Using ammonia for refrigeration is effective, saves resources and is generally safe. However, because gas leaks cannot be completely prevented, ammonia monitoring is essential. An ammonia detection system reduces the risk of production downtime, protects employees, and can save seconds that make a difference in case of an alarm.
A NATURAL REFRIGERANT
Ammonia: Dairies, bottling plants, butcheries and production plants with a freezing tunnel cannot do without it. Low temperatures must be generated wherever food is produced or processed – during storage, for preservation, or for special technologies and processes.
Chemically known as NH₃, ammonia has been used for this purpose for over a hundred years—used for the first time by Carl von Linde in 1870 to refrigerate beer. Today, the alkaline compound of nitrogen and hydrogen dominates the refrigeration system market now more than ever. This is because the natural refrigerant has a number advantages. Ammonia has a good volumetric cooling capacity and is a very effective refrigerant due to a heat of vaporization of 1368 kJ/kg.
Only a small amount of ammonia needs to be moved in the system—approximately 13 to 15 percent compared to fluorocarbons—to generate a high refrigeration capacity. This makes NH₃ very economical. But not only is the installation of an ammonia-based refrigeration system about 10 to 20 percent cheaper than similar systems, operating costs are also significantly lower.
Another advantage is that ammonia is more environmentally friendly than liquid hydrocarbons: Its global warming potential (GWP) and ozone depletion potential (ODP) equal zero.
RISKS INVOLVED WITH AMMONIA REFRIGERATION
While ammonia is one of the most widely produced chemicals—more than 130 million tons are produced annually, and growing—the substance poses significant hazards.
Ammonia is a flammable gas and can form flammable or potentially explosive compounds in dry air when in a gaseous state. The concentration threshold is between 15 and 28 vol. percent. However, the required ignition temperature is rather high—at least 1202 °F/650 °C. (www.eurammon.com)
NH3 is generally lighter than air and reaches the atmosphere with a high diffusion velocity. In confined spaces and containers, it can displace any available oxygen. NH3 has a high affinity for moisture and quickly forms compounds with the moisture in the atmosphere. If ammonia leakages occur, water curtains are used to damp down vaporous ammonia. The gas then sinks to the ground in the form of ammonia aerosol, which is visible as a white mist.
Ammonia has a relatively low flammability and only burns continuously if there is a flame. If the steam content in the air exceeds 11 vol. percent, flammable and potentially explosive mixtures are no longer possible. Despite the extensive use of ammonia around the world, explosions and fires purely caused by NH3 are very rare. Any known incidents occurred almost exclusively in closed rooms or containers. Thus, there is only a risk of explosion if the concentration in unventilated rooms exceeds the explosion limit (15 vol. percent) or if high-energy ignition sources are present.
Maintenance work at ammonia refrigeration systems requiring welding, soldering or cutting must be performed with extreme caution: existing oil mists can lower the explosion limit of NH3/air mixtures. NH3 systems should therefore be purged with air or a non-flammable gas prior to starting the welding work in order to remove residual ammonia.
Another safety concern is the corrosive effect of ammonia on iron, copper, zinc, tin and their alloys. Leaks can damage parts of the refrigeration system and cause uncontrollable gas leakages. Corrosion caused by ammonia not only causes perforation in the form of holes, but also as corrosive cracks. These cracks are often very subtle and difficult to detect.
2.- Toxic dangers to humans
Ammonia is a respiratory poison that has a strong irritating and corrosive effect—mainly in the liquid state. The strongest reaction occurs with moist body surfaces such as the mucous membranes and eyes, causing painful chemical burns. Inhaling ammonia can damage the respiratory tracts and lungs.
If liquid ammonia comes in contact with the cornea, it can cause blindness. Because the temperature of liquid ammonia is below -27.4 °F / -33 °C, it causes severe frostbite if it comes into contact with the skin. Symptoms of minor ammonia poisoning include a feeling of suffocation and breathing difficulties, dizziness, a burning sensation in the throat, increased salivation, stomach ache and vomiting. Extended exposure to liquid ammonia damages the respiratory and digestive organs. In some cases, serious symptoms may appear hours after inhalation. Contaminations at a level above IDLH(300 PPM) or higher are life threatening. Nevertheless, fatal accidents involving ammonia are rare. Due to its distinctive acrid smell, which can be noticed in very low concentrations starting at 5 ppm, humans normally notice it well below the threshold of a harmful concentration.
This has both advantages and disadvantages. Ammonia’s low odor threshold can cause panic among the employees, even if the concentration is not dangerous. This is also one reason why reliable and precise detection is essential: it helps to prevent uninformed and impulsive reactions.
AVOIDING ACCIDENTS THROUGH EARLY LEAKAGE DETECTION
Even if the ammonia refrigeration systems are professionally and properly maintained, accidental ammonia leakage can occur for many reasons— such as corrosion, leaking valves, operating errors. Cracks frequently develop when liquid residual ammonia is locked in pumps or lines during standstill, which warms up and expands as a consequence. Other critical scenarios include filling, maintenance and cleaning.
In fact, experts estimate that leakage losses in complex industrial refrigeration systems amount to 2 to 17 percent every year, depending on the age and condition of the system.
It is crucial to detect leakages quickly. On one hand, any effort for required refilling should be kept to a minimum to avoid damage to the system and the products stored in refrigeration. On the other hand, any interruption of production costs money—whether it’s a real alarm due to a hazard or merely a false alarm. This is why ammonia leakages in particular need to be detected with high precision.
PROPER INSTALLATION OF FIXED GAS MEASUREMENT EQUIPMENT
In ammonia-based refrigeration systems, the transmitters of a gas measurement system are normally installed in the compressor room and near the valve stations to monitor the ends of pipes. The sensors in the compressor room should generally be positioned at the ceiling, because ammonia weighs about half as much as air and rises quickly after a leakage. Even if workers cannot smell ammonia in the lower section of the room, the concentrations at the ceiling can be significantly higher. When installing the transmitters, make sure that the sensors are not positioned directly in the airflow to or from the evaporator.
Extreme temperature fluctuations are prevalent in the compressor room. Moisture can develop during air heating and cooling due to condensation, which can affect the performance of sensitive devices. Moisture can also be generated during defrosting procedures at the evaporator. If possible, sensors should be positioned in locations that provide the best protection from these or other potential disruptive factors.
Because of freezing and fluctuating temperatures, moisture and corrosion, not every sensor is suitable for the challenging environment of a refrigeration system. For a reliable measurement performance, it is critical that the gas detectors are robust enough to withstand the wide range of temperatures and other prevailing challenges. In addition, the sensors must have a high level of precision in detecting ammonia.
A basic level with a low background concentration is often present in ammonia refrigeration systems. In order to avoid frequent false alarms, the defined alarm thresholds should be slightly above the Threshold Limit Value (TLV) level. A high quality sensor will support reliable and precise measurements and help minimize false alarms and production downtime.
For reliable and precise measurements, a gas detection system must be calibrated regularly and receive proper maintenance at specific intervals. This is even more important if the system operates under challenging conditions, as it does with ammonia refrigeration. It pays off to invest in higher quality service. Technicians with specialized expertise are faster at noticing and fixing effects of aging and signs of wear. As a result, experienced technicians increase the reliability of ammonia detection devices and extend their lifespan.
TRUSTED SAFETY SOLUTIONS
In the majority of ammonia accidents, the hazardous substance occurs in gaseous form. If injured persons are in the contaminated area, their rescue is obviously the highest priority. Respiratory protection independent of the ambient air is essential for firefighters and other emergency personnel. One important component is a head piece, which also protects the eyes from contact with the contaminated atmosphere. In many cases, gas-tight chemical protective suits are required in the hazardous area.
If liquid ammonia leaks from refrigeration systems, the situation becomes more problematic. Upon contact with the skin, the temperature of the liquid, which is -27.4 °F /-33 °C or lower, causes frostbite. The level of severity depends on the contact surface and the amount of liquid. Basic protective clothing cannot withstand the freezing cold liquid which corrodes through the material. When starting the work, wearing a chemical protective suit made of cold-resistant material is recommended. To protect the skin from frostbite, warming work clothes and woolen sock liners and gloves should be worn under the suit.
Selecting the right filter
Emergency personnel who work on a damaged system to detect leakages should be provided with personal gas warning devices for safety reasons. For work that involves the risk of leaking or rapidly escaping ammonia— such as opening the refrigeration circulation system—personal protective equipment should be used.
In the case of an alarm, escape apparatus that are independent of the ambient air support an organized evacuation of the plant. Filtering escape devices that protect workers from toxic gases also help protect escaping persons.
Special requirements need individual solutions
Ammonia-based refrigeration systems present a particular challenge for safety management. Gas measurement equipment, personal protective equipment, and escape equipment all require individual solutions that consider all plant-specific factors.
Both new installations and the integration of new components into existing infrastructures require fundamental knowledge of basic chemical and physical principles, technical parameters, and legal requirements—which makes for a highly complex planning process. Expert advice will be worth it every step of the way: during acquisition, installation, commissioning, and maintenance.