Operation absorption refrigerators operating on gaseous fuel requires a deep understanding of the thermodynamic processes occurring inside the system. A special place in the history and practice of autonomous cooling is occupied by units adapted for operation on coke oven gas, which is characterized by a specific chemical composition and special combustion requirements. When technical documentation or task conditions mention the parameters of gas supply at a temperature of 80°C and a pressure in the range of 100-600 mm Hg, we are talking about critical operating modes of a generator or preliminary preparation of the fuel mixture.
Understanding the physics of the process allows you not only to correctly configure the equipment, but also to avoid emergency situations associated with violation tightness or change in the phase state of the refrigerant. Unlike household compressor models, here the driving force of the cycle is the thermal energy obtained by burning gas. That is why control over the temperature of the coolant or the gas itself, as well as the pressure in the supply system, becomes a key factor in the efficiency of the entire cooling cycle.
In this material we will analyze in detail how these parameters affect the operation of the absorption machine, why the pressure range is 100-600 mm Hg. Art. works for certain types of burners and injectors, and what role does gas preheating to 80°C play? This knowledge is necessary for engineers, technicians and owners of specialized equipment to ensure stable operation of refrigeration units in conditions where the electrical network is not available.
Fundamental diagram of the operation of an absorption refrigerator on gas
The basis of the operation of any absorption unit is a continuous cycle of circulation of the working mixture consisting of a refrigerant (usually ammonia) and absorbent (an aqueous solution with the addition of an inert gas, such as hydrogen or helium). Unlike compressor systems, where vapor compression is carried out by a mechanical piston, here this function is performed by thermochemical compressor. A gas burner burning coke oven gas heats the bottom of the generator, causing the solution to boil and release ammonia vapor.
The process begins with a weak solution of ammonia in water rising through the generator tube. Under the influence of heat, the components separate: ammonia goes into a gaseous state, and water remains in a liquid state, flowing back. Next, the ammonia vapor passes through a reflux condenser, where partial condensation of water vapor occurs, and pure ammonia enters the condenser. There it cools, turns into liquid and enters the evaporator through the throttle.
In the evaporator, due to the low partial pressure of hydrogen, liquid ammonia boils intensely, taking heat from the refrigeration chamber. Then the mixture of ammonia and hydrogen vapor enters the absorber, where it meets the weak solution flowing down from above. Ammonia is absorbed by water, and hydrogen, being lighter, is returned to the evaporator. The cycle is closed, and to maintain it, a constant supply of heat to the generator is required.
Why hydrogen?
Hydrogen is used in the system as a buffer gas. It does not participate in the chemical reaction, but creates the necessary partial pressure, allowing ammonia to boil at low temperatures without creating a vacuum in the entire system.
The effectiveness of this process directly depends on the stability of combustion and the accuracy of maintaining the temperature regime. Any fluctuations in fuel supply or changes in its physical properties (temperature and pressure) immediately affect the performance of the installation. Therefore, the parameters specified in the request are not just numbers, but working coordinates for setting up the system.
Physico-chemical characteristics of coke oven gas
Coke oven gas is a by-product of coking coal. This is a complex mixture of flammable gases, the main components of which are hydrogen (about 50-60%), methane (20-30%), carbon monoxide and various hydrocarbons. The most important characteristic for refrigeration equipment is the heat of combustion, which for coke oven gas is lower than for natural gas, but higher than for blast furnace gas. This requires special calibration of burner nozzles.
The density of coke oven gas is significantly less than the density of air, which creates the risk of upward leaks and requires special attention to the tightness of the upper connections of the pipelines. In addition, the gas may contain impurities of sulfur and naphthalene, which, during combustion or cooling, can contaminate nozzles and heat exchangers. That is why the requirements for filtration and preparation of gas before combustion in refrigeration units are extremely high.
When it comes to a gas temperature of 80°C, we are faced with a non-standard, but technically sound parameter. Typically the gas is supplied at ambient temperature. However, heating gas to 80°C can be used in specific industrial conditions or in heat recovery systems for:
- 🔥 Preventing condensation of heavy fractions and naphthalene in pipelines during sudden expansion.
- ⚙️ Increasing combustion efficiency in special low-temperature burners where pre-activation is required mixtures.
- 🌡️ Compensation for heat loss in long supply lines, so that by the time of combustion the gas maintains its design parameters.
The use of hot gas requires the use of heat-resistant materials for seals and gaskets, since standard rubber at 80°C can quickly degrade, losing elasticity and tightness. This is a critical point when upgrading or servicing old equipment.
Pressure analysis: range 100-600 mm Hg. Art.
Pressures in the range of 100 to 600 mmHg (approximately 1330–8000 Pascals or 0.013–0.08 atm) are characteristic of low pressure systems, often found in older absorption plants or natural draft systems. For comparison, the pressure in a domestic gas network is usually about 130 mmH2O. Art. (about 9-10 mm Hg), which is significantly lower than the indicated values.
Such a wide range (100-600 mm Hg) may indicate the system is operating in transient modes or the presence of a pressure regulator that ensures supply stability during fluctuations in the main network. In the context of coke oven gas, which is often used in industrial settings or in autonomous gas supply systems (gas holders), the pressure can vary depending on the fullness of the tank and the ambient temperature.
If the gas is supplied at a pressure of 600 mm Hg. Art., this requires installing a high-quality gearbox in front of the refrigerator burner. Directly applying such pressure to a standard nozzle will lead to flame separation, soot, and a sharp drop in the efficiency of the refrigerator. On the contrary, a pressure of 100 mmHg. Art. may be the minimum necessary to overcome the hydraulic resistance of the injector and ensure stable combustion.
⚠️ Attention: Pressure 600 mm Hg. Art. (about 0.08 bar) significantly exceeds the operating pressure of household gas appliances. The use of such pressure without a reducer is strictly prohibited and can lead to an explosion of the burner device or a fire.
Pressure control is carried out using pressure gauges, graduated in appropriate units. Modern systems often use electronic sensors that convert these values into signals for safety automation, which will shut off the gas supply when the parameters go beyond acceptable limits.
The influence of temperature and pressure on the efficiency of the cycle
The relationship between the temperature of the incoming gas, its pressure and the efficiency of the refrigerator is described by the laws of thermodynamics and hydrodynamics. The supply of gas at 80°C affects its density: hot gas is less dense than cold gas. With a constant volume flow through the nozzle, the gas mass flow will decrease, which can lead to underheating of the generator and a drop in cooling capacity.
However, if the system is configured for mass flow (which is more correct), then gas heating can be compensated by appropriate pressure adjustment. Range 100-600 mm Hg. Art. allows flexible flow control. At high pressure (600 mm Hg), even hot gas will be supplied in sufficient quantity to maintain boiling. At low temperatures (100 mmHg), heating can be a critical factor in ensuring the required speed of the mixture in the airlift of the generator.
The table below shows the approximate effect of changing parameters on the thermal load of the generator (assuming constant burner geometry):
| Gas parameter | Pressure (mm Hg) | Temperature (°C) | Impact on the process |
|---|---|---|---|
| Normal | 130 (water column) ~ 9.5 | 20 | Normal operation |
| High pressure | 600 | 20 | Risk of flame separation, noise, soot |
| Heated gas | 100-600 | 80 | Decreased density, change in volume calorific value |
| Low pressure | < 50 | 20 | Underheating, lack of circulation, refrigerator failure |
It is important to note that coefficient of performance (efficiency) absorption machine strongly depends on the boiling temperature in the generator. The optimal generator wall temperature is usually 150-170°C. If the incoming gas is already at a temperature of 80°C, this reduces the thermal load on the walls of the heat exchanger, but the main contribution is made by the heat of combustion. Therefore, the main task is to ensure the correct gas-air ratio.
Setting and adjusting the burner device
Adjusting the burner to operate on coke oven gas with the specified parameters requires precise adjustment of the air damper and inlet pressure. Since coke oven gas contains a significant amount of hydrogen, the combustion rate of such a mixture is higher than that of natural gas. This means that the primary air must be supplied more intensely to avoid the flame slipping into the burner.
The adjustment process begins by checking the pressure at the inlet to the reducer. If it is 600 mmHg. Art., it is necessary to set the gearbox to the operating value specified in the refrigerator’s passport (usually this range is 130-200 mm of water column, which in terms of a column of mercury will be about 10-15 mm). If the system is designed specifically to operate at a pressure of 100-600 mmHg. Art. (for example, in industrial installations), the adjustment is made by changing the cross-section of the throttle opening.
To ensure stable combustion at a gas temperature of 80°C, it is necessary to check the materials of the mixing chamber. The flame should be even, blue, without yellow tongues (a sign of incomplete combustion) and without detachment from the divider. The yellow color of the flame when working with coke oven gas often indicates a lack of air or the presence of impurities that require cleaning the nozzle.
☑️ Setting up the burner
Particular attention should be paid to the ignition system. When using hot gas, the spark gap may require correction, since the electrical conductivity of the hot gas mixture is different from the cold one. In some cases, easier ignition may be observed, but there is also an increased risk of popping noises if an attempt is unsuccessful.
Maintenance and operational safety
Operation of equipment using coke oven gas, especially at non-standard temperatures and pressures, requires strict adherence to safety rules. Coke oven gas is toxic due to its carbon monoxide content. Even a small leak in a confined space can be deadly. Therefore, rooms with such equipment must be equipped with effective supply and exhaust ventilation and gas pollution sensors.
Regular maintenance includes:
- 🧹 Cleaning of nozzles and jets from soot and tarry deposits characteristic of coke oven gas.
- 🔍 Checking the integrity of heat exchangers for microcracks caused by thermal expansion when working with hot gas.
- 🛠️ Monitoring the operation of safety automation (flame, draft and pressure sensors).
When working with pressure up to 600 mm Hg. Art. All threaded connections must be made using high-quality seals that are resistant to gas and temperature. The use of lead white or ordinary tow without impregnation is unacceptable, since the gas easily penetrates through microscopic channels.
⚠️ Attention: If you smell gas or signs of poisoning (headache, nausea), immediately turn off the fuel supply, open the windows and leave the room. Do not turn on electrical appliances and do not use open fire.
Also, periodic flaw detection of metal allows you to prevent emergency situations associated with depressurization of the system under pressure.
Frequently asked questions (FAQ)
Is it possible to connect a refrigerator designed for natural gas to coke oven gas?
Without modification - no. Coke gas has a different calorific value, density and burning rate. It is necessary to replace the injectors (nozzles) with analogues with a larger cross-section, as well as adjust the primary air. In some cases, it may be necessary to replace the burner itself.
Why is the gas heated to 80°C before entering the refrigerator?
This is not done at home. Heating up to 80°C can be used in industrial systems to prevent the formation of condensate (naphthalene) in pipelines or to use waste heat from technological processes. Under normal conditions, the gas should be at room temperature.
Which means a pressure of 100-600 mm Hg. Art. in the instructions?
This is the operating pressure range at the inlet to the control system or burner for specific industrial models. For household appliances, this value is too high (usually 130 mmHg ≈ 9.5 mmHg). It is necessary to carefully read the device passport so as not to damage the equipment.
How often should you clean the burner when operating on coke oven gas?
Due to the presence of impurities and resinous substances, it is recommended to clean it at least once a year, and with intensive use - every 6 months. A sign of the need for cleaning is a change in the color of the flame to yellow or the appearance of soot on the heat exchanger.
Is coke oven gas dangerous for rubber seals?
Yes, some components of coke oven gas and high temperature (80°C) can destroy ordinary rubber. It is recommended to use seals made of special gas-resistant and heat-resistant materials, such as fluorine rubber or paronite.