Compressors are machines capable of moving gases from a lower pressure to a higher pressure. Fluids can only move spontaneously in the opposite direction, we mean, from a higher pressure to a lower one. That is why we need to employ a machine in this type of service.
Industrial compressors are installed in the context of Compression Systems, composed by the compressor itself and several auxiliary equipments and devices, like pipes, vessels, valves, heat exchangers, instruments, controllers and eventually others.
A driver machine is responsible for supplying the power required for gas compression. Coupled to compressor’s shaft, the Compressor Driver can be chosen from the following types:
- Eletric motors
- Steam turbines
- Gas turbines
- Combustion motors
A compression system works receiving gas from upstream (supply) and pushing it to downstream (delivery).
Generally speaking, the supply comes from one or more gas generation processes and it is delivered to one or more gas consumption processes. Thus the compressor can be seen as a link between processes that take place upstream and downstream of it.
Simple Compression Systems are equipped with only one compression unit and move the gas along a unique path (only one gas source and one gas destination).
Some more complex compressors arrangements are commonly used in industries, and will be described later.
Compressors internally divided in multiple sections separated by side streams, for instance, belong to the complex compression systems category.
1.2 Implications of the Gas Compressibility
Unlike the liquid pumping processes, gas compression is unavoidably accompanied by two important collateral effects:
- Increase of gas density
- Increase of gas temperature
Both effects are consequences of gas compressibility
The increase of gas density (or reduction of specific volume) derives from the application of compression forces to the surface of gaseous substances, characterized by easily deformable elastic molecular structures. This effect goes unnoticed in pumps because of the almost absolute incompressibility of the liquid.
Part of the energy transferred by compressors to the compressed gas is received and stored as molecular internal energy. Temperature is an indication of the internal energy level of a substance. This is the reason why some increase of gas temperature is observed during the compression.
Gas temperature increase is something to be taken into account during the compressor design because entails a potential threat to the integrity of the machine and also to the compression system as a whole.
Thus, could we say that temperature increase is an undesirable occurence under any point of view?
Not exactly. Look at figure below. It depicts the typical shape of a compression process in the PxT diagram of a pure substance.
We can see that pressure and temperature grow together so that the resulting process path deviates progressively from the saturation line. Otherwise, the perspective of condensation would transform the compressor design into a very challenging task.
On rare occasions the gas temperature increase can be harnessed to the benefit of the application (such as when combustion processes follows compression) or even to enable an operational goal (for example in refrigeration systems, as will be shown right ahead).
1.3 The Zeroth Principle
The operational characteristics presented by a compressor in a compression system depend not only on its geometry and settings, like size and rotation. There are other factors related to the service being carried out that can exert a strong influence on the compressor performance. According to the zeroth principle, these factors, known as service parameters of the compression, are essentially four. Three of them refer to the conditions in which the gas is continuously generated by an upstream process, and a fourth one that comes from downstream process operation:
- Suction Pressure (Ps)
- Suction Temperature (Ts)
- Gas Composition
- Discharge Pressure (Pd)
The subscripts (s) and (d) stands for supply (or suction) and delivery (or discharge) respectively.
The zeroth principle is one of the most relevant concepts in compression theory.
Each set of service conditions leads to a given operating point of a compressor, resulting in a well defined set of performance parameters of the machine, among which the main are:
- Gas flow rate
- Specific work
- Power consumption
- Compression efficiency
- Discharge temperature
- Heat exchanged
In many cases, the speed of rotation is changeable, as if introducing a fifth degree of freedom in the compressor-system relationship.
Changing the rotation, we can move the compressor to another operating point, under the same set of service conditions.
Rotation speed is just one of the tools used for adjusting the operating point of the compressors, and perhaps the most convenient of all, but there are other options, as we shall see later.
1.4 Steady State versus Transient State
Steady State is a condition commonly assumed in studies of fluid systems. It means that the fluid properties - pressure, temperature, speed, ... - at each section remain unchanged over the time (although these properties do change along gas path). Transient State is obviously the opposite.
Steady State is a very convenient condition for a system. It derives from the preservation of a desirable balance between system inputs and outputs. This makes less likely the occurance of a operational disruption caused by some parameter going out of its acceptable range.
Compression systems would never operate rigorously in steady state if it had not been for the assistance of automatic controllers. These devices exert continuous interventions in some of the system parameters, extinguishing the transients as soon as they appear. Moreover, steady state hypothesis makes easier the flow calculations and the analysis of system behavior.
1.5 Elementary Considerations about Compressors Performance Parameters
1.5.1 Gas Flow Rate
In a simplified compression system analysis, we describe the gas movement by means of a single spatial coordinate, so that general flow properties (pressure, temperature, speed, etc ...) are considered as having each one a unique value for all points belonging to a given flow section (the term “section” here refers to an area perpendicular to the flow direction).
The fluid flow intensity through a given section of an equipment can be expressed in either massic or volumetric basis. We mean the mass flow rate (M) — amount of mass passing through the section in one unit time — or the volumetric flow rate (Q), if the volume of fluid projected through the section is used, instead of the mass.
When the compression system operates in steady state, the mass flow rate is the same in any two consecutive sections, provided there are no inflows and outflows in-between. This conclusion comes from the well-known Kirchhoff’s Law. The volumetric flow rate, by contrast, varies continuously according the flow properties. The relationship between mass flow and volumetric flow at the same section is given by the local fluid specific volume (v):
Incredibly, neither the former nor the latter are commonly used for gas flow rate specifications. In engineering practice, gas flow rates are much more often expressed in standard volumetric flow rates (Q0), or “volumetric rate referred to standard conditions of pressure (P0) and temperature (T0)”, like normal cubic meters per hour, for instance. Caution is necessary when interpreting the meaning of this parameter. It represents the volumetric flow rate assumed by the gas if it was hypothetically carried to a section where pressure and temperature exhibited the standard values, but maintaining the same original mass flow rate. When expressing flow in Nm3/h, the conventionally accepted standard is 100 kPa of pressure and 0°C of temperature. Since there is no reason for these gas properties to appear together anywhere in the system, this normalized volumetric flow rate Qo is not an actual volumetric flow rate. Anyway, according to the definition, dividing the standard flow by the specific volume of fluid at standard conditions (vo), we get the mass flow rate through the system:
The specific volume at standard conditions for a gas with molecular weight MW can be approximated by:
sendo v0 expresso em m3/kg, T0 em K e P0 em kPa.
As the standard specific volume is itself a constant for each gas stream, the standard volumetric flow rate is proportional to the mass flow rate, and consequently is also constant for the entire flow (provided, of course, the steady state prevails). This explains why it’s repeatedly affirmed that the standard flow rates are, in fact, indirect measurements of mass flow rate, as long as the gas composition does not change.
Application Example: An operational plant was revamped with the aim of obtaining an increase of load. Before the intervention, the maximum gas charge of the unit was 80000 normal m3/h. After some change of settings, the plant was restarted and under full load, the following conditions were measured:
- Volumetric (actual) suction flow rate: 65000 m3/h
- Suction pressure: 150 kPa
- Suction temperature: 300 K
- Gas molecular weight: 28,0
- Based on these data, was there any improvement in the gas charge capacity of the plant?
- If so, in what amount?
- How would the potential uncertainties about the molecular weight value affect this conclusion?
The comparison between load capacities of process plants should be executed on massic basis. However, the use of standard flow rates is also valid, if there is no change in the gas molecular weight.
We can find the load in normal m3/h related to the new operating conditions by means of:
There was an increase in load capacity slightly above 10%, presuming the invariability of the molecular weight. In case of a different molecular weight, the the same standard flow rate will represent another mass flow rate, as shown by the following expression:
1.5.2 Specific Work
Specific Work (w) is the amount of work per unit mass which has to be transferred to the compressor in order to perform the gas compression. When dealing with Turbocompressors (we shall soon see what that means), only a very small part of this energy is dissipated in mechanical friction or employed to drive auxiliary equipments. The meaningful part is assigned to the gas that passes through the machine and is named Head (H), although the term “actual specific work” can be used as well. Turbocompressors have their behaviour strongly related to this quantity.
In Thermodynamics context, work is energy somehow related to movement. In our case, work comes from the moving parts of the compressor and goes to the gas flowing inside it.
We can’t talk about an absolute amount of work associated to the compression, because we are dealing with a continuous process. We have to consider the amount of work per mass unity, or specific work (head). In a steady state process, the head would be a constant, of course.
According to the Energy Conservation Law, the specific work supplied to a compressor must be equal to the rise of the energy per unity of mass stored in the gas flowing through the machine, provided the heat exchanges are negligible. Then, a fundamental question arises: how do flowing fluids store energy?
HEAD = OUTLET FLOW ENERGY - INLET FLOW ENERGY
One part of the work done is stored as molecular internal energy of the gas. When compressed, the gas suffers volume contraction, acting like a spring, so that the pressure forces exert work on the molecules, increasing their energy (remember that roughly speaking, work is the product of a force acting on a body and the displacement caused in consequence).
The other part of the work done is employed in the gas transport, allowing it to move from a lower (suction) pressure to a higher (discharge) pressure. The work is exerted against the forces originating from the pressure field and incorporated by the gas as potential flow energy. (A minimal amount of work could also be associated to changes in kinetic energy or gravitational potential energy of the gas, but both are generally irrelevant).
We could dive into a large discussion about this matter but we think it’s not the place nor the moment to do that. We will limit us to say that the sum of the two forms of energy transfer before mentioned corresponds to the variation of the gas thermodynamic property known as Specific Enthalpy (or simply Enthalpy). Then we have:
HEAD = OUTLET FLOW ENTHALPY - INLET FLOW ENTHALPY
HEAD = ENTHALPY INCREASE
Enthalpy (h) is an immeasurable property. Though, it is a thermodynamic property, and so its value can be obtained as function of gas composition, pressure and temperature, by means of thermodynamic calculations.
Power (W) is the rate of mechanical work per unit time. It represents the true energy demand of a compressor.
When a compressor is working in steady state, the power supplied by the driver is equal to the power absorbed by the compressor (except by some energy dissipation due to mechanical friction).
Suppose two compressors A and B working in independent applications. The head applied by the system to Compressor A is much higher than the head applied to compressor B. Thus may we say that compressor A needs a bigger driver than compressor B?
Of course not. Head is energy per mass unity of gas. Power also takes in account the intensity of gas flow passing through the compressor. It is the product of head and mass flow rate:
Both these quantities exert equally important influences on the power, as we can see.
There is also a relationship between power and the efforts resulting from driving action, involving the torque (τ) applied to compressor’s shaft and the speed of rotation (N):
This formula explains why every fluid machine have an specification of maximum transmissible power by the shaft. The true purpose is to limit the torque. Calculating the power on the shaft and measuring the rotation are much simpler tasks than directly assessing the torque applied to the shaft.
It doesn’t matter if a compressor is able to execute a given service and if it succeed in delivering the specified gas flow rate. It must do it in a reasonably efficient way. Efficiency can be understood as rational use of energy. There are two different effects incorporated into the parameter referred to as Total Compression Efficiency(η).
One of them is energy degradation. That’s as we call the conversion of a most useful form of energy (mechanical energy) into a less useful form (internal energy), by the action of dissipative effects. This role is played by fluid friction and turbulence inherent to every flow phenomenum. In a practical point of view, energy degradation represents compression energy waste, giving rise to the definition of a Thermodynamic Efficiency (ηth).
The other part of the Total Compression Efficiency refers to energy losses originated by mechanical friction between machines parts in contact, leading to the definition of a Mechanical Efficiency (ηmec).
The efficiency can give us a good insight about the quality of compressors performance. But have in mind that it is a quantity affected by many factors, whose calculation requires great care.
1.5.5 Discharge Temperature
Gas temperature increases observed during compression can bring undesirable consequences, so this parameter must be maintained under continuous monitoring. The maximum acceptable temperature is one of the most important limiting factors of compressing units operation, sometimes forcing the use of serially disposed machines interspersed with gas coolers.
Suction and discharge pressures, suction temperature, gas compressibility and thermodynamic efficiency are the main parameters affecting compressors discharge temperature. For each set of service conditions (Ps, Ts, Pd and gas composition) there is an expected value for thermodynamic efficiency and consequently an expected value for discharge temperature as well. If a discharge temperature higher than the predicted one results from the gas compression, some kind of machine degradation might be in progress.
The maximum allowable discharge temperature of a turbocompressor is targeted towards the application, normally ranging between 200°degC and 300°degC.
1.5.6 Heat Exchanges
Thermodynamics shows that gas cooling during a compression process brings us two important advantages. For the same set of service conditions, cooled compression would lead to:
- Reduction in the final compression temperature
- Reduction in the specific work demanded by the compression
Unfortunatelly, most of compressors types (such as turbocompressors, certainly) provide very poor conditions for thermal transfer and therefore are hardly ever equipped with internal cooling apparatus. It wouldn’t be profitable.
The natural thermal losses (to the environment) cause negligible effects on the compression processes, so non-cooled compressors should be considered essentially adiabatic.
We would dare say that, in real world, the reasons for building a cooled compressor would be others than the two mentioned above. Probably the intention to limit the compressor’s casing temperature or something else.
Compressors users are ordinarilly interested in gas pressure rise, not in temperature rise. Therefore, there is no reason to deliver hot gas thru a piping circuit of an industrial plant.
Compression systems frequently use external cooling with the main objective of limiting the gas discharge temperature. In these systems the compression is divided into steps that are interspersed by cooling the gas in heat exchangers.
The compression steps can be performed in independent machines or in a unique machine divided in sections. The second option is more economically attractive , of course.
It can be shown that the arrangement with external cooling entails some lowering of power consumption if compared to the compression in a unique step.
The aftercooler shown in the above scheme can be encountered in almost all industrial compression systems, with the obvious function of cooling down the compressed gas to the atmospheric temperature or close to it.
Aftercooling is of special importance when the compressed gas is a mixture containing easily condensable components. As we can see in a liquid-vapour equilibrium diagram, the dew point (condensation temperature) of a substance increases with pressure, making feasible the condensation even at high temperatures.
The aftercooler induces the gas condensing and allows the collection and extraction of the liquid generated, avoiding the occurrence of this process in the pipes. Liquid accumulations in gas piping lead to potential risk of damage.
This explains why even highly volatile components of a gas can be kept in vapour phase if the pressure is low and become liquid when compressed and cooled sequentially, giving raise to a liquid phase. The higher the dew point, the easier it will be to cause condensation by heat removal.
1.6 Turbocompressors Working Principles
There are two principles which underlie the conception of all kinds of compressors for industrial use. These principles distinguish the compressors in two categories designated as volumetric and dynamic.
In volumetric compressors, the increase of pressure is achieved by reducing the volume occupied by the gas inside a compression chamber. At least three steps are involved in the operation of such kind of machines. In the beginning, a certain amount of gas is admitted into the chamber, through its suction end. Then the chamber is closed and undergoes some volume reduction, causing pressure increase. Finally, the chamber is opened and the gas is delivered to consumption through the compressor discharge end. This so-called operational compressor cycle is repeated during each rotation of the machine shaft. The gas is transported in small portions per each cycle, causing the total flow rate to be strongly dependent on the machine size and rotation.
Reciprocating Compressor, the first built compression machine, and still the most used type of compressors worldwide.
Dynamic compressors, also called turbocompressors, execute the compression in two steps. Firstly, the gas passes through a rotating device, called impeller, where it’s given the amount of energy needed to perform the compression. However, because of reasons intrinsically related to the compression process, the energy is transferred to the gas in two distinct forms: Kinetic Energy and Enthalpy. The enthalpy is the energy form related to the pressure rise that occurs still inside the impeller. The other portion, received in the form of kinetic energy, causes the gas to be accelerated when travelling throughout the impeller channels.
The rotor of a domestic fan is an example of impeller. Differently from what happens in compressors, almost the total amount of energy given to the air in a fan is kinetic energy. Practically no enthalpy gain. Happily, because to create an airstream is everything what we intend a fan to do.
When dealing with compressors we are interested basically in pressure gain, not in speed gain. Then, as soon as the gas is thrown out by the impeller, the conversion of its kinetic energy into enthalpy has to be accomplished. With this purpose, a second element, called diffuser, is employed. The diffuser is a non-moving piece designed with the appropriate geometry for the mentioned energy conversion. The diffusion process is achieved merely by passing the flow through some type of channel or duct having increasing cross section area.
In the figure beside we see a diffuser with indication of the proper flow orientation capable of promoting deceleration and pressure growth. Actually, this will happen only when handling fluids moving with subsonic speeds. Anyway, that’s the normal situation we face in compressors operation.
In spite of being a two steps process, the compression is developed inside turbocompressors in a continuous way, what makes possible to attain very high flow rates, in comparison to the volumetric machines.
1.7 Turbocompressors Types
1.7.1 The Centrifugal Compressors
The turbocompressors are categorized into two species: Centrifugal (also known as Radial) and Axial.
The impeller of a Centrifugal Compressors is composed by a pair of discs having a number of curved blades (or vanes) in between.
The gas admitted through the center opening of the impeller is forced into rotary motion by the blades and consequently moves towards the disc periphery by the action of the centrifugal force. At the same time that the gas is continuously accelerated, it flows oriented by the divergent channels formed by the blades, which shape is responsible for the diffusive action that occurs yet inside the impeller. (Notice that each pair of neighbouring blades belonging to an impeller forms a divergent channel responsible by some diffusive effect).
Between the external periphery of the impeller and the internal casing surface of the centrifugal compressor there is an empty space through which the gas travels with no force or momentum acting upon it. According to mass and momentum conservation principles, the gas resulting trajectory is spiral-shaped along which the velocity decreases at the same time the pressure increases. This space is called annular diffuser, and it is used to induce the diffusion process immediately after the gas is discharged from the impeller, in order to slow down the flow before the friction and turbulence losses become excessive.
Most of the single-staged centrifugal compressors, like the one pictured here, still have external divergent discharge nozzles, where a supplementary diffusion process takes place (casing diffuser).
Turbocompressor of centrifugal type driven by an eletric motor with a gear box in between.
The large majority of centrifugal compressors employed in industrial processes are multistage machines. Although they are provided with the same basic components than the one sole stage machines, their appearence and characteristics differ in some meaningful aspects.
1.7.1 The Axial Compressors
Axial Compressors implement the same principle as the centrifugals adopting a completely different geometry. They use a massive rotor like a drum with several sets of blades circumferentially disposed and regularly spaced along the longitudinal direction. When the shaft is rotated, each one of these sets of moving blades acts as successive impellers, transferring enthalpy and kinetic energy to the gas, almost ever in similar amounts.
As the rotor is installed in its place inside the machine, each pair of contiguous sets of rotor blades gets separated by a set of stationary blades, distributed along the internal periphery of the casing (stator). These stator blades have their profile and their position designed to act as diffuser passages. This arrangement causes the kinetic energy gained in the previous set of rotor blades to be converted into enthalpy by the next set of stator blades. Therefore, a pair of adjacent row of blades, one moving and another stationary, constitutes the axial stage.
1.8 Turbocompressors Application Ranges
Single-stage centrifugal compressors are very rarely applicable, because their low capacity of increasing pressure compared to the normal requirements of industrial compression processes. Up to ten stages mounted in a unique shaft have been employed in some applications. Notice that, by reason of the radial movement imposed to the gas, from the impeller inner opening to its periphery, the amount of energy transferred per centrifugal stage is much higher than the corresponding to axial stages, both running in their maximum allowable rotation speeds. In axial compressors, the gas motion describes a helical path around the rotor, maintaining a constant distance from shaft centerline, without harnessing the centrifugal force contribution. To compensate, the trajectory imposed by the axial geometry is less prone to energy degradation, enabling the attainment of high efficiencies even when working with very large flow rates.
From technical point of view, the selection of the most suitable type of compressors for a given application is basically dependent on two parameters:
- Inlet volumetric flow rate
- Discharge pressure
The size, geometry and speed of rotation are very important turbocompressors design factors, strongly related to the inlet volumetric flow rate specified for the machine. The discharge pressure, for its turn, is the maximum pressure to be supported by the compressor casing, so that it’s the fundamental design factor related to mechanic strength and leakage avoidance aspects.
The large majority of the centrifugal compressors worldwide installed operate with inlet volumetric flow rate from very low up to about 6000 m3/min. Discharge pressure limit is around 700 bar.
Centrifugals are the most versatile compressor ever conceived, and that’s the reason why they are currently the most demanded compression machine for modern industrial applications.
Axial compressors were initially developed to be part of the jet engines employed in aircraft propulsion. These engines are composed by three main equipments: a compressor, a combustion chamber and a turbine.
A compressor to be used in this application has to meet a very stringent requirement: It must be able to produce very high flow rates with very high efficiencies. High flow rates are needed to allow the generation of enough amount of thrust. Furthermore, high efficiencies are needed because the compressor driving power is subtracted from the gross energy developed by the engine. As previously pointed out, the gas flows through axial compressors without abrupt changes of direction and therefore those machines are deemed to have the highest thermodynamic efficiency among all. Their energy consumption may be up 10% lower than the amount consumed by a centrifugal compressor designed for the same service.
During the period of great technological development that took place after the Second World War, the axial compressors formerly employed in aircraft propulsion started to be considered for industrial applications. Logically, the systems characterized by extremely high flows and low pressures (conditions that normally appear together) offered the best opportunities for that novelty. Coincidently or not, air is the gas handled in the two more important applications that fit this profile, encountered in Steel Industries (Blast Furnaces) and Refinery Plants (Catalytic Cracking Units).
Although considered a very important machine because its size and the high technology involved in its design, very few axial compressors are actually found in industry, because of its low applicability. They are customarily designed for inlet volumetric flow rate ranging from 100000 m3/min to about 1000000 m3/min. The allowable discharge pressure, however, is limited to about 6 bar.
1.9 Compression Service Specification
The Service Specification (or Process Specification) is a document that establishes the performance requirements of a compressor. The purpose of this specification can be either the purchase of a new compressor or simply the definition of a new set of operating conditions resulting from a revamping study of an existing system. This document is prepared by the plant designer, who is not, in general, a compressor specialist, but someone with a deep knowledge on the plant operation.
The essential elements of a service specification are:
- Required service conditions
- Required gas flow rate
If the purpose is the acquisition of a new compressor, the service specification is only a minimal part of the procedures needed to formalize the purchase order. The final request have to be issued by a machinery engineer, and includes a plethora of additional information such as the desired type of compressor, its main constructive characteristics, acceptance testing requirements, standards to be met, scope of supply, plant available utilities, documents to be provided, bid conditioning and rules for submitting proposals and so on.
A single compression system design may generate several service specifications for the compressor, due to the existence of different operation modes demanded for the plant. Obviously, the installed machine should be able to satisfactorily meet all the requirements. Almost always the higher desired flow rate comes together with the higher pressure ratio (discharge pressure divided by suction pressure), characterizing the design or dimensioning operational condition. Some kind of control device must be installed in order to allow the compressor to operate at off-design or partial load conditions. The act of manipulating compressors working point with the purpose of reaching more convenient operating conditions is called Capacity Control, an essencial resource for industrial systems.
Now, in order to give the reader a brief insight on how to build a compressor service specification, let’s study the case of a mechanical compression refrigeration system. This is a very good example because its simplicity allows us to understand all the basic aspects related to compression systems functioning.
Application Example:The objective of a refrigeration system is to remove heat from a product, generally a fluid stream. The system works with a pure substance called refrigerant which is made to flow cyclically through a group of interconnected equipments. The four most important are depicted in the following figure:
The Evaporator is a heat exchanger through which the product to be cooled flows transferring heat to the refrigerant. Most commonly it is a shell and tube heat exchanger where the product goes in the tubes and the refrigerant partially fills the shell.
The Compressor collects the vapour formed into the evaporator shell and move it to the condenser.
The Condenser is also a shell and tube heat exchanger where the refrigerant is brought back to liquid phase by water cooling.
The Expansion Valve is used to modulate the refrigerant transfer from condenser at high pressure to the evaporator at low pressure, avoiding the occurrence of high liquid levels in the latter.
In refrigeration cycles is absolutely mandatory that the refrigerant fluid doesn’t change its temperature while receiving heat in the evaporator. Only in this way is possible to maintain a high temperature difference between the two fluids in contact, in such way the thermal exchange effectiveness is guaranteed. This situation can only be attained if (a) the refrigeration fluid is a pure substance and (b) the pressure is kept constant.
Due to compression, the refrigerant vapor temperature rises, allowing the heat rejection to the flowing water in the condenser. The hot vapour becomes a colder liquid collected in the bottom of the condenser. Afterwards, this liquid refrigerant passes the expansion valve and turns back into the evaporator as a very cold liquid mixed with a small portion of vapor in equilibrium, completing the cycle.
From the compressor point of view, the evaporating process is the upstream or generating gas process; meanwhile the condensing process is the downstream or consuming process. Perhaps no other arrangement could provide such comprehensive and insightful example of a compression system as a whole.
The figure below shows the refrigeration cycle in a pressure-enthalpy diagram. The two constant temperature lines refer to the vaporization and condensation temperature levels, as we can see. The corresponding saturation pressures are respectively the low and the high cycle pressures.
The starting point for refrigeration systems design is the thermal load, i.e. the amount of thermal energy lost by the product to be cooled. Assuming that we you want to cool down a given mass flow rate Mp of a certain product from the temperature Tmax to the temperature Tmin, the resulting thermal flux(Φ) is:
where c stands for the product’s specific heat capacity (specific heat at constant pressure for gases or simply specific heat for liquids). Remember that the thermal flux is equivalent to energy divided by time.
Thanks to the heat exchangers designers’ experience it is possible to determine the suitable refrigerant vaporization temperature to assure the removal of the thermal flow previously calculated. This temperature is the same as the compressor suction temperature, the first service parameter of this machine to be established.
The next step is to choose of the refrigerant fluid to be used. Some industrially made substances exhibit the required features for this type of application. Among all these features, stands out the saturation pressure corresponding to the evaporation temperature. It should be slightly above atmospheric pressure in order to avoid the operation at sub-atmospheric pressures but not so high to the point of overloading the equipments that work in the low pressure side of the system. Any available refrigerant that fulfills those conditions happens to be a candidate for the intended application. The saturation pressure would be specified as evaporator operating pressure (which matches, except for small piping losses, the compressor suction pressure).
Let’s see some numbers: Suppose we want to build a refrigerant cycle where the evaporation temperature is -20°C. The vapour pressure of water at this condition is 0.02337 bar, very low for our purpouse, of course. Propane (2.444 bar) and ammonia (1.9019 bar) seems much more suitable for that. And the industrial refrigerant R134a (1.327 bar) would be better yet. If we intended to use methane, we would get very disappointed to find out that the mentioned temperature is above the critical point of this substance, so the liquid-vapor equilibrium at this temperature would be unfeasible.
Moving forward with the design of the refrigeration cycle, the condensing pressure is chosen so that the corresponding saturation temperature of the refrigerant is sufficiently higher than the temperature of the available cooling media, therefore providing adequate heat transfer intensity. Obviously this pressure should not be so great as to overload the compressor unnecessarily. Typically, pressures between 30 bar and 40 bar would be necessary to lead the condensing temperature above 80°C, depending on the refrigerant used.
Remembering what we have said previously about the essential elements of a compressor service specification, they are:
- Required service conditions
- Required gas flow rate
So, lastly we must decide about the mass flow rate Mc required for compressor, which corresponds to the refrigerant mass flow rate through the cycle. Taking the refrigerant specific enthalpy change during the evaporation process (h1-h4) and also the thermal flux(Φ) offered by product to be cooled, we get:
Thus we are now able to issue the compressor service specification. But the most important teaching brought about by this lesson refers to the acknowlegement that the compressor service specification must be in complete harmony with the design of the industrial plant where the machine will be installed.
So far we have been concerned about the compressor specification regarding the rated operating condition, also called dimensioning operating condition. Generally speaking, this condition is the one that serves as basis for the compression system equipments manufacturing. Particularly in case of refrigeration systems, this condition corresponds to the highest thermal load requirement. But industrial systems very seldom operate all the time under the conditions they were designed for. Therefore it’s worthwhile to inform the equipment suppliers about possible alternate operational scenarios, specially those capable of guiding the system to some kind of extremum or limitation. It is advisable, for instance, to specify the partial load condition resulting from the minimum thermal requirement. This information has a meaningful importance especially for the design of the cycle control and protection system.
Creating a process specification referring to an off-design operating condition is not as simple as the procedure used to obtain the nominal specification. The difference lies in the fact that the nominal condition is arbitrary, since it serves as design basis, while the off-design condition should be seen as an “abnormal” functioning of an industrial plant already conceived.
To be clearer, let’s look at the case of the condensing pressure in the refrigeration cycle. As we have seen, this pressure is almost arbitrarily chosen during the set up of the nominal operating conditions, and this value most be considered during the condenser design. Differently, in order to build the off-design specification related to a reduced thermal load, one needs to solve a problem which consists in determining the equilibrium pressure of an “existing” or “already defined” condenser. Actually, the condenser doesn’t even exist yet, but none of its design parameters has been left undefined. As the condensing pressure is not controlled, it will be certainly lowered so that the resulting smaller temperature differential between the refrigerant and the cooling water causes condensation in a lower rate, if maintained the same heat exchange area.
The most frequently observed cause of operational troubles related to compressor specifications is the unpredictability of the service conditions. Sometimes the compressor is submitted to conditions very different from those specified. Even though industrial physical or chemical transformation processes generally show large operational flexibility, a poor machine performance can result.
If you arrived here, congratulations. You finished the first lesson of our course, dedicated to a very basic introduction about Turbocompressors subject. If you want to go ahead, look for the second topic, entitled “The Gas Nature”. There you will find the explanations about the three fundamentals aspects related to the behaviour of gases that you must take in account when analyzing compression systems.