Virtual sensor for water content in oil circuit

Abstract

A method for monitoring an oil-injected screw compressor configured to compress aspirated air by returning oil from an oil separator vessel (11) to a compression chamber (12) of a compressor block (30), for condensate formation in the oil circuit due to a too low compression discharge temperature (VET), determines a water inlet mass flow {acute over (m)}ein(t) and a water outlet mass flow {acute over (m)}aus (t) for a point in time t and determines generated condensate flow Δ{acute over (m)}w (t)={acute over (m)}ein(t)−{acute over (m)}aus (t) on the basis of difference formation.

What is claimed is: 
     
         1 . A method for monitoring an oil-injected screw compressor configured to compress aspirated air, in which oil from an oil separator tank ( 11 ) is fed back to a compression chamber ( 12 ) of a compressor block ( 30 ), for condensate formation in the oil circuit due to a low compression discharge temperature (VET),
 wherein a water inlet mass flow {acute over (m)} ein  (t) and a water outlet mass flow {acute over (m)} aus  (t) are determined for a point in time t and generated condensate flow Δ{acute over (m)} w (t)={acute over (m)} ein  (t)−{acute over (m)} aus  (t) is determined on the basis of a difference formation.   
     
     
         2 . The method according to  claim 1 , wherein a condensate mass m K  which has been generated in this time interval is calculated from a summation of the condensate flows occurring in a time interval, wherein the calculation preferably takes place as follows: m K (t)=m K (t−dt)+Δ{acute over (m)} w *dt (for example with dt=1s). 
     
     
         3 . The method according to  claim 1 , wherein the total generated condensate mass m K  (t) is calculated from a known initial value of the condensate mass m K  (t 0 ) by numerical integration of the condensate flow Δ{acute over (m)} w  over time. 
     
     
         4 . The method according to  claim 2 , wherein the total condensate mass m K  (t) generated at a point in time t is compared with a limit value m K,max1  and upon exceeding the value m K,max1 .
     m   K ( t )> m   K,max1      
       an intervention is made in the control unit of the screw compressor. 
     
     
         5 . The method according to  claim 4 , wherein, at m K  (t)>m K,max1 , a target value for the compression discharge temperature (VET) is increased by a predetermined amount, in particular by a predetermined temperature value, e.g. VET soll,neu ==VET soll,alt +5 K. 
     
     
         6 . The method according to  claim 2 , wherein the total condensate mass m K  (t) generated at a point in time (t) is compared with a limit value m K,max2  and wherein upon exceeding the value m K,max2        m   K ( t )> m   K,max2      
       an error message is issued. 
     
     
         7 . The method according to  claim 1 , wherein the water inlet mass flow {acute over (m)} ein  (t) is calculated from the inlet air mass flow {acute over (m)} Luft  (t) with the assumption of a relative humidity of 100%. 
     
     
         8 . The method according to  claim 1 , wherein the water inlet mass flow {acute over (m)} ein  (t) is calculated from the inlet air mass flow {acute over (m)} Luft  (t) assuming a relative humidity with a fixed value F, with 70%≤F<100% or a relative humidity dependent on the intake temperature T Ans . 
     
     
         9 . The method according to  claim 7 , wherein for determining the inlet air mass flow {acute over (m)} Luft  (t)
 a system-specific delivery characteristic, which depends in particular on the current speed n for screw compressors regulated by frequency converters, and preferably further also   the pressure p akt  in the oil separator tank ( 11 ),   the ambient pressure p amb , and   intake temperature T Ans  are included.   
     
     
         10 . The method according to  claim 9 , wherein for determining the water inlet mass flow {acute over (m)} ein  (t)
 the inlet air mass flow {acute over (m)} Luft (t),   the intake temperature T Ans , and preferably also   the ambient pressure p amb , and   a value for relative humidity, in particular a relative humidity value of 100%,   are included.   
     
     
         11 . The method according to  claim 9 , wherein for determining the water outlet mass flow {acute over (m)} aus  (t)
 the inlet air mass flow {acute over (m)} Luft (t),   the pressure p akt  in the oil separator tank ( 11 ),   the temperature T 2  after the oil separator tank ( 11 ), and preferably also   a value for relative humidity, e.g. a value of 100%,   are included.   
     
     
         12 . An oil-injected screw compressor ( 10 ) comprising a control unit ( 40 ), in particular a central control unit, adapted to perform the method according to  claim 1 .