
referring to the opposition’s equipment
or, very often, the existing 15-year-
old system, which is in perfectly good
working condition, but which they
intended to replace. In this example,
‘high-efficiency’ is relative to a very low
base. Does a 10% increase in efficiency
compared to this old system qualify
as high efficiency? Perhaps 20%? This
doesn’t make sense, so let’s stop over-
using the phrase ‘high-efficiency’.
TOP 5 EFFICIENCY KILLERS
The vast majority of systems have a
savings potential of 10% to 30%. Of
those opportunities, 80% are operational
in nature, in other words, no capital
investment is required and, therefore,
this is a desirable return on investment.
Notwithstanding this, there are many
challenges and barriers to the market.
These are some of the most common:
Business as usual. There is little or no
incentive for most role players in the
HVACR industry to directly change the
current business model because efficiency
is seldom measured and therefore, cannot
be used as a performance benchmark.
“If it ‘ain't broke, don’t fix it”. Run-to-fail
maintenance strategies are common-
place and result in avoidable failures and
lost efficiency. We see many examples
where a loss of efficiency due to (for
example) small maintenance issues have
lost efficiency for several years.
“I have a Scada/BMS/Dashboard
system; therefore, I know the efficiency
of my system at all times”. Most of
these systems generate data about
temperature, pressure and setpoint alarms
– they do not calculate the efficiency.
“My system is effective; therefore, it is
efficient” — a common misconception,
especially amongst end-users. If your
COLD LINK AFRICA •
January/February 2020
office is at the required air temperature
(effective), it does not necessarily mean it
is efficient.
“The system was commissioned
when it was installed; therefore, it is
efficient.” The majority of systems are
either not adequately commissioned
or not commissioned at all (the phase
‘retro-commissioning’ has now become
a commonly-used term). Proper
commissioning is one of the most
important contributors to the efficient and
reliable operation of any cooling system.
MEASURING YOUR PLANT
The system identifies inefficiencies,
documents baseline performance
and provides early warning for system
weaknesses. You can compare the cost
of energy optimisation against calculated
savings once a performance baseline is
documented, ensuring your investment
is paying back. Studies and practical
experience show that the system can
reduce energy consumption by 10-30%.
As 20% of world energy consumption is
used by refrigeration processes, these
energy reductions directly affect global
carbon dioxide emissions.
HOW IT WORKS
The ClimaCheck measuring tool uses a
process called the ‘internal method’.
This means that the performance and
efficiency of any vapour compression
cycle can be calculated on-site and in
real-time with non-invasive temperature
and pressure probes. Think of it as
taking a laboratory to the chiller or
refrigeration system.
The internal method is based on
the possibility to thermodynamically
determine the refrigeration process by
assessing the specific enthalpy changes
in various parts of the refrigerant system.
(Berglöf, 2004).
QUANTITIES THAT MAY NEED TO BE
MEASURED ARE:
•
•
•
Surface temperatures in the
refrigerant system
Refrigerant high (condensing) and
low (evaporating) pressure
Electric powers (compressors, pumps,
fans, etc. according to system
boundary for calculation of COP.
Knowing the temperature, pressure and
type of refrigerant at a particular location,
it is possible to get the value of the
specific enthalpy in a table or chart or to
calculate the value by means of known
correlation or by means of programmes.
In a simple, one-stage refrigeration cycle
measurement of the condensing and
evaporating pressures, the outlet and
suction temperatures of the compressor
and the sub-cooled liquid temperature
after the condenser will suffice. These
measurements, i.e. two pressures and
three temperatures provide sufficient
information to visualise the process
by means of points 1, 2, and 7 on the
diagram. Assuming isenthalpic expansion
point 8 will also be known.
Figure 13 The refrigeration process in
a diagram of specific enthalpy versus
pressure.
By means of computerised equipment,
it is possible to use the measured pressures
and temperatures to calculate the
specific enthalpies on the saturation curve
as well as in the regions of superheated
vapour and sub-cooled liquid. When the
process has been mapped it is possible
to calculate the heating or cooling
coefficient of performance, e.g.
•
For cooling the coefficient of performance
COPc in a corresponding way is given by:
•
The method can either be used to directly
determine the coefficient of performance,
using a known value of the loss factor,
or to study performance changes after
calibration by means of a parallel external
measurement or simply to assess relative
changes. CLA
Equation 28
Where h 2 - h 7 represents the heat
delivered by the refrigerant to the
condenser and h 2 – h 1 represents the
work supplied to the refrigerant by the
compressor. Unfortunately, the reality
is a little more complex as not all of the
compressor work will result in a specific
enthalpy increase from point 1 to point 2
owing to thermal losses to the ambience.
In a simple, hermetic compressor, the
losses P. loss may be expressed as a fraction
f of the motor power input P e,m . This
enables a calculation of the refrigerant
mass flow rate q m,R according to:
•
Equation 32
As 20% of
world energy
consumption is
used by refrigeration
processes, these
energy reductions
directly affect global
carbon dioxide
emissions.
Equation 29
Then the heat pump heating coefficient
of performance COP h is given by
•
Equation 30
i.e.
REFERENCE :
•
•
Equation 31
Anna-Lena Lane, Jessica Benson, Lina Eriksson,
Per Fahlén, Roger Nordman, Caroline Haglund
Stignor, Klas Berglöf, Guy Hundy. (2014).
Method and guidelines to establish System
Efficiency Index during field measurements
on air conditioning and heat pump systems.
SP Technical Research Institute of Sweden;
Energiteknik, p. 13, 31, 32.
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