Most of the considerations for the installation of natural gas engine-driven air compressors are similar to those for electric air compressor systems. However, there are a few installation considerations that are unique to engine-driven air compressors. Of course, all installations must comply with local, state, and federal codes and standards, and also must follow the manufacturer’s recommended installation procedure for a specific product. Installation of engine-driven air compressors requires the following special considerations:
- Fuel Supply System [Go]
- Ventilation and Combustion Air [Go]
- Heat Rejection [Go]
- Jacket Water System [Go]
- Lubricating Systems [Go]
- Exhaust System [Go]
- Heat Recovery System [Go]
The primary components of the fuel supply system are shown below. To prevent damage to the natural gas piping system, a flexible connector should be installed between the engine and the rigidly supported gas line in the mechanical room. The gas piping material should be black iron.
Fuel Supply System Components
Generally, a primary and secondary gas regulator are used in series to adjust fuel supply to the engine. The primary regulator, which provides gross pressure regulation of the natural gas supply, is typically provided by the utility. A fuel supply pressure of 2 psig is generally required for naturally aspirated engines; however, the manufacturer should be consulted for specific supply pressure requirements.
Standard turbocharged engines are now available from most manufacturers with low supply pressure requirements, although higher pressures (approximately 20 psig) used to be the standard. A manual gas shut-off cock is installed upstream of the primary regulator. A secondary regulator fine-tunes the amount of flow to the engine. This regulator is usually supplied by the engine manufacturer. It is recommended that a filter be installed in the fuel supply to remove 99% of all particles larger than one micron. A dedicated gas meter for the engine is useful for recording fuel consumption and corresponding energy cost savings.
As part of the safety features designed into the gas train, redundant fuel shut-off valves automatically ensures that the fuel flow to the engine is stopped when the engine is shut down either normally or in an emergency condition. The shut-off valve is actuated by an electric solenoid. For added assurance, two solenoids are installed in series between the primary and secondary regulators. Factory Mutual (FM), Underwriter’s Laboratory (UL), and all other governing codes and utility requirements should be consulted for specific gas train requirements.
Ventilation air must be supplied to the engine for proper combustion and to remove the heat radiated from the engine. Three to six percent of the energy input to the engine is lost to the surroundings from the surface of the engine. The heat lost from other sources (exhaust and jacket water piping) can double the amount of heat released into the engine room. In confined areas, sufficient air flow must be provided to the engine such that the temperature rise does not exceed 15°F to 20°F. In multiple engine applications, this temperature rise is sometimes permitted to be higher. However, the manufacturer should be consulted for requirements for the ambient conditions. Generally, combustion air represents a small percentage of the total air required.
Proper ventilation is required for several reasons. First is the need to maintain a comfortable work environment for the operating and maintenance staff. Secondly, engine power is adversely affected by high inlet air temperatures. In addition, ventilation protects electrical components and equipment from temperatures that could jeopardize their performance.
Ventilation air must flow properly through the mechanical room to provide effective cooling of the engine. Ventilation air should be routed such that air circulates in proximity to the engine. Ideally, the engine should be oriented to allow the air to traverse along the entire length of the engine to maximize the cooling effect. Inlet air should be introduced low to allow it to sweep along the base of the engine; exhaust air should be evacuated high on the opposite end from the intake.
Heat rejection systems generally consist of a cooling tower and pump which remove heat from aftercoolers and oil coolers. In engine-driven systems, they also remove heat that is not recovered for useful purposes from the engine jacket water cooler. This type of system is shown schematically in the figure below. Because cooling tower systems are open to the atmosphere, heat exchangers should be used to isolate water circuits and prevent fouling of the engine. Additionally, filters or strainers should be used to keep the fouling of the heat exchangers to a minimum.
The efficient operation and thermal performance of a cooling tower depends on both mechanical maintenance and cleanliness. Basic maintenance of the cooling tower includes:
- Periodic inspections of the mechanical equipment to ensure that is in a good state of repair,
- Periodic draining and cleaning of wetted surfaces and areas of alternate wetting and drying to prevent the accumulation of dirt, scale, or biological organisms,
- Periodic cleaning of debris collected in the strainers,
- Proper treatment of the circulating water for biological control and corrosion, in accordance with accepted industry practice, and
- Systematic documentation of operating and maintenance functions
Heat Rejection System
(Click to enlarge)
Jacket water circuits are used to control the engine’s operating temperature. The water circuit pumps the water for heat rejection or for heat recovery. The heat can be recovered, productively used, carried away by an air-cooled radiator, dissipated in a cooling tower or raw water system, or rejected into the engine room. Circulating water systems must be kept clean because the internal passages of the engine are not readily accessible for service. Generally industrial engines cannot be drained and flushed effectively without major disassembly, although the opposite is true of automotive-derivative engines. Coolant fluids must be non-corrosive and free from salts, minerals, and certain chemical additives that can deposit on hot engine surfaces or form sludge in relatively inactive fluid passages. Limiting fresh water make-up is the initial step to maintaining clean coolant surfaces. Softened water or mineral-free water is effective for initial fill and make-up. The coolant system should also be tight and leak-free.
All engines use the lubricating system to reduce friction of hot moving parts and remove some heat from the machine. Some configurations only cool the piston skirt with oil; other designs remove more of the engine heat with the lubricating system. The operating temperature of the engine may be significant in determining the proportion of the engine heat removed by the lubricant. Between 5% and 10% of the total fuel input is converted to heat that must be extracted from the lubricating oil which results in the need for the oil cooler shown in the figure above. Oil coolant temperatures high enough to permit economic use in a process such as domestic water heating may be feasible.
Radiator-cooled units generally use the same fluid to cool the engine water jacket and the lubricant; thus, the temperature difference between the oil and the jacket coolant is not significant. If the oil temperature rises in one region of the engine, the heat may be transferred to other engine oil passages and then removed by the jacket coolant. When the engine jacket temperatures are much higher than the lubricant temperatures, the reverse process occurs and the oil removes heat from the engine oil passages.
The characteristics of each lubricant, engine, and application are different, and periodic laboratory analysis of oil samples is needed to establish optimum lubricant service periods. The following factors should be considered in selecting an engine:
- High-quality lubricating oils are generally required for operation at oil temperatures between 160°F and 200°F, with longer oil life expected at lower temperatures. Moisture may condense in the crankcase if the oil is too cool, which reduces the useful life of the oil.
- Copper piping should be avoided in oil-side surfaces in coolers and heat exchangers to reduce the possibility of oil breakdown caused by contact with copper.
- A full flow filter provides better security against oil contamination than one that filters only a portion of circulated lubricating oil and bypasses the rest.
Engine exhaust systems remove products of combustion from the engine and reduce exhaust noise through the installation of mufflers or silencers. Some heat recovery devices double as exhaust silencers. Each engine should be installed with its own individual exhaust and silencer system. Routing of exhaust piping should be short and direct in order to minimize system back pressure which impedes engine performance. Excessive back pressure causes loss of engine power, poor fuel economy, and high exhaust valve temperatures.
The exhaust pipe and silencer should be insulated for high temperature (1,200°F) service. The insulation is required to protect plant personnel, prevent exhaust gas from condensing, and limit heat transfer to the mechanical room. Exhaust systems with heat recovery should be constructed to resist corrosion, and an end cap should be installed on the exhaust pipe of all systems to prevent rain and debris from entering the system.
When heat recovery systems and catalytic converters are both used on a project, the heat recovery device should be located downstream of the catalyst. Because the catalyst raises the temperature of the exhaust gas stream, more heat is available to be recovered at its outlet.
Heat can be recovered in an engine from four locations: exhaust gas, jacket water, lubricants, and turbocharger. The major source of heat is exhaust gas, although only part of the exhaust gas heat can be salvaged due to the limitations of heat transfer equipment and the need to prevent flue gas condensation. Many heat recovery boilers designs are based on a minimum exhaust temperature of 250 °F to 300°F to avoid condensation and acid formation in the exhaust piping. Final exhaust temperature at part load is important for generator sets that operate at part load most of the time. Depending on the initial exhaust temperature, approximately 50% to 60% of the available exhaust heat can be recovered.
Jacket cooling passages for reciprocating engines must remove about 30% of the heat input to the engine. To avoid thermal stress, the temperature rise through the engine jacket should not exceed 15°F. Flow rates must be kept within the engine manufacturers’ design limitations to avoid erosion (excessive flow) or inadequate distribution within the engine (low flow). Low-temperature limit controls prevent excessive system heat loads from seriously reducing the operating temperature of the engines. This is done because engine castings may crack if they are suddenly cooled because a large demand for heating temporarily overloads the system. A heat storage tank is an excellent buffer for such occasions because it can provide a very high heat rate for short periods to protect the machinery serving the heat loads. The heat level can be controlled with supplementary heat input such as an auxiliary boiler.
Lubricant heat exchangers should maintain oil temperatures at 190°F, with the highest coolant temperature consistent with the economical use of the salvaged heat. Engine manufacturers usually size their oil cooler heat transfer surface on the basis of 130°F entering coolant water and without provision for additional heat gains that occur with high engine operating temperatures. The cost of obtaining a reliable supply of lower temperature cooling water must be compared with the cost of increasing the size of the oil cooling heat exchanger and operating at a cooling water temperature of 165°F. In applications where engine jacket coolant temperatures are above 200°F and where there is use for heat at 155°F to 165°F, the heat from the lubricant can be recovered economically.
Turbochargers on natural gas engines require 1.) the engine fuel supply be medium pressure gas (12 to 20 psi), and 2.) rather low aftercooler water temperatures (90°F or less) for high compression ratios and best fuel economy. Aftercooler water at 90°F is a premium coolant in many applications because the usual sources are raw domestic water and evaporative cooling systems such as cooling towers. Aftercooler water at temperatures as high as 135°F can be used, although the engine output will be somewhat reduced. Using a domestic water solution may be expensive because the coolant is continuously needed while the engine is running, and the heat exchanger designs require a large amount of water even though the load is less than 200 Btu/hp-hr. This “once-through” system may also have environmental permitting problems with the heater discharge. A cooling tower can be used, but unless required for other reasons, it will increase initial costs. Also, it requires winter freeze protection and water quality control. If a cooling tower is used, the lubricant cooling load must be included in the tower design load for periods when there is no use for lubricant heat.
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