User guide

Application Guide | Gas detection in refrigeration systems
4 DKRCI.PA.S00.A1.02 | 520H12772 © Danfoss | DCS (MWA) | 2018.05
Introduction Gas detection and leak detection are two distinct
activities that covers the same topic, but the
methods are very different.
Gas detection covers the analysis of air samples
to determine whether they contain refrigerant
gas. Leak detection is a systematic inspection
of a refrigeration system to determine whether
it is leaking. The terms gas detection and leak
detection are not interchangeable, and must not
be mixed.
Leak detection equipment is normally hand-
held equipment carried by people, and used for
detection of leaks in refrigeration systems. There
are several types of leak detectors available,
ranging from simple techniques like soapy water
to sophisticated electrical instruments.
Gas detection equipment is usually used in
a fixed installation with a number of sensors
located in areas where refrigerant might be
expected to accumulate in the event of a plant
leak.
These locations depend upon the layout of
the machinery room and adjacent spaces, on
the configuration of the plant and also on the
refrigerant in question.
Before selecting the appropriate gas detection
equipment, a number of questions have to be
answered:
• Which gases have to be measured and in what
quantities?
– Which sensor principle is the most suitable?
– How many sensors are needed?
– Where and how should they be positioned
and calibrated?
• Which alarm limits are appropriate?
– How many are required?
– How is the alarm information processed?
This application guide will address these
questions.
Sensor technology The choice of sensor technology for refrigerant
gas detection will depend on the specific target
refrigerant gas and ppm range required. Danfoss
offers a range of different sensor technologies
EC - Electrochemical sensor Electrochemical sensors are mainly used for toxic
gases and are suitable for ammonia.
They consist of two electrodes immersed in an
electrolyte medium.
Time
Tolerance rang
e
Sensitivity
max.
min.
Max. operating time
before calibration
“High” gas concentration
“Low” gas concentration
An oxidation / reduction reaction generates an
electric current that is proportional to the gas
concentration.
They are very accurate (+/- 2%) and tend to be
used mainly for toxic gases, which cannot be
detected otherwise, or where high levels of
accuracy are needed (fig. 1).
Danfoss offers specific EC sensors for ammonia
in ranges up to 0-5.000 ppm with an expected
lifetime of 2 years, depending on exposure to
target gas.
Exposure to large ammonia leaks or constant
background ammonia will shorten the sensor life
(fig. 2). EC sensors can be re-calibrated as long as
the sensitivity of the sensor is above 30%.
They are very selective and rarely subject to
cross-interference. They may react to sudden
large humidity changes but settle quickly.
Fig. 1: Sensitivity of electrochemical sensors
to match most commonly used refrigerants,
appropriate ppm ranges, and safety requirements
for refrigeration systems.