Water Cooled Chillers – WPS

with Screw Compressors

The WPS range of water cooled liquid chillers has semi-hermetic screw compressors and is available with either the low GWP refrigerant, R513a or the older refrigerant R134a.

Depending on the refrigerant used, the available capacities for the WPS chillers are between 129kW and 2,993kW.  Models up to 857kW are ≤ 1m wide and can fit through a door way.

WPS Water Cooled Chiller – Overview

To ensure the highest level of quality and reliability in our Chillers, all manufacturing and testing procedures are carried out according to ISO 9001:2015 Quality Management system and ISO 14001:2015 Environmental Management system.

  • Suction lines are insulated with closed cell foam insulation, then wrapped with a special protective material and then finally epoxy coated.  

Unit Construction

  • Welded structural C-channel base painted with mono-component catalysed primer sprayed paint.
  • Base is equipped with welded brackets for heavy duty lifting lugs
  • Easily accesible system components


  • Suction gas-cooled semi-hermetic screw compressor
  • Variable slide valves allow the chillers to match actual load conditions for best part load performance
  • Double-walled pressure compensated rotor housing, adding to stability and additional sound attenuation
  • Proven long-life bearings with pressure unloading
  • Optimised oil management with built-in directly flanged on oil separator
  • Long life fine filter mesh and magnets on oil circuit
  • Pressure relief bearing chamber ensures minimum refrigerant dilution in the oil
  • Large volume motor suitable for part winding or Star Delta starting, with integrated embedded sensors in each winding
Semi-hermetic Screw Compressor

Semi-hermetic Screw Compressor

Compressor Jacket

  • The compressor jackets consist of 9.5mm thick closed cell rubber sound insulation material inside a sound deflecting vinyl cover to provide superior sound reduction for screw compressors

Standard Compressor Compartment

  • A standard compressor compartment is constructed from gauge 16 (1.5mm) thick galvanised steel sheet metal insulated with 9.5mm thick closed cell rubber insulation.  All sheet metal is painted with Petra powder paint

Advanced Compressor Compartment

  • The advanced compressor compartment is constructed from a double wall, heavy gauge galvanised steel
  • The wall is gauge 14 (2mm) outer skin and gauge 22 (0.7mm) perforated inner skin
  • There is a 50mm thick fibreglass insulation with a density of 48kg/m³ between the inner and outer skins


  1. The above options are available in certain combinations in order to achieve the lowest possible sound emissions.  
  2. Indicative dB(A) values for each sound reduction option, and combinations of  options, are available in our VIP Guest area.  Please click here to Sign In or Register for access
  3. Please take note that the compressor compartment installation might change the overall chiller dimensions


  • High efficiency direct expansion (DX) shell and tube type barrel coolers with inner grooved tubes for improved efficiency
  • Barrel coolers are tested and stamped for refrigerant side design pressure of 1000kPa and for a maximum water side working pressure of 1500kPa
  • These working pressures comply with the applicable sections of the ASME standard and the European codes of ISPESL AND TÜV
  • The evaporators are equipped with internal water baffles in the shell.  They are fabricated from brass or plastic for maximum corrosion resistance
  • The shell and tube evaporators are provided with water vents and drain connection plugs and are insulated with 19mm closed cell foam insulation

Shell & Tube Evaporator

Shell & Tube Evaporator

  • Available for conditions down to 0°C, -18°C or -29°C
  • Electrical resistance heating tape is wrapped around the shell to help protect the evaporator fluid contents fro freezing at low and very low temperature conditions
  • A continuous power supply to the chiller is necessary to maintain operation of the heater 24/7
  • This accessory will not protect any external water pipework connected to the unit and additional frost protection measures may be required
  • The shell & tube evaporator can be insulated with closed cell foam insulation of 25mm, 38mm or 50mm
  • The insulation density is 48 kg/m³ with a K-factor of 0.035 W/(m.°K)
  • The evaporator can be clad in either aluminium, stainless steel or painted galvanised steel (gauge 22 [0.7mm])
  • Cladding is applied over the barrel insulation

Shell & Tube Condenser 


  • High efficiency shell and tube condenser with inner grooved tubes to optimise efficiency
  • Condensers are tested and stamped for refrigerant side design pressure of 1500kPa and a maximum water side working pressure of  1500kPa
  • These working pressures comply with the applicable sections of the ASME standard and the European codes of ISPESL and TÜV
  • The condensers are provided with water vents (purge) and drain connection plugs
Shell & Tube Condenser

Shell & Tube Condenser

Refrigeration Circuit

Components of each refrigeration circuit:

  • Independent refrigeration circuit per compressor
  • Liquid line solenoid valve
  • Liquid line shut off valve
  • Liquid line moisture indicator sight glass
  • Replaceable core type filter
  • Fully charged with refrigerant
  • High safety pressure switch (capsule type; factory pre-set)
  • Electronic expansion valve
    • Electronically operated step motoer flow control valves, intended for the precise control of liquid refrigerant flow.
    • Synchronised signals to the motor provide discrete angular movement which translates into precise linear positioning of the valve piston
    • Easily interfaced with microprocessor based controllers
Petra Electronic Expansion Valve

Petra Electronic Expansion Valve

  • Liquid, discharge and suction pipes are all hard drawn copper pipes that are formed using automated CNC pipe bending machines in Petra’s own facility.  This minimises pipe-brazed joints which in turn increases system reliability
  • All refrigerant circuit copper piping is epoxy painted
  • The hot gas bypas system consists of a mechanical valve capacity regulator used to adapt compressor capacity to actual evaporator load
  • It is installed in a bypass line between the high and low pressure sides of the refrigeration system and is designed for hot gas injection into the evaporator just after the expansion valve
  • The hot gas bypass valve is UL listed, file SA7200
  • The hot gas bypass valve allows additional capacity reduction for units operating below the minimum step of unloading for the compressor
  • Of the hot gas bypass is installed on the lead compressor only, the ‘lead/lag’ function (for the compressor) will be eliminated
  • Optional pressure gauges for monitoring the refrigeration discharge and suction pressure
  • In addition, the gauges can be used to verify the refrigerant charge and proper system performance
  • The gauges are Bourdon type, oil filled stainless steel housing
  • Oil filling provides greater protection of the gauge internal components from corrosive atmospheres
  • The gauges have dual scale, both PSI and BAR
  • The water flow switch is supplied as a loose item for field installation
  • CE approved safety interlock to prevent operation operation of the chiller without evaporator water flow
  • The water flow switch functions as a safety that assumes water flow rate with the shell and tube (Barrel) evaporator and sends a signal to shut off the chiller if no water flow exists due to pump failure or other causes
  • It is a paddle type flow switch
  • The paddle consists of three segments that can be removed or trimmed and sized to fit the water pipe size
  • The paddle is made of copper alloy 


  • Compressor electronic current monitor and overload protection through controller
  • Free terminal for remote ON/OFF connection
  • Free terminal for general alarm output
  • Control voltage is 200-240V for all components
  • Single point power connection for each electrical panel
  • Circuit breaker for each compressor
  • Starting contactors for compressors
  • ON/OFF switch for each compressor
  • Control circuit breaker for short circuit protection
  • Short cycling protection for compressors (time delay)
  • Control transformer supplies all unit control voltage from main unit power supply to internal components such as (but not limited to) solenoid valves, compressor motor protector, compressor crank case heater and microprocessor controller
  • Microprocessor controller for full management of chiller operation and safety circuits
  • Power supply monitor (phase failure relay) used to protect the power circuit against over or under voltage conditions and against phase loss or loss reversing conditions 

  • NEMA 3X with IP 54 minimum enclosure standard electrical panel
  • Two separate panels, one for power and the other for control
  • Electrical panel is eqipped with a heavy gauge galvanised steel access door
  • Panel is painted with oven baked polyester electrostatic powder paint
  • Each door is equipped with external handle with key and tooled latch
  • Doors contain sealing heavy duty clip-on bulb gasket between the door and the panel that provides effective sealing
  • All doors have multiple hinges
  • Each door has a door retainer to keep the door open during service
  • Each door has a built-in pocket to accomodate a laminated wiring diagram and IOM documents
  • Power factor correction is used to improve the power factor level
  • Maximising the power factor improves system reliability, minimises voltage drops and gives better power quality
  • Advance safety capacitors with harmonic filters and a main microprocessor controller are provided to manage the required capacity for the capacity stages
  • Only one capacitor panel is needed for the chiller regardless of the number of compressors or fans
  • The power factor correction capacitor is usually installed in a separate electrical box, but this depends on the unit size.  Please call us to discuss if this is of interest to you
  • The earth leakage relay is a safety device used in electrical installations with high earth impedance to prevent shock
  • It detects small stray voltages on the metal enclosures of electrical equipment and interrupts the circuit if a dangerous voltage is detected
  • An earth leakage relay can be supplied for the whole unit power supply, or for each compressor
  • The main disconnect switch with door interlock is used to de-energise the power supply to the chiller during service or repair works
  • It has an external handle that is installed on the the electric panel door
  • The switch has to be de-energised to open the electric panel door
  • The disconnect switch can be supplied with either an integrated fuse or a non-fuse type
  • Ampere and volt meters are used to measure the power current and the voltage consumption
  • An ampere meter is used for each phase
  • The volt meter is used to measure the voltage of power supply between each phase and the other, and between each phase and neutral
  • The chiller maybe supplied with a dual power connection
    • One power entry for the compressors
    • The second power entry for the rest of the unit
  • Each power connection can be equipped with a separate main disconnect switch
  • External overload for each compressor
  • External overload for each condenser fan motor
  • Circuit breaker for each condenser compressor
  • Circuit breaker for each condenser fan motor
  • NEMA 4X electrical panels made from galvanised steel
  • NEMA 4X electrical panels made from stainless steel


Petra’s microprocessor controller is engineered to meet the most demanding requirements of all control and data monitoring applications.  It can be set up to perform different functions for cooling and, where applicable, heating applications.

However, in order to render the microprocessor conroller and peripherals easy to use, only the necessary functions for the safe sequencing of operation, and alarm functions are enabled as standard.

In standard execution, whilst using the microprocessor for chiller control, it communicates with the standard conventional control such as high pressure, flow switches etc.  This allows service technicians to inspect and maintain such devices in the usual way.

Main Functions of the Microprocessor Controller

  • Temperature control on water inlet
  • Fan control
  • Complete alarm managment
  • Compressor time management
  • Pump management
  • Pump time managment
  • Automatic lead/lag function
  • Auto restart after power failure
  • Connection to remote terminals
  • Temperature reading °C
  • Anti-recycling timer to limit the number of compressor starts per hour
  • Smart defrost managment (for heat pump units only)

Start Up

For initial start up the following conditions must be met:

  • Control circuit breaker must be switched to on
  • Energise the microprocessor control through keypad, remote start/stop, schedule or BMS command
  • Chilled water pump running
  • Flow has been proven
  • All safety conditions satisfied

When the water in temperature is above the target set point, the first compressor will start after the call for cooling is received.

The control strategy is designed to modulate the chiller capacity to maintain the control sensor reading within the specified control band.  To accomplish this, the microprocessor controller will constantly monitor the control value, its rate of change and position in relationship to the control band and make adjustments accordingly.

Capacity Control

The capacity control logic will control the sequence of operation for the compressor as follows:

  • If the chilled water-in temperature is above the set point, the chiller’s capacity control logic will ask for more capacity by adding a cooling step.  Once the step control has increased, the capacity control logic has a time delay before allowing the new step to increase again.  This time delay depends on how far the temperature is from the target set point
  • If the chilled water-in temperature is within the control band, special logic functions will maintain the chiller within the control band
  • If the chilled water-in temperature is below the control band, the chiller’s capacity logic asks for less capacity by subtracting from the steps.  Once the step has been decreased, the capacity control logic has a time delay before allowing more steps to be decreased again

Low Suction Unloading and Holding

  • This protection is activated when the set point (LOW SUCTION UNLOAD) is active
  • The purpose of this protective feature is to take corrective action prior to a safety being tripped

High Discharge Pressure Unloading & Holding

  • This protection initiates when the set point (HIGH DISCHARGE UNLOAD) is active
  • The purpose of this function is to take corrective action prior to a safety being tripped
  • The system will begin unloading the compressor until the discharge pressure drops below the calculated value

Operating Schedules

  • Two operating schedules per day and eight holidays are supported by the microprocessor controller software
  • Each schedule has a start and end time
  • It the time and day of the microprocessor controller clock is within these limits then the schedule is ‘True’ and the system will be allowed to run
  • If not ‘True’ the system will be off

Soft Load Function

  • To ensure soft starting of the compressors, the initial operation will be at part load with gradual loading based on load demand
  • This start method prevents sudden load change and saves energy


Displayed Data

  • Leaving / Entering water temperature
  • Ambient temperature
  • Compressor discharge pressure / temperature
  • Compressor suction pressure / temperature
  • Compressor drawn current
  • Suction / discharge superheat
  • Compressor load percentage
  • Saturated suction / discharge
  • Compressor oil diff
  • Compressor Timers
  • Digital input status
  • Output relays status
  • Protections status
  • Historical alarm
  • Schedule
  • Adjustable setpoint
 Safeties and Alarms
  • Cut out and unloading
  • High discharge pressure
  • High discharge temperature
  • Low suction pressure
  • Low suction temperature
  • Freeze state
  • High ampere state
  • Low discharge pressure
  • Unsafe suction pressure
  • Unsafe discharge pressure
  • Flow switch (no flow protection)
  • Phase loss protection
  • Low differential oil pressure
  • Unsafe oil pressure
  • Low oil level
  • Motor temperature
  • Low motor amps
  • Probe error alarm

Within the hardwired structure, there are six features, as follows:

  1. Unit start/stop command
  2. Compressor run status
  3. Compressor trip status
  4. Condenser fan run status
  5. Condenser fan trip status
  6. General alarm
Options for Mk VII Controller Only

BMS Card

This card provides connection for:

  1. Modbus RTU – RS4 85 (standard)
  2. PCO WEB TCP / IP (optional)

    TCP/IP -SNMP BAC NET – email Modbus

  3. PCO NET (BAC NET MSTP RS4 85) (optional)
  4. LON Board (optional)
  5. Konnex Board (optional)

Petra BMS Card

Options for Mk VII Controller Only USB in Microprocessor Controller

  1. USB Host: to connect a standard USB dongle for:
    1. SW application upgrade
    2. Download Pco logs
  2. USB device: to connect a PC (without external converter) for:
    1. SW application upgrade
    2. Download Pco logs
    3. Configure and monitor the unit by commissioning tool

Petra USB in Microprocessor Controller

Other Optional Accessories and Chiller Upgrades

  • Pressure relief valves are installed in accordance with the requirments of the ASME (Boiler and Pressure Vessel Code Section VIII, Division 1)
  • The pressure is relieved by allowing the pressurised fluid to flow from an auxilliary passage out of the system to the atmosphere
  • The relief valve is designed to open at a pre-determined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits
  • Heat Pump Chillers that may be used to provide heating in the winter are also available.  Please contact us for more information
  • Simultaneous heating and cooling chillers are also available to provide heating and cooling to different areas at the same time.  Please click here for more information, or get in touch here if this is of interest to you
  • The 120V single phase power supply is provided through the transformer that may be connected before or after the non-fused main disconnect switch
  • GFI socket is used to operate electrical devices on site, such as laptops, tablets and mobile phones

Control Transformer

  • The control transformer is to supply power input to auxillary compnents at 120 or 220 volts, such as (but not limited to) bulk head lights or the GFI Outlet

WPS Range of Air Cooled Chillers

The WPS range of Water Cooled Chillers is available with either refrigerant R513a or R134a.

The model references,  capacities, dimensions, sound data and operating weight is given in the tabs above, but for more detailed information please call us on 0118 988 0101 or sign in here to download the specifications

Please contact us for further information regarding the water cooled chiller operating parameters for R513a.

Capacity  kW*EERESEERSEPRSSCEEL (mm)W (mm)H (mm)Sound Power (dBA)Operating Weight (kg)

*All stated kW capacities are based on 29.4/35°C entering/leaving condenser water temperature, 12/7°C entering / leaving cooling water temperature and 50Hz power supply.  Models WPS 45 to WPS 275 (Door Way Series) are also available as Extended Chassis models.