As technology has evolved, so has the refrigeration industry. What were once holes in the ground filled with ice and snow have transformed into the modern high-efficiency compression machinery we have become so familiar with today. However, as common as these devices have become, the design process remains a challenge. This is where a combination of scientific knowledge, experience and creative initiative comes into play. While there are, of course, several variations in terms of the application of each design step, the guidelines presented here could be applied not only to refrigeration compressors, but also to compressors used in many other processes and industries.
There are a number of steps to consider throughout the compressor design process, and each step has to relate back to the original design concept. Experience has shown that having a starting concept and an end goal in mind is imperative. Namely, before you can begin the process, you need to know where you are starting and where you want to end up. With this in mind, before we can even get started with preliminary design, blade profiling and analysis of computational fluid dynamics (CFD), it is important to take out a piece of paper and start brainstorming. Consideration of the different refrigeration technologies (cycles), is always a great place to start, so we can ensure we will design the best compressor for the application. The cycle will directly impact the rest of the compressor design decisions, so this is not a step that can be bypassed. This article’s discussion begins with cyclic compression.
In modern days, power generation planners are faced with the challenge of pushing out the most energy from fuel while at the same time minimizing cost and emission. However, finite fuel also generates mass concerns regarding the reserve left to be used in nature. Consequently, people are continuously looking for an economical and highly efficient solution.
To this date, combined cycle gas turbine applications are found to be the best solution to the problem. The application is known to be highly efficient, have favorable energy conversion rates, comparatively lower start up time compared to conventional steam cycles and able to squeeze more power from the same amount of fuel.
The term, “mixed flow compressor”, refers to a type of compressor that combines axial and radial flow paths. This phenomenon produces a fluid outflow angle somewhere between 0 and 90 degrees with respect to the inlet path. Referred to as the meridional exit angle, the angled outflow of this mixed flow configuration possesses the advantages of both axial and centrifugal compressors. Axial compressors can produce higher order efficiencies for gas engines, but they have relatively low-pressure ratios unless compounded into several stages. Centrifugal compressors can produce high-pressure ratios in a single stage, but they suffer from a drop in efficiency. The geometrical distinction of mixed flow compressors allows for higher efficiencies while maintaining a limited cross-sectional area. The trade-off for a mixed flow compressor when introduced to aero gas turbines is that there is an associated weight increase due to the longer impellers needed to cover this diagonal surface. However, when related to smaller gas turbines, the weight increase becomes less significant to the overall performance of the engine.
Turboexpanders are used in a number of applications, including floating LNG (liquefied natural gas), LPG (liquefied petroleum gas) / NGL (natural gas liquids), dew point control, and ethylene plants. Used as a highly efficient system that takes advantage of high pressure, high-temperature flows, the turboexpander both produces cryogenic temperatures and simultaneously converts thermal energy into shaft power. Essentially, a turboexpander is comprised of a radial inflow expansion turbine and a centrifugal compressor combined as a single unit on a rigid shaft. The process fluid from a plant stream will run through the expansion turbine to both provide low-temperature refrigeration and convert thermal energy to mechanical power as a byproduct. First, the gas will radially enter the variable inlet nozzles (or guide vanes) of the turbine, which will allow for a localized increase in fluid velocity prior to entering the turbine wheel. The turbine wheel will accept this high-temperature, high-pressure, accelerated gas and convert it into mechanical energy via shaft rotation. The primary product of a turboexpander manifests at the outflow of this turbine. After the process gas passes through the turbine wheel, this gas has expanded so dramatically that it produces cryogenic temperatures colder than any other equipment in the plant.
The useful mechanical energy converted from this system is generally used to drive a centrifugal compressor positioned on the opposite end of the shaft. In the case of this expander-compressor setup, the mentioned turboexpander technology avoids the excessive use of fuel consumption seen in other systems, and significantly decreases the CO2 footprint of the overall design. As well, there are various examples of turboexpanders that use an expander-generator setup, which converts the mechanical energy from the turbine into direct electrical power. Turboexpanders have come a long way in the last 40 years. With the advent of magnetic bearings and more advanced sealing systems, turboexpanders have been able to handle shaft speeds in large and small machines of up to 10,000 rpm and 120,000 rpm, respectively. Moreover, innovations in specific CFD modules for turbomachinery have allowed turboexpander systems to achieve efficiencies upwards of 90%.
Many people speculate about the confusion on what is considered a compressor, a blower, or simply a fan. In essence, each of these turbo-machines achieve a pressure rise by adding velocity to a continuous flow of fluid. The distinctions between fans, blowers, and compressors are quite simply defined by one parameter, the specific pressure ratio. Each machine type, however, utilizes a number of different design techniques specific to lower and higher-pressure applications. As per the American Society of Mechanical Engineers (ASME), the specific pressure is defined as the ratio of the discharge pressure over the suction pressure (or inlet pressure). The table shown below defines the range at which fans, blowers, and compressors are categorized.
Similarities between the design of fans and blowers occur near the lower end of a blower’s range. As well, many design parallels exist between high-pressure blowers and compressors. For the article, we will be investigating the different design characteristics of centrifugal blowers. Blower selection depends on a number of factors including operating range, efficiency, space limitations, and material handled. Figure 1 shows a number of different impeller blade designs that are available for centrifugal blowers.
The integrally geared compressor, also known as a multi-shaft compressor, is a technology that has been around since the 1960s, but remains underdeveloped. Usually seen in applications in the industrial gases industry, integrally geared compressors (IGCs) can range in size from small product machines to steam turbine driven high-horsepower, high-flow compressors for air separation plants. These compressors modular construction principle, consisting of as many as eight different stages, allows for implementation in a large number of varied customer processes. The main advantages of IGCs in the industrial gases industry is the compact design and smaller installation footprint, efficiency increases due to the use of multiple speeds for separate impellers, and overall lower operational and installation costs.
One of the key design differences between the standard inline compressors and the IGCs is that the integrally geared compressor makes use of both closed AND semi-open impellers. The reason for the use of open impellers in IGCs are the higher strengths due to better manufacturing techniques, speed of manufacture, and the inherent lower costs. However, the main drawback to having an open impeller in your system is that in the event of impeller rub, the damage to the compressor would be significantly worse than with a closed impeller.
As turbomachinery technology continues to advance in efficiency as well as overall power, many engineers want an estimate on how long these manufactured machines will operate. Specifically, in high-temperature and high-flow turbomachinery applications, one of the main sources of performance degradation can be attributed to increases in surface roughness. Gas turbine and compressor blades in particular experience a substantial amount of surface degradation over their lifetime.
There are many mechanisms that contribute to surface degradation in airfoils and annulus surfaces. Foreign particles adhering to the material surface (or fouling) is generally caused by any increase in contaminants such as oils, salts, carbon, and dirt in the airflow. Corrosion occurs when there is a chemical reaction between the material surface and the environment that causes further imperfections on the machine surfaces. Additional mechanical factors such as erosion and abrasion will play a part in a machine’s surface degradation as well.
One of the most prominent steps of complete gas turbine design is turbine-compressor matching. There are three major components to a gas turbine: compressor, combustor, and turbine. Although all of the components are designed individually, each of the components needs to correspond within the same operating condition range since all are integrated into one cycle. Consequently, an optimal design of each component must fit the requirement of other component’s optimal parameters. Corresponding operating points for each component must be found at equilibrium with the engine, thus the overall performance of gas turbine can be reached within the defined range of parameters.
The idea behind component “matching” process is to find flow and work compatibility between corresponding components. Based on the mechanical constraints, gas generator speed and firing temperature of a gas turbine have limitations depending on: ambient temperature, accessory load and engine geometry. The match temperature chosen should be the ambient temperature which reach both upper limits at the same time. Pressure ratio needed to allow a certain gas flow is also one of the most prominent parameters that has to be taken into consideration. Designers need to make sure that the gas flow through the power turbine from gas generator satisfy the pressure ratio needed for compressor power requirements. Gas generator can easily show an altered match temperature due to some conditions i.e: reduction in compressor efficiency (due to fouling, etc), change of thermodynamic properties of combustion product, gas fuel with lower or higher hearing value, etc. Match parameters of an engine could also be altered by changing the flow characteristics on the first turbine nozzle.
Using characteristic map/curve as well as thermodynamic relationships of turbine and compressor, calculations can be performed to identify the permitted operating range. It must be taken into consideration that all calculated value must match the value from map data.
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Even though energy consumption for HVAC and refrigeration system is considerably smaller than most technology applications, energy savings is still desired for many reasons: cleaner technology, saving cost, fuel economy and many more. Improvements in insulation, compressor efficiency and optimization of the cycle can be implemented to achieve better performance. Installation of variable speed drives is one way to optimize the potential of HVAC system.
Although has been implemented to various HVAC components, variable-speed drive is considerably still one of the “newer” advancements in the compressor industry. These devices are able to precisely control the motor speed and trim/balance systems. Variable speed control compressor gives end-users the comfort of matching the speed to what is needed at the time; giving precise temperature control with less cycling and longer run times. With longer run times, the technology also helps to remove moisture and relative humidity during the summer; or on the other hand during the winter by increasing the speed of compressor, system are able to deliver hotter air.
Compared to fixed compressor, where there are only two options for end-users to set: maximum capacity or completely off; variable speed drives gives the end-user an ability to adjust power output to compressor. The technology also comes with the benefit of less energy wasted from off and on cycle, precise load matching and low amp gradual compressor motor startup; therefore, improving the efficiency on certain conditions.
Coupling variable speed drives to centrifugal compressor alter the behavior of the component. Although, not always requiring smaller energy (i.e at or near full load) compared to fixed speed compressor, installation of VSD could really benefit the users in terms of power consumption (i.e at part lift), to optimize even further implementation of both compressor types would benefit both conditions.
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A turbocharger (TC) has to provide a required pressure ratio for efficient combustion and operation of an internal combustion engine (ICE). The turbocharger consists of a turbine and a compressor sides on the same shaft. The turbine utilizes the energy of exhaust gases while the compressor forces the air into the engine. The compressor with a wide operating range is a strict requirement in the automotive industry because the unit has to operate across all of the ICE regimes.
Even though any compressor has a design point, the ability to operate at low and high mass flows is critical for TC compressors. To satisfy the operating range requirement, a designer tries increasing mass flow at choke and decreasing mass flow at surge. This is quite a challenge. For smaller mass flow rates, the impeller outlet and diffuser should be optimized. The choice of a vaneless diffuser is always justified by increased flow range at the cost of efficiency.
To increase the right-most mass flow limit, a designer optimizes the compressor inlet. The common practice is to design blades with large inlet metal angles. Increase in inlet angles open larger area for the flow to pass. This, in turn, leads to large incidence angles at design point. Therefore, many TC compressors are designed with large positive incidence in the design point. The incidence angle increases for every speedline going toward the surge line. Incidence distribution on a TC compressor map is shown in the figure below. It is equal to +12 deg (with respect to tangent) in the design point.
Design point: An operating condition where a compressor reaches maximum efficiency
Compressor Map: Pressure versus mass flow characteristic at different rotational speeds and isoefficiency contours
Speedline: Dependence of pressure on mass flow rate for a given shaft speed
Surge: Left-most point on a compressor map for a given shaft speed
Choke: Right-most point on a compressor map for a given shaft speed
Incidence: The difference between inlet flow and metal angles. If an incidence is small, the flow has less resistance to enter the impeller.
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