Abstract:
Provided is an air compressor which exhibits improved reliability by resolving problems relating to drainage discharge defects, and exhibits improved energy efficiency. The air compressor comprises: a compressor body which compresses air; a compressed air flow path through which the compressed air from the compressor body flows; a heat exchanger which is provided to the compressor flow path so as to cool the compressed air from the compressor body; and a drainage pipe ( 62 ) which branches from the compressed air flow path so as to connect to the exterior, and through which drainage condensed from the compressed air cooled in the heat exchanger flows. A strainer ( 65 ) which removes foreign matter contaminating the drainage is provided to the drainage pipe ( 62 ). An on-off valve ( 66 ) at the downstream side of the strainer ( 65 ), and a pressure sensor ( 41 ) at the upstream side thereof, said sensor detecting pressure inside the drainage pipe ( 62 ), are each provided so as to resolve drainage discharge defects.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an air compressor. 
       BACKGROUND ART 
       [0002]    As an air compressor which has a mechanism of discharging a drain generated after compressing and cooling air, there is one described in Patent Literature 1. This publication describes that “pressure detecting means (first pressure detecting means)  52  for detecting a pressure in a drain discharge circuit  40  at this position is provided between a strainer  42  and a diaphragm  41 ” in the drain circuit (e.g., see paragraph 0046) and “when the pressure in the fluid circuit between the diaphragm and the strainer falls to or below a reference pressure set to a predetermined pressure which is less than the pressure when the strainer is not clogged, it is determined that the strainer is clogged (e.g., see claim  1 ). 
       CITATION LIST 
     Patent Literature 
       [0003]    PATENT LITERATURE 1: JP-A-2007-231740 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    A drain-discharge-related structure and operation will be described by taking an oil-free screw compressor that compresses air in two stages as an example of air compressor. In the oil-free screw compressor that compresses air in two stages, the compressed air which is compressed to a predetermined pressure and heated to a high temperature by a low pressure stage side compressor body is temporarily subjected to intermediate cooling by an intermediate cooler before being sent under a pressure to a high pressure stage side compressor body, and therefore condensed water (drain) is generated here. Therefore, the drain needs to be removed by a drain separation apparatus such as a drain separator so that the drain is not mixed into the high pressure stage side compressor body. To remove the drain generated, according to Patent Literature 1, a branch circuit (“drain discharge circuit 40” in Patent Literature 1) is provided which has a mechanism for discharging the drain to the outside. 
         [0005]    This branch circuit is provided with a strainer, a diaphragm (orifice) and a valve as a mechanism for discharging the drain to the outside. This branch circuit is configured so that one end thereof is open to the atmosphere and compressed air containing the drain is discharged to the outside. Here, pressure detection means such as a pressure sensor is provided between the strainer and the orifice to detect a pressure between the strainer and the orifice. Then, clogging of the strainer is determined in comparison with a certain reference pressure and a control signal is outputted. 
         [0006]    However, the example in Patent Literature 1 is intended to detect clogging of the strainer, and when clogging of other parts such as orifice or pipe or defect in execution of work on an external pipe is detected, the pressure between the strainer and orifice does not decrease, and therefore there may be a situation in which it is not possible to detect errors. For this reason, reliability of the air compressor may deteriorate in association with defects or nonconformities of the drain discharge part in general. 
         [0007]    Furthermore, although this branch circuit causes the circuit that supplies compressed air to communicate with the outside air, since the branch circuit is always slightly open, there is a problem that the compressed air always flows to the outside. In this case, the air compressor involves a problem with energy-saving properties. 
         [0008]    The present invention has been implemented in view of the above-described problems and it is an object of the present invention to provide an air compressor capable of solving problems related to a so-called drain discharge defect, improving reliability and improving energy-saving properties. 
       Solution To Problem 
       [0009]    In order to attain the above described object, an air compressor of the present invention is provided with a compressor body that compresses air, a compressed air flow path through which the compressed air from the compressor body flows, a heat exchanger provided on the compressed air flow path to cool the compressed air from the compressor body, a drain pipe which branches from the compressed air flow path to communicate with an outside and though which drain condensed from the compressed air cooled in the heat exchanger flows, a strainer provided in the drain pipe to remove foreign substances mixed in the drain, an on-off valve provided on a downstream side of the strainer in the drain pipe, and a pressure sensor provided on an upstream side of the strainer in the drain pipe to detect a pressure in the drain pipe. 
       Advantageous Effects of Invention 
       [0010]    According to the present invention, it is possible to provide an air compressor that improves reliability and energy-saving properties. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a system diagram of an air compressor. 
           [0012]      FIG. 2  is a diagram describing a detailed configuration of the drain discharge section shown in  FIG. 1 . 
           [0013]      FIG. 3  is a diagram illustrating an electromagnetic valve operation command and a pressure. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0014]    Hereinafter, a specific embodiment of an air compressor of the present invention will be described with reference to the accompanying drawings. Note that in respective drawings, parts assigned identical reference numerals indicate identical or corresponding parts. 
         [0015]      FIG. 1  is a system diagram indicating the air compressor according to the present embodiment. An example in  FIG. 1  adopts a two-stage scheme using two screw-type compressors provided with a screw rotor in the compressor body and shows a water-cooling type two-stage oil-free screw compressor whose cooling system is a water cooling system. 
         [0016]    The water cooling type two-stage oil-free screw compressor according to the present embodiment includes a first-stage low pressure stage side compressor body  3  and a second-stage high pressure stage side compressor body  4 , and an intercooler  5  is provided between the low pressure stage side compressor body  3  and the second-stage high pressure stage side compressor body  4 , and an aftercooler  7  is provided on the discharge side of the high pressure stage side compressor body  4 , and these devices are connected via an air pipe making up a compressed air flow path. Furthermore, the intercooler  5  and the aftercooler  7  are constructed of a water-cooling type shell-and-tube type heat exchanger (shell-and-tube cooler). These components  3 ,  4 ,  5  and  7  and pipes connecting these components form a compressed air flow path. 
         [0017]    Since the oil-free screw compressor according to the present embodiment does not contain any oil content in the compressed air, generation of a drain in the compressed air flow path is likely to cause rusting in the device in the compressor unit (screw compressor) and the compressed air flow path. Therefore, treatment of the drain is important in reliability of products. 
         [0018]    In  FIG. 1 , reference numeral  50  denotes a case of a compressor unit (screw compressor) and the aforementioned first-stage low pressure stage side compressor body  3 , second-stage high pressure stage side compressor body  4 , intercooler  5 , aftercooler  7  or the like are arranged in the case  50 . The low pressure stage side compressor body  3  and the high pressure stage side compressor body  4  are driven by one main motor  10  via a gear  9 . The compressor unit has a structure in which when the motor  10  is started and the compressor bodies  3  and  4  are driven, air flows along the compressor air flow path as shown by void arrows in the drawing. 
         [0019]    That is, when the motor  10  is driven, outside air is guided to the inlet side of the low pressure stage side compressor body  3  via an intake pipe, and taken into the low pressure stage side compressor body  3  via a suction filter  1  and an intake throttle valve  2 . The air taken into the low pressure stage side compressor body  3  is compressed and discharged by a pair of screw rotors, this compressed air (compressed gas) is guided from the first stage discharge pipe to an inlet header  6   a  of the intercooler  5 , passes through a tube (heat transfer pipe) of the intercooler  5  and flows to an outlet header  6   b  side. When flowing through the tube, the compressed air is cooled by cooling water flowing outside the tube. The cooled compressed air passes through a second-stage intake pipe from the outlet header  6   b  and is taken into the high pressure stage side compressor body  4 . 
         [0020]    The low pressure stage side compressor body  3  boosts the pressure of the intake air to a predetermined intermediate pressure (e.g., on the order of 0.20 MPa) and the resultant high temperature (e.g., approximately 160° C.) compressed air is cooled in the intercooler  5  to, for example, “cooling water temperature+approximately 13 to 20° C.” and taken into the high pressure stage side compressor body  4 . 
         [0021]    The air taken into the high pressure stage side compressor body  4  is pressure-boosted to a predetermined pressure (e.g., 0.70 MPa) and the resultant high temperature compressed air flows into an inlet header  8   a  of the aftercooler  7  via a discharge pipe, passes through a tube (heat transfer pipe) of the aftercooler  7  and flows to an outlet header  8   b  side. When passing through the tube, the compressed air flown into the aftercooler  7  is cooled by cooling water flowing outside the tube to, for example, “cooling water temperature+approximately 13° C.” and is then supplied to the demanding side of the compressed air via the discharge pipe. 
         [0022]    Note that a demister  13  is incorporated in the outlet header  6   b  of the intercooler  5  and the outlet header  6   b  has a structure that also functions as a drain separator. That is, when the compressed air is cooled in the intercooler  5 , a drain is generated, but the drain generated is configured to be separated from the compressed air by the demister  13  and discharged out from the compressor unit  50  via a drain pipe  62 . 
         [0023]    The aftercooler  7  is also configured in substantially the same way as the intercooler  5  and a drain generated in the aftercooler  7  is configured to be discharged out from the compressor unit  50  via a drain pipe  63 . 
         [0024]    Another cooling system also exists. A water-cooling type oil cooler  14  is provided along a circulation path of a lubricant that flows isolated from the compressed air flow path for cooling the lubricant that lubricates bearing parts of the compressor bodies  3  and  4 , and the gear  9 . Although the circulation path of the lubricant is not shown in the diagram, there is a lubricant pipe that connects a gear box  11  that houses the gear  9  and the oil cooler  14 . 
         [0025]    Next, the cooling water pipe system through which cooling water flows will be described using  FIG. 1 . The cooling water enters the compressor unit  50  via a cooling water inlet pipe  57  and is divided into a first path that flows into the intercooler  5 , a second path that flows into the aftercooler  7  and a third path that flows into the oil cooler  14 . 
         [0026]    The cooling water that flows along the first path flows into the intercooler  5  via a first cooling water pipe  58 , cools the compressed air flowing through the tube of the intercooler  5 , then passes through a cooling jacket provided in the second stage compressor body  4 , cools the high pressure stage side compressor body  4  and is then discharged out from the compressor unit  50  from the cooling water outlet pipe. 
         [0027]    The cooling water that flows along the second path flows into the aftercooler  7  via a second cooling water pipe  59 , cools the compressed air flowing through the tube of the aftercooler  7 , passes through a cooling jacket provided in the low pressure stage side compressor body  3 , cools the low pressure stage side compressor body  3 , and is then discharged out from the compressor unit  50  from the cooling water outlet pipe. 
         [0028]    The cooling water that flows along the third path flows into the oil cooler  14  via a third cooling water pipe  60 , cools the lubricant there and is then discharged out from the compressor unit  50  from the cooling water outlet pipe. 
         [0029]    As already described, in the intercooler  5 , the air pressure-boosted to an intermediate pressure (e.g., on the order of 0.20 MPa) and heated to a high temperature is cooled to, for example, the order of “cooling water temperature+13° C”, and therefore in most cases, the water content in the compressed air is condensed and a drain is generated. This drain flows into the outlet header  6   b  which also functions as a drain separator, separated from the compressed air by the demister  13 , passes through the drain pipe  62  and is discharged out from the compressor unit  50 . 
         [0030]    Here, an intercooler  5  side drain discharge section (part A at bottom left in  FIG. 1 ) will be described in more detail. 
         [0031]      FIG. 2  is a diagram describing a detailed configuration of the part A shown in  FIG. 1 . The drain discharge section is constructed of a check valve  64 , a Y-type strainer  65 , an electromagnetic valve  66  and an orifice  67  connected to the drain pipe  62  in that order, and a drain discharge port at an end thereof is open to the atmosphere. Furthermore, a pressure sensor  41  is placed upstream of and immediately before the Y-type strainer  65  (that is, upstream of and immediately before an object to be checked for clogging). 
         [0032]    Here, purposes of the respective components of the drain discharge section will be described. 
         [0033]    The check valve  64  is placed for the purpose of preventing outside air from flowing into the compressed air flow path from the part A because the pipe of the low pressure stage compressor outlet has a negative pressure during an unload operation of the compressor, and therefore, the check valve  64  may be placed at any position of the drain discharge section. 
         [0034]    The strainer  65  is intended to prevent foreign substances from being mixed into the electromagnetic valve  66  and the orifice  67  placed downstream. Therefore, the strainer  65  needs to be provided upstream of the electromagnetic valve  66  and the orifice  67 . 
         [0035]    The electromagnetic valve  66  is an on-off valve, on/off of which is controlled by a control unit  42  and is provided for the purpose of opening/closing the drain discharge section. Modes of the on-off control will be described later, and the electromagnetic valve  66  is provided downstream of the strainer  65  as described above. The orifice  67  is intended to adjust the amount of discharge of the compressed air with which the drain is mixed and is provided as required. When the amount of discharge need not be adjusted, the orifice  67  may be omitted. 
         [0036]    In addition to these basic components, the present embodiment further provides a three-way switching valve  68  upstream of the strainer  65  and a bypass pipe  69  that branches from the three-way switching valve  68  to bypass the respective components  65  to  67 . The three-way switching valve  68  is configured to be switched, when the strainer  65  is cleaned, so as to be enabled to change the flow direction to the bypass pipe  69  side. 
         [0037]    When a drain discharge defect occurs in the above-described configuration, the drain generated in the intercooler  5  is taken into the high pressure stage side compressor body  4  together with the compressed air. The drain taken into the compressor body promotes rusting in the casing in the compressor body, increasing the possibility that the rotor may bite the rust, producing non-conformity such as fixed stagnation between the rotors or fixed stagnation between the rotor and the casing. 
         [0038]    Causes for drain discharge defects may be assumed to be clogging of the strainer  65 , operation defect of the electromagnetic valve  66 , clogging of the orifice  67  and failure in normal discharge of the drain due to insufficient execution of work on the drain pipes or the like. 
         [0039]    A drain also occurs in the aftercooler  7  in the same way as the intercooler  5 . For this reason, a drain discharge section (part B at the top left in  FIG. 1 ) is provided in the same way as the intercooler  5 . However, since the pressure in this part never becomes negative during an unload operation for reasons related to the structure of the present embodiment, there is no need to provide a check valve such as the part A. When a drain discharge defect occurs, this may not directly affect the device in the compressor unit  50 , but foreign substances may be mixed in the supply line toward the demanding side and may affect devices on the downstream side connected to a discharge pipe  56 . Therefore, control similar to that of the on-off valve  66  in the part A is performed. 
         [0040]    Next, the on-off control of the electromagnetic valve  66  will be described.  FIG. 3  is a diagram illustrating an operation command of the electromagnetic valve  66  and a pressure detected by the pressure sensor  41 . Here, the drain discharge section (part A) of the intercooler  5  will be described as an example. 
         [0041]    Although keeping drain discharge open all the time is a method generally used, keeping drain discharge open all the time means discarding compressed air to the exterior by an amount corresponding thereto, and it is therefore preferable to adopt intermittent discharge using an electromagnetic valve or the like from the standpoint of energy-saving. The present embodiment performs intermittent discharge by controlling on-off of the electromagnetic valve  66 . 
         [0042]    Through a signal from the control unit  42 , the electromagnetic valve  66  is controlled so as to repeat ON/OFF; being opened for a time t 1  (2 seconds, for example) and closed for a time t 2  (30 seconds, for example). If no non-conformity such as clogging occurs and normal discharge is performed, the pressure in the pressure sensor  41  becomes like a (during normal discharge) shown by a solid line in  FIG. 3 . 
         [0043]    However, if clogging occurs on any one of the components on the downstream side such as clogging of the strainer  65 , the following problem occurs. That is, even when the electromagnetic valve  66  is opened, if the strainer  65  is closed, the electromagnetic valve  66  is not open to the atmosphere, and therefore the pressure detected in the pressure sensor  41  becomes like a broken line b (during discharge defect) with no variation. Therefore, if there is no pressure drop during output of an open signal to the electromagnetic valve  66 , the system determines a discharge defect, and performs control such as displaying an alarm on a display section of the control unit of the compressor or determines a failure and causes the main motor  10  of the compressor to stop. 
         [0044]    The principles of detection of a discharge defect due to clogging are as described above, but since a pressure change is small even during normal discharge, steps may be taken to prevent erroneous normal/defect decisions such as providing a certain time condition (defined time t 3 ), outputting a plurality of open signals to the electromagnetic valve  66  within the time t 3 , comparing an open signal output count N 1  with a pressure drop count N 2 , and determining as a discharge defect only when the pressure drop count N 2  is equal to or less than a preset count NO (smaller than N 1 ). Note that during an unload operation, the pressure of the drain pipe (part A) on the intercooler  5  side becomes negative and no drain is generated, and therefore steps may be taken such as the intercooler  5  side making a determination only during a load operation. 
         [0045]    Even when the electromagnetic valve  66  which is opened when power is on is used here, if the electromagnetic valve  66  does not operate due to a failure of the electromagnetic valve  66  or the like, it remains closed and there is no pressure change, which can thereby be determined as a discharge defect. 
         [0046]    According to the present embodiment described so far, the following effects are obtained. 
         [0047]    First, adopting the configuration without using the drain discharge section which is kept slightly open all the time can provide a configuration advantageous from the energy-saving standpoint. 
         [0048]    In the configuration example described in BACKGROUND ART, even if an electromagnetic valve is provided in the diaphragm (orifice) to improve energy-saving properties, if the electromagnetic valve has an operation defect and remains closed, there is no means for determining the defect, whereas the present embodiment can solve this problem as well. 
         [0049]    That is, by adopting the aforementioned configuration and control, it is possible not only to determine clogging of the strainer  65  but also to detect, in the event of a failure of the electromagnetic valve  66 , defects in general such as clogging of the orifice  67 , drain discharge defect caused by execution of piping by the customer, and can thereby contribute to improvement of product reliability. 
         [0050]    When the pressure sensor  41  fails, it is also easy to detect the failure. That is, in the present embodiment, since the pressure sensor  41  is attached on the upstream side of the strainer  65 , the pressure sensor  41  always detects the pressure of the compressed air (normally detects 0.2 MPa in the example shown in  FIG. 3 ). Therefore, the sensor detects a pressure corresponding to the operating condition of the compressor during a normal operation, and can also easily determine the presence or absence of a failure of the pressure sensor  41 . 
         [0051]    Note that the present invention is not limited to the above-described embodiment, but includes various modifications. For example, the above-described embodiment has described the present invention in detail in an easy to understand way, and the embodiment is not necessarily limited to one provided with all the components described above. Some of the components of the embodiment may be replaced by other components or omitted, and other components may be added to the components of the embodiment. 
         [0052]    For example, since the present invention needs only to be an air compressor requiring screw-type compressor drain discharge, a variety of embodiments can be assumed by changing the above described compression scheme to another, for example, changing the cooling scheme from a water cooling scheme to an air cooling scheme, changing the number of compression stages from a multi-stage scheme to a single stage scheme, changing an oil-free scheme to a refueling scheme or changing a screw scheme to a scroll scheme. 
       REFERENCE SIGNS LIST 
       [0053]      3 : low pressure stage side compressor body,  4 : high pressure stage side compressor body,  5 : intercooler,  6   a : inlet header,  6   b : outlet header,  7 : aftercooler,  8   a : inlet header,  8   b : outlet header,  42 : pressure sensor,  62 : drain pipe,  64 : check valve,  65 : strainer,  66 : electromagnetic valve,  67 : orifice,  68 : three-way switching valve,  69 : bypass pipe