System Overview #
innovateIT Car Wash Equipment LLC has developed a high-efficiency reverse osmosis system to provide spot-free rinse service for express car wash applications.
The system uses “Flow on Demand” control algorithm to adjust pump pressure based on number of cars being washed, operator demand, and condition of the membranes.
The VFD pump allows the system to respond to the car wash demand for RO water and is sized to generate RO needed to match the demand throughout the day. This approach allows the use of a reduced-size RO storage tank which is fully integrated into the RO stainless steel stand.
In addition, the VFD pump allows the unit to produce full flow (10 GPM – Standard Flow, 15 GPM – High Flow) down to 40F feed water temperature to ensure consistent flow for the wash even in extremely frigid winter climates.
fig. 2 – 1 – Feed water temperature vs. RO system output
Principles of Operation #
NOTE: Most car washes operate with varying feed water temperatures over the course of the year. This unit is designed to ensure that the unit produces adequate RO for rinsing cars across a wide range of feed water temperatures and levels of dissolved solids.
The system is designed to automatically generate the required RO for the demand of the tunnel (up to 24,480 GPD with the Standard Flow system or 37,440 GPD with the High Flow system.
The unit uses the Grundfos VFD Pump along with a PLC controller to match the RO production to the RO usage. This approach produces high quality water with maximum operating efficiency.
The tank level sensor (pressure sensor) is used to generate the request for RO production flow rate which varies based upon the RO storage tank level. This feature allows the system to increase flow as the tank level drops. When matched to properly sized nozzles in the tunnel, the unit is designed for up to 200 cars per hour continuously.
The variable flow approach based on tank level allows the use of a smaller RO storage tanks which integrate directly to the RO system. The result of this feature allows the pump to operate at lower pressures for longer periods, increasing membrane life and reducing pump cycling. This avoids short cycling the pump and allows unit to run efficiently even during periods of low or intermittent carwash tunnel volume.
An electrical control box is installed on the unit which includes the PLC, HMI, and mechanical disconnects from the main power supply, to convert the 480VAC three-phase power to 24 VDC to power the HMI, PLC, and air solenoids and water quality TDS sensor (24Volt).
The control box also includes a motor starter for the re-pressurization pump which uses RO water from the storage tank and delivers RO flow to the tunnel. The re-pressurization pump is turned on based on an RO request signal from the tunnel controller and does not integrate into the RO production pump controller.
A flow switch and an air operated solenoid valve have been added to the re-pressurization pump output to protect the pump from a dry running condition. This feature will turn off the re-pressurization pump and register a fault if the flow switch fails to close.
The electrical inputs and outputs (I/O) has been selected to minimize the number of sensing devices while providing the ability to diagnose failure of the equipment.
System Operation #
The following input and output signals are used to provide the controller information to operate and troubleshoot the system. These sensors are shown on the system schematic (Appendix 2).
Pump Inlet Pressure Switch PRS1 #
- 0-5 psi adjustable pressure switch which is used to detect an issue with the water supply. The switch is normally open and if this switch fails to close after start-up, it will turn off the RO pump and register a system fault. A PS1 fault will shut down the RO Production Pump and log a fault in the data log.
RO Pump Pressure Sensor PRS2 #
- This pressure sensor is used to protect the system from operating over the setpoint pressure.
- The system can produce 10 GPM (Standard Flow) or 15 GPM (High Flow) of RO below 200 psi at a water temperature above 40F.
- This pressure set point is to allow the use of membranes that might be lower-pressure or cold-water membranes.
- This situation may occur as the membranes reach the end of life or if feed water drops below 40 F.
RO Product Flow Sensor – ROFLO #
- This sensor provides the feedback signal to the pump controller to adjust RO Pump speed and pressure to achieve the desired RO flow rate. The programmed flow rate is adjustable on the PLC settings screen.
- This sensor provides a 4-20 mA signal to the controller and the calibration is set in the controller software. This allows the operator to adjust the requested flow based on the tank level. As the tank level decreases, the flow is set to increase. This approach allows the unit to generate RO at the lowest energy cost to meet the flow required by the tunnel.
- ‘RO Production’ turns on when the tank level drops to 75% and fills it back to 95%.
- Verify the RO pump turns on when the tank level drops to 75% tank level.
- Once the re-pressurization pump is commanded off, the unit will keep running continuously until the level reaches 100%. The system will wait 15min then initiate a RO MEMBRANE FLUSH CYCLE before shutting down.
- During the flush cycle, the RO pump circulates a few gallons of RO water to the feed side of the membranes. RO water pulls deposits from the surface of the membranes. The flush water (RO water) remains in the dirty (concentrate) side of the membranes until the next production cycle.
- Using this method, the feed (concentrate side) of the membranes will always be cleaning the membranes between the RO production cycles.
The tank is divided into three sectors which allow flexibility in set-up. The objective is to allow the unit to adjust flow based on tank level with lower flow rates as the tank is nearing 100% full and higher flow rates as the tank approaches empty.
The ‘RO Production Turn On’ level is adjustable and will normally be set in the 60% range. In the screen shot above the operator has set this number to 80%. Based on the settings above the unit will ramp from 0 GPM to 8 GPM when the tank reaches 80% full condition.
Tank Level Sensor #
- The tank sensor is a low-pressure sensor that measures the static water head pressure based on the depth of water in the tank.
- The 0-5 psi sensor is scaled to a 4-20 mA signal. This signal is used to request flow rate based on the tank level. This signal is an input to the PLC which is used to modify the flow request based on the tank level.
Recirculation Flow Switch #
- The recirculation flow switch verifies that the recirculation loop flow (the combined flow of the recirculation flow and reject flow) is operating at more than 12 GPM of flow during operation.
- This switch protects the membranes from fouling, verifying that Valve C is open and the recirculation and reject flow orifices are not blocked during RO production. If the reject flow orifice or the recirculation orifice becomes blocked it would increase RO recovery and potentially damage the membranes. The switch will not close if the RO pump pressure is less than 70 psi.
Re-Pressurization Pump Flow Switch #
- The switch closes as RO pressure at the pump flow reaches 5 GPM.
- This switch verifies that when the RO Request for water is sent from the tunnel controller, Valve D has opened, the delivery pump has turned ON, and RO water is flowing to the tunnel.
- If this switch fails to close it will turn off the re-pressurization pump to protect it from running in a dead-head or dry running condition. (It is recommended that you have a spare flow switch and cable to allow quick repair if the switch fails. PN: Flow Switch- RC-FS VK309)
System Functions #
The following diagram shows the system in START mode. Each of following functions are designed to ensure a reliable and consistent supply of RO water for the spot-free rinse portion of the wash.
fig. 2.3 – 1 – HMI Screen in START mode
Stop Mode #
- The unit will always start-up in the STOP mode.
- In this mode, power is supplied to the RO system, and the 24vac signals are energized. However, the pumps and solenoid valves are all commanded off. The HMI will show a red dot in the mode button on the main screen.
Start Mode #
Press START on the HMI (fig. 2.3.2 – 1)
- The system will go through a short initialization of 5 to 10 seconds.
- START mode will produce RO water until the tank is full (Set maximum tank %)
- The RO pump will continue to operate until the tank reaches the set tank level %.
- If the unit is inactive for 15 minutes, the unit will perform a flush cycle.
fig. 2.3.2 – 1 – Start mode button
NOTE: In the event of either low RO flow from the system (due to failed membranes or pump problems) or if the demand from the tunnel exceeds the flow from the RO system, there is a municipal water line that uses a valve (E) to fill the RO tank if the water level drops drop below 25%. This is to protect the re-pressurization pump and ensure rinse water is delivered to the tunnel.
Tunnel Signal Command for RO Water Delivery to Tunnel #
This signal is controlled directly by the tunnel system controller using a 24 Volt DC relay signal to the R.O. System Controller and is used to turn on the Re-Pressurization Pump.
This signal switches on and off for each car in the tunnel based on the conveyor speed. The re-pressurization pump is limited to 180 cycles per hour (maximum continuous cycle rate).
RO Storage Tank #
The stainless steel RO storage tank is integrated into the stainless steel stand. Municipal water flows to a solenoid outside the tank to automatically supply municipal water to the tank if there is a failure in the system.
Note: The solenoid supplies water if the RO pump or membranes have reduced RO flow.
WARNING
The 24V DC signal may be present in the system controller even when the controller power has been manually disconnected from the 3-phase power source. You must turn off the 24 V electrical signal (RO Request) from the RO system controller before performing maintenance or repair to the system controller. Failure to de-energize this circuit could lead to equipment damage or personal injury.
System Performance #
The stainless steel RO storage tank is integrated into the stainless steel stand. Municipal water flows to a solenoid outside the tank to automatically supply municipal water to the tank if there is a failure in the system.
RO systems are designed to provide spot-free rinse water to the car wash tunnel and are rated by the volume of RO water produced per day. Membranes are extremely sensitive to feed water temperature and will typically lose approximately 2% of flow for every 1F drop in feed water temperature at constant pressure.
The innovateIT R.O. System is conservatively rated for consistent performance from 40F to 80F feed water temperatures. The system utilizes oversized membranes operating at conservative flux ratings combined with daily RO flush cycles to achieve high-quality water and long membrane life.
The unit provides 10 GPM (Standard Flow)/15 GPM (High Flow). The maximum commanded flow at low tank levels is 18 GPM (Standard Flow)/26 GPM (High Flow) are programmed. These max flow ratings can only be achieved when the water temperature is 77F or above.
The RO volume (gallons per car) should be set for the coldest expected feed water which eliminates the need to adjust the flow to the tunnel throughout the year to adjust for changes in municipal water temperature variation. The pressure required to generate full flow will range from 70 psi (@ 80°F) to 200 psi (@ 40°F).
The Grundfos VFD pump has been oversized to allow this wide range of temperature capability while using Dupont (formerly Dow Filmtec) membranes which ensure high quality (low TDS RO water). The VFD pump also allows pumping at the correct pressure and flow to generate the needed RO flow to support the tunnel demand.
Although the unit has a 7.5 HP pump, most of the time this system will be running at much lower power. The electrical energy used to generate the RO increases as the RO membrane flow increases.
The result of generating RO at the rate required to satisfy the tunnel demand results in reduced energy consumption per gallon of RO produced. The membrane’s performance varies as the feed water temperature changes. This means the RO pump pressure must increase to maintain flow as feed water temperature drops.
Over time, the flow and pressures may vary. Lower flow rates would indicate potential scaling or fouling of the membranes and higher RO flows or increasing RO TDS readings may indicate either membrane damage (caused by chlorine damage) or a membrane O-ring seal leak.
The TDS of the RO water ranges between 0 and 15 PPM with new membranes. New membranes process chemicals in the RO membrane material, and the initial 60 minutes of operation the TDS will go from 200 ppm or higher down to the normal 0-15 ppm level.
This represents normal start-up for new membranes. If after an hour of operation, the TDS is still running above 15 ppm the operator should pull a sample from the bottom of each membrane to determine if a seal was cut during assembly of the membranes into the membrane housings.
In addition, TDS should be checked after the RO pumps runs for a couple of minutes as when the unit sits overnight or when unit is stopped during the day as there will be “TDS creep” as salts will migrate across the membranes when unit is sitting in standby mode.