28 Product Developments Catch cascading carbamate quickly
Erik Karlsen of HB Products, a supplier to Thermofrost Cryo, looks at leaks in a cascade system operating with carbon dioxide and ammonia.
THE WORD ‘CASCADE’ comes from cascade waterfalls where the water descends a series of steps. In a cascade refrigeration system, a low temperature
refrigerant (CO2) is used in the lower cascade step.
The condenser of this step is used as an evaporator of the upper cascade step, which contains another refrigerant (NH3).
There are several good reasons to install a cascade refrigeration system. Typically, this is done when very low temperatures must be obtained. In principle, this could be done in one step, but when the temperature difference between condenser and evaporator becomes too high, the ‘economic’ working range of the refrigerant will be exceeded. A cascade refrigeration plant minimises this difference, increases the yield and therefore lowers the operating costs.
Leaks in the heat exchanger
The pressure in the CO2 cycle is significantly higher than the pressure
in the NH3 cycle. Therefore, a leak in the heat exchanger will cause CO2 to enter the NH3 cycle. Provided that there is no water in the system, the following chemical reaction will occur:
CO2 + 2NH3 NH2COONH4 The reaction product is ammonium carbamate and it is a salt with a tendency to build up. They are transported with the refrigerant and will settle in compressors and valves. Additionally, it may accumulate in the evaporator and reduce the efficiency. As seen in the reaction equation, one
mole CO2 and two mole NH3 produce one mole NH2COONH4. Converted into mass, this means that approximately 44g
carbon dioxide and 34g ammonia produce 78g ammonium carbamate. At -10°C and a pressure of 26.49bar (boiling point) 44g carbon dioxide in the gas phase takes up a volume of approximately 0.8L. Crystalline ammonium carbamate has a density of 1.6g/cm3 which – with the 78g – takes up nearly 50mL. Although this calculation is a good starting point, one should not assume that, in case of a leakage, only 0.8L
of CO2 will enter the NH3 cycle. As mentioned before, the
CO2 pressure is significantly higher than the NH3 pressure, and depending on the size of one or more
leakages more or less CO2 will enter the NH3 cycle and initiate the reaction described above.
The principle of a cascade system with CO2 and NH3. a sensor which detects ammonium
If the CO2 is not detected in time, there is a risk that ammonium carbamate
– caused by the high flow velocity in the refrigerant cycle – can enter valves and compressors very fast.
Pressure and temperature
It would be obvious to measure the CO2 with a CO2 sensor, and there are several sensors are available for this.
However, the problem is that a CO2 sensor does not detect any CO2 when the reaction has already taken place. In this
case one might never find out if there is a leak in the heat exchanger before it’s too late.
To solve this problem, a Danish company, HB Products, has developed
Goodness, gracious, great balls of ice!
WHEN BARTENDERS NEED something to give their drinks an extra special edge, ice machine manufacturer Hoshizaki has a highly original solution – ice balls. The Hoshizaki IM-65NE-Q is a 26kg per day capacity machine with 17kg of storage producing balls of ice rather than cubes and is the only machine of its type in the world. Apart from the obvious novelty factor, ball ice has some very real advantages over cubes. It makes a stunning visual statement – especially in hi-ball glasses. It melts even more slowly than Hoshizaki’s slow melting cube ice, and its rounded surface prevents sticking. As is common to all Hoshizaki IM ice machines, it is designed with hygiene as a priority and utilises a closed cell ice making system that has an automatic rinse and flush cycle. This means that, after every new batch of ice, the water reservoir will drain, rinse and then refill with fresh water.
Importantly, each ball is made individually by a dedicated jet spray to ensure the highest possible ice quality. The easy-to-clean storage bin has high density foam injected insulation to slow down the ice melting, while a gasket on the insulated door ensures a tight fit to prevent particles entering the storage bin that could contaminate the ice.
ACR News September 2014 Visit ACR News online at
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carbamate in a NH3 system through the capacitive principle.
The detection is based on the fact that the relative resistance of ammonium carbamate is much higher than of gaseous carbon dioxide and ammonia. Experiments show that the chemical reaction is very fast, but the same experiments also show that the ammonium carbamate can be detected at
the same moment as the CO2 enters the NH3 system. This is indicated by a rapid increase in capacitance.
HB Products has developed this sensor to help solve this problem. Additionally, the detection signal can also be connected
to an emergency stop in the NH3 cycle, which is activated before any damages occur.
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