METHOD AND APPARATUS FOR ESTIMATING OPERATING STATE OF POSITIVE DISPLACEMENT COMPRESSOR

FIELD

The present disclosure relates to compressors, and more particularly to estimating an operating state of a positive replacement compressor.

BACKGROUND

Positive displacement compressors, such as screw compressors, are a common type of flow and pressure producing devices that may be driven with an electric motor and a frequency converter. A compressor is able to generate pressure to a fluid by compressing the fluid in a working chamber defined by a rotating male and female rotor of the compressor, for example. At the beginning of a working cycle of the compressor, the rotors may suck fluid through an inlet hole into the working chamber. When the working cycle proceeds, the inlet hole may be closed, and an amount of fluid corresponding with a stroke volume of the compressor is enclosed inside the working chamber. When the working cycle proceeds, the working chamber starts to contract because of the motion of the rotors. At the end of the working cycle, the fluid is led through an outlet hole. The inlet and outlet holes can be opened without a separate control means during the rotor's rotating motion and, thus, pressure created in the compression space remains constant. However, the output pressure of the compressor unit may not remain constant as it may be affected by pressure relief valves and other components of the compressor unit, for example.

In some applications, monitoring the output pressure may be desirable. The output pressure may be monitored by using a pressure sensor, for example. However, a pressure sensor may increase the cost of the system. Therefore, a sensorless estimation of the output pressure may be desirable. Estimation of the output pressure may be based directly on manufacturer-provided data, for example. However, manufacturers of compressors typically publish operational values only for some rotational speeds of the compressor. This may effectively prevent the estimation of the output pressure at different speeds.

BRIEF DESCRIPTION

An object of the present invention to provide a method and an apparatus for implementing the method so as to alleviate the above disadvantages. The objects of the invention are achieved by a method and an arrangement which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The present disclosure describes a novel method for estimating an operating state of a positive displacement compressor, such as a screw compressor or a reciprocating compressor. The operating state may represent the output pressure of the compressor. In addition, the operating state may further represent a mass flow rate of the compressor.

The method is based on performing identification runs on the compressor system at different rotational speeds. Based on the identification runs, characteristics curves representing the relation between mechanical power and output pressure at a rotational speed may be formed. The current operating state may then be estimated on the basis of the characteristics curves, the current mechanical power and the current rotational speed.

The characteristics curves may be based on the estimated (or measured) rotational speed and mechanical power of the compressor. If the method is implemented on a frequency converter, estimates of the rotational speed and the mechanical power may be directly available from the frequency converter, for example.

Since the operating state is estimated with identified characteristics curves and information on the rotational speed and the mechanical power, no pressure sensor is required in the compressor system.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

• Figures 1 a and 1 b show working cycles of a compressor at different output pressures;
• Figure 2 shows a practical example of the mechanical power consumption of an exemplary screw compressor unit at three rotational speeds;
• Figure 3 shows a simplified flow diagram of an exemplary embodiment of an identification phase according to the present disclosure;
• Figure 4 shows an example of rotational speed vs. mass flow rate characteristics for an exemplary screw compressor;
• Figure 5 shows exemplary identification runs on a compressor;
• Figure 6 shows an exemplary situation where a pressure ratio is estimated at a current rotational speed that corresponds with the rotational speed of one of the identification runs; and
• Figures 7a to 7c show exemplary illustrations of different phases of calculating an interpolated characteristics curve.

DETAILED DESCRIPTION

The present disclosure describes a method for estimating an operating state of a positive displacement compressor pressurizing a pressure system. The operating state represents an output pressure of the compressor. The operating state may be in the form of a pressure value or a pressure ratio value, for example. The operating state may further comprise information on the produced mass flow rate. The method may be implemented on a frequency converter that supplies an electric motor of the positive displacement compressor, for example. The compressor may be a screw compressor and a reciprocating compressor, for example.

In the present disclosure, an internal pressure in the working chamber of the compressor at the end of the compression phase is referred to as a nominal pressure p_nom, and the pressure outside of the compressor unit is referred to as an output pressure p_out of the compressor. A pressure difference between the nominal pressure p_nom and a desired output pressure p_out changes the shape of the compressor's working cycle curve in a volume V vs. pressure p diagram. This is demonstrated in exemplary embodiments in Figures 1a and 1 b.

Figure 1 a shows a working cycle where the output pressure p_out is lower than the nominal pressure p_nom. During a suction phase 1, a working chamber of the compressor is filled with a fluid at an input pressure p_in. Next, in compression phase 2, the fluid in the working chamber is compressed to the nominal pressure p_nom. In a following discharge phase 3, the fluid is discharged from the working chamber. Since the output pressure p_out is lower than the nominal pressure p_nom, the pressure in the working chamber drops rapidly to the level of the output pressure p_out. Figure 1b shows a working cycle where the output pressure p_out is higher than the nominal pressure p_nom. During the discharge phase 3, the pressure in the working chamber rises to the level of the output pressure p_out because the output pressure p_out is higher than the nominal pressure p_nom.

Depending on the pressure difference between the nominal pressure and a desired output pressure, extra energy may be required for achieving the desired output pressure. Because of this extra energy, a power requirement of the compressor may be represented as a linear function of the desired output pressure.

Output pressure produced by the compressor unit is not usually informed by the manufacturers as extensively as the mass flow rate at different rotational speeds, for example. Typically, just the nominal pressure p_nom of the compressor and the related power consumption are informed in the compressor datasheets for different rotational speeds. In addition, actual power consumption characteristics of the compressor unit may be affected by the surrounding system having a pressure relief valve and a pressure vessel, for example. The actual output pressure outputted by the compressor may not necessarily correspond to the nominal pressure of the compressor. Because of this and rotational-speed-dependent compression losses, published power consumption information at the compressor nominal pressure may not be sufficient for estimating the actually produced output pressure.

Figure 2 shows a practical example of the mechanical power consumption of an exemplary screw compressor unit. The mechanical power consumption is shown as a function of output pressure at different rotational speeds n₁ to n₃. In Figure 2, the compressor has a minimum pressure valve of 4 bar gauge pressure unit at its output. The compressor is connected to a pressure vessel, which has a safety valve of 8 bar gauge pressure (the nominal pressure p_nom of the compressor is also 8 bar).

When the output pressure p_out is below 4 bar, the minimum pressure valve keeps an internal pressure after the compressor constant. The actual compressor power demand depends on the pressure after the compressor and, thus, the power demand stays constant when the output pressure remains below the 4 bar limit pressure. However, when the output pressure increases over 4 bar, the output pressure p_out and the internal pressure after the compressor are the same. Figure 2 shows that between 4 and 8 bar there is a constant increase in the mechanical power consumption as a function of the output pressure as the pressure vessel is constantly pressurized until the output pressure reaches 8 bar (the maximum pressure allowed by the safety valve).

In order to be able to estimate the operating state of a compressor, regardless of possible restrictions set by a surrounding pressure system, a method according to the present disclosure may be used. The method may comprise an identification phase and an estimation phase.

The identification phase comprises performing a plurality of identification runs to pressurize the pressure system, and determining a characteristics curve for each identification run. Each identification run is performed at a different rotational speed, and each characteristics curve represents relations between the output pressure and a mechanical power consumption of the compressor at the rotational speed of the identification run. The estimation phase may comprise determining current rotational speed and mechanical power of the compressor, and determining the operating state of the compressor on the basis of the calculated characteristics curves, the current rotational speed and the current mechanical power.

Figure 3 shows a simplified flow diagram of an exemplary embodiment of an identification phase according to the present disclosure. During the identification phase, three identification runs are carried out to pressurize the surrounding system (such as a pressure vessel) to a certain end pressure level. The identification runs are carried out at evenly-spaced, constant rotational speeds n₁, n₂, and n₃. However, embodiments of an identification phase according to the present disclosure are not limited to evenly-spaced rotational speeds. Further, embodiments of an identification phase according to the present disclosure are not limited to using three identification runs but any plurality of identification runs may be used.

In Figure 3, each of the rotational speeds is referred to by an increasing index i which starts from index 1 in starting step 31. In the following step 32, the rotational speed reference n_ref is set to the value of the ith rotational speed and, in the following step 33, the compressor is started and operated at the ith rotational speed. Next, in step 34, an estimated motor rotational speed n_est, an estimated shaft torque T_est and an estimated mechanical power consumption P_est are measured/determined. Estimates of motor rotational speed and the mechanical power consumption P_est may be available from a frequency converter controlling the motor of the compressor, for example. The estimates of the motor rotational speed n_est and shaft torque T_est may be used for determining an estimated mechanical power consumption P_est of the compressor unit, for example. The estimates n_est, T_est, and P_est may be time stamped and stored to the memory.

Next, in step 35, a condition check is made to determine whether an end pressure is reached. This can be accomplished in various ways, some of which are discussed later in this disclosure. If the set end pressure is not reached, the method loops back to step 34, and continues storing the time-stamped estimates. This loop is repeated at a sufficient sampling frequency (e.g. 10 Hz) until the pressure reaches the set end pressure.

The method then continues to step 36 in which the compressor is stopped. Next, in step 37, the value of the index i is compared with an end value representing a desired number of identification runs. In Figure 3, three identification runs are performed. If index i in step 37 does not indicate that the current identification run is the last one, the method continues to step 38 in which the identification phase waits for the pressure vessel to be emptied and increases index i by one. The method then loops back to step 32 where the rotational speed reference n_ref is set to the value of the new ith rotational speed, and the method continues the same way as the previous time.

However, if index i indicates in step 37 that the current identification run is the last one to run, the identification phase ends. After completing the identification runs, data stored during the identification runs may be used for determining the characteristics curves in the form of actual mechanical power consumption P versus output pressure p_out characteristics of the compressor and surrounding system at each rotational speed of the identification runs.

In the following, some embodiments of determining the characteristics curve in an identification phase according to the present disclosure are presented in more detail.

For example, in one embodiment of the identification phase the determining of each characteristics curve may comprise determining a first intermediate function and a second intermediate function, and calculating the characteristics curve on the basis of the first and second intermediate function.

Each first intermediate function may represent the mechanical power as a function of time during the identification run. A first intermediate function may be formed by gathering time-stamped data samples of the mechanical power during an identification run and by generating the first intermediate function on the basis of the gathered data samples, for example.

Each second intermediate function may represent the output pressure as a function of time during an identification run. If the power consumption at a nominal pressure is known, the second intermediate function may be determined by determining a mass flow rate during the identification run, calculating a volume of the pressure system by using the determined mass flow rate values, and formulating the second intermediate function by using the determined mass flow rate and the calculated volume.

The mass flow rate may be determined on the basis of a known (estimated) rotational speed and known device characteristics, for example. Compressor manufacturers may publish their device characteristics according to ISO 1217 standard Annex E5. This typically means that the maximum and minimum volume (or mass) flow rates and related rotational speeds are published for the compressor together with three or more evenly spaced volume (or mass) flow rates within the maximum and minimum flow rates. Since positive displacement compressors typically produce a certain mass flow rate at certain rotational speed and their relationship can be assumed to be linear, an estimate of the produced mass flow rate may be generated on the basis of the following equation: $$Q_{m,\mathit{est}}=n_{\mathit{est}}k+C,$$ where n_est is an estimate of the rotational speed, k is the ratio of published mass flow rates to rotational speeds and C is the possibly occurring offset in the required rotational speed to provide mass flow.

Figure 4 shows an example of rotational speed vs. mass flow rate characteristics for an exemplary screw compressor. In Figure 4, three data points representing measured mass flow rates and their respective rotational speeds n₁ to n₃ are shown. Figure 4 shows a linear relationship between the rotational speeds and the mass flow rates, which is typical for screw compressors. An estimate Q_m,est of the mass flow rate may be determined by selecting the current rotational speed n_est, and by finding the estimate Q_m,est of the mass flow rate on the basis of the linear relationship shown in Figure 4.

Alternatively, if the produced mass flow rate is known only at a single rotational speed (indicated with subscript ₀), an estimated current mass flow rate Q_m,est may also be calculated with the following equation: $$Q_{m,\mathit{est}}=\frac{n_{\mathit{est}}}{n_0}{Q}_{m,0}\mathrm{.}$$

However, since the efficiency of a screw compressor may notably decrease at reduced rotational speeds, estimation accuracy of (2) may be limited near a zero rotational speed.

When the mass flow rate is known, the volume of the surrounding pressure system (e.g. the pressure vessel and related piping) can be calculated. For example, the volume may be calculated on the basis of the determined mass flow rate, a specific gas constant, pressures and the temperatures at the start and at the end of the identification run, and the duration of the identification run. Assuming that the mass flow rate remains constant (i.e. the compressor is driven at a constant rotational speed) during an identification run, the volume V of the surrounding pressure system may be calculated, for each identification run, with the following equation: $$V=\frac{Q_m\mathrm{\Delta }\mathit{tR}}{\frac{p_2}{T_2}-\frac{p_1}{T_1}}$$ where R is the specific gas constant, Δt is the duration of identification run, p is the pressure, and T the temperature. Gas temperature can be approximated to remain constant and to be the same as the surrounding air temperature, if no further exact information is available. If there is no accurate information on R, dry air value of 287 J/kg·K can be used. Subscript ₁ refers to the start situation and subscript ₂ to the end situation for the identification run. The start pressure p₁ may be assumed to be equal with the (known) surrounding atmospheric pressure if the pressure vessel is emptied before each identification run. Identification runs may be stopped when the compressor nominal pressure p_nom is reached. Thus, the end pressure p₂ may be equal to the nominal pressure p_nom. If a mechanical power at the nominal pressure p_nom of the compressor is known (from device characteristics provided by the manufacturer, for example), the nominal pressure p_nom and, thus, also the end pressure, may be detected on the basis of the mechanical power. When the identification runs have the same starting pressure and end pressure, a constant volume V of gas may assumed to be compressed into the pressure vessel during each identification run.

When the volume V during an identification run has been determined, the second intermediate function representing output pressure as a function of time may be calculated. The second intermediate function may be formulated on the basis of the determined mass flow rate, the calculated volume, the specific gas constant R, the temperatures T₁ and T₂ at the start and at the end of the identification run, the pressure p₁ at the start of the identification run, and the duration of the identification run as follows, for example: $$p_{\mathit{out}}\left(t\right)=\frac{T_2{\mathit{RQ}}_m}{V}t+\frac{T_2{p}_1}{T_1}$$

This value may also be used for calculating a compressor pressure ratio Φ that represents the ratio of compressor output and input pressure: $$\varphi =\frac{{\mathit{p}}_{\mathit{out}}}{{\mathit{p}}_{\mathit{in}}}$$

When the first intermediate function and the second intermediate function have been defined, the characteristics curves for compressor output pressure behavior may be determined based on them. For each identification run at a different rotational speed, a characteristics curve representing the mechanical power consumption as a function of compressor output pressure may be defined. For example, for each stored time-stamped sample of the mechanical power in the first intermediated function, a respective value of the output pressure p_out or pressure ratio Φ may be determined on the basis of the second intermediate function represented by equations (4) and (5), for example. Each characteristics curve may be formulated by pairing the values of estimates P_est and p_out with the same time stamps, for example.

Alternatively, another embodiment of the determining of the characteristics curves may be used if the compressor is connected to a pressure system (such as a pressure vessel) that is configured to operate within known pressure limits. For example, the compressor may comprise a minimum pressure valve with a known minimum pressure and a maximum pressure valve (e.g. a safety valve) with a known maximum pressure. The minimum pressure valve and the maximum pressure valve define two known reference pressure points for the compressor output pressure. These reference pressure points may be used for determining the characteristics curves. This approach may be desirable if the compressor mechanical power consumption at its nominal output pressure is not available, for example.

The characteristics curves may be calculated on the basis of a plurality of identification runs also in this embodiment. Each identification run may be used to define a first intermediate function representing the mechanical power as a function of time during the identification run and a second intermediate function representing the output pressure as a function of time during the identification run. Each first intermediate function may be formed by gathering time-stamped data samples of the mechanical power during an identification run and by generating the first intermediate function on the basis of the gathered data samples.

The main function of the minimum pressure valve may be to ensure sufficient pressure production by the compressor before its connection to the surrounding system (such as the pressure vessel). In practice, the opening of the minimum pressure valve and the increased production of mass flow is visible as an increase in the first intermediate function during the identification run. Correspondingly, the opening of a safety valve means that the compressor cannot increase its output pressure anymore, which should also be visible in the first intermediate function.

Figure 5 shows exemplary identification runs on a compressor. In Figure 5, three curves represent three first intermediate functions at three different rotational speeds n₁ to n₃. In Figure 5, the minimum pressure valve opensat 4 bar, and the maximum pressure valve opens at 8 bar. Each first intermediate function shows a distinct change in the slope of the curve both when the minimum pressure valve is deactivated and when the maximum pressure valve activated. The first 60 seconds of measurements were reserved for starting the compressor and ensuring that the compressor unit works at a constant rotational speed and provides constant pressure into the surrounding system. However, depending on the compressor unit, the start-up time may also be shorter or longer.

The changes in the slope of the first intermediate function can be utilized in determining the second intermediate functions. Each second intermediate function may be determined by determining time instants of the output pressure reaching the known minimum and maximum pressure. The time instants may be determined by detecting the step-wise changes in the slope dP_est/dt of the first intermediate function, for example. On the basis of the determined time instants, the known minimum pressure and the known maximum pressure, the form of the second intermediate function can then be determined. The first time instant where dP_est/dt increases step-wise (e.g. around 100 seconds in Figure 5) may be considered to indicate the deactivation of minimum pressure valve's operation (e.g. at 4 bar gauge pressure). The following time instant with step-wise decrease in dP_est/dt may correspondingly be considered to indicate the activation of safety valve's operation (e.g. at 8 bar gauge pressure).

When the time instants of reaching the two reference pressure points are known, the exact shape of the second intermediate function can be determined. For example, the pressure may be considered to rise linearly between the known minimum pressure and the known maximum pressure in the second intermediate function. The characteristics curves p_outP may then be calculated on the basis of the first and second intermediate function. Depending on the characteristics of the compressor within the pressure range, a characteristics curve may be modelled to have a linear fit curve between the known reference pressure points.

The known maximum pressure (defined by a maximum pressure valve, for example) may be utilized as the end pressure for the identification runs. This applies also to the embodiment where the output pressure is determined by calculating the mass flow and the volume (e.g. as shown in Equations (1) to (5)). Instead of determining the end pressure by detecting a known mechanical power consumption corresponding with the nominal pressure, reaching the end pressure may be detected by a step-wise change in the slope of the first intermediate function, as discussed above.

In yet another embodiment of determining the characteristics curves, the identification runs may also be performed without the information on the compressor mechanical power consumption at its nominal pressure. This embodiment is applicable to a situation where the compressor is controlled with the on/off pressure control method, i.e. where the compressor is controlled such that when the output pressure drops below a known low pressure limit (e.g. at 4 bar of gauge pressure), the compressor is started, and when the output pressure rises above a known high pressure limit (e.g. at 8 bar of gauge pressure), the compressor is stopped. Only information on applied pressure limits (i.e. a minimum pressure limit and a maximum pressure limit) is required, so that mechanical power consumption at these pressure limits can be identified for each rotational speed used in the identification runs. The characteristics curve for each identification run may be identified on the basis of the mechanical power consumptions at the two known pressure limits. A first mechanical power of the compressor at the low pressure limit and a second mechanical power of the compressor at the high pressure limit may be determined, and a linear portion of the characteristics curve may be formed on the basis of the known low and high pressure limits, and the first and second mechanical power.

When the identification phase is finished and the characteristics curves have been determined with the identification runs, the current operating state may be estimated in the estimation phase. In the estimation phase, the current rotational speed and mechanical power of the compressor may be determined on a frequency converter, for example. Then, based on the determined characteristics curves, the current rotational speed and the current mechanical power, a present operating state representing an output pressure of the compressor may be determined. This kind of estimation phase is applicable to all embodiments of the identification phase disclosed above.

If the current rotational speed is the same as one of the rotational speeds applied during the identification runs, the estimate for the present output pressure or pressure ratio may be directly determined from the respective characteristics curve. This is demonstrated in Figure 6 for the pressure ratio. Figure 6 shows an exemplary situation where the pressure ratio is estimated at a current rotational speed corresponding with a rotational speed n₂ of one of the identification runs. In Figure 6, the present pressure ratio p_est corresponding with the current mechanical power P_est at the rotational speed n₂ may be directly determined from the characteristic curve p_outP at the rotational speed n₂.

If the current rotational speed is between the rotational speeds used in the identification runs, an interpolated characteristics curve at the current rotational speed may be calculated on the basis of the characteristics curves of the identification runs. The present operating state may then be determined on the basis of the interpolated characteristics curve and the current mechanical power.

For example, calculating of the interpolated curve may comprise interpolating a plurality of intermediate curves at different, constant output pressures on the basis of the calculated characteristics curves. Each intermediate curve may represent the relations between the rotational speed and the mechanical power at a single constant output pressure. Then, for each intermediate curve, a mechanical power at the current rotational speed may be calculated, and the interpolated characteristics curve may be formed on the basis of the calculated mechanical powers.

Figures 7a to 7c show exemplary illustrations of different phases of calculating the interpolated characteristics curve. In Figure 7a, three characteristics curves p_outP at three different rotational speeds n₁ to n₃ are shown. The characteristics curves define three data points (shown as circles) at a first output pressure p₁ and at a second output pressure p₂. Based on these data points, two intermediate curves are interpolated. Each intermediate curve represents a relation between the rotational speed and the mechanical power at one of the output pressures p₁ and p₂.

Figure 7b shows an intermediate curve at output pressure p₁. The curve is shown as a rotational speed vs mechanical power (nP) curve. The nP curve may be formed on the basis of the data points by using spline interpolation, for example. With each intermediate curve, an estimate of the mechanical power at a current rotational speed may be determined for the respective output pressure. In Figure 7b, a triangle shows the mechanical power at the current rotational speed n_est and output pressure p₁.

When the intermediate curves have been calculated and mechanical powers have been estimated at the current rotational speed on the basis of the intermediate curves, the interpolated characteristics curve may be formed. Figure 7c shows an interpolated characteristics curve (dashed line) at the current rotational speed n_est. In Figure 7c, two points (shown as triangles) define the mechanical powers at pressures p₁ and p₂. The interpolated characteristics curve is formed as a linear function passing through these points.

In the exemplary illustrations of Figures 7a to 7c, three characteristics curves were used and two intermediate curves were formed on the basis of the characteristics curves. However, a method according to the present disclosure is not limited to these specific numbers of characteristics curves and intermediate curves but any plurality of both may be used.

In some embodiments of a method according to the present disclosure, the operating state may further comprise information on the produced mass flow rate. Thus, the method may comprise determining the present mass flow. The present mass flow rate may be determined on the basis of present (estimated) rotational speed and known device characteristics, for example. Equation (1) or (2) may be used for determining the present mass flow, for example.

The present disclosure further describes an apparatus for estimating an operating state of a positive displacement compressor pressurizing a pressure system. The apparatus comprises means for carrying out a method according to the present disclosure. The apparatus may be a frequency converter, for example. The frequency converter may supply an electric motor of the positive displacement compressor, for example. The frequency converter may comprise a computing device, such as a processor with a memory, a DSP, or an FPGA, that is configured to carry out a method according to the present disclosure. Estimates of amotor rotational speed and mechanical power consumption may be available from the frequency converter, for example. The estimates of the motor rotational speed and shaft torque can be used for determining an estimated mechanical power consumption of the compressor unit, for example.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.