4 - Refrigerant Pumping - Monika Witt Presentation PDF

Refrigerant Pumping and low charge Ammonia Refrigeration systems

TH. WITT Kältemaschinenfabrik GmbH

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Trends and drivers


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Regulation Safety Reduce the risk of personal injury, by •  designing safe systems •  following safe procedures for maintenance •  educating the operation staff

Quality •  Reliable refrigeration systems will increase food and product quality •  Maintaining a cold chain will avoid waste


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Environment Sustainable refrigeration systems •  Use natural refrigerants with no ODP and no/low GWP •  Make efficient use of materials and operating fluids (e.g. less lubricants, less water consumption) •  Consume less energy


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Cost and profitability Profitable refrigeration systems •  Allow for reliable operation •  Operate energy efficient •  Reduce cost for operating supplies (no leakages, less lubricants, less water consumption) •  Reduce down time •  Increase productivity


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Reduced refrigerant charge Possibilities to reduce the refrigerant charge •  Design of a low pressure receiver instead of high pressure receiver system (LP receiver systems have 10 – 20% less charge comprared to HP receiver systems) •  Compact design with short piping, design with water cooled condensers •  Use of low charge components, particularly condensers •  Use of indirect systems


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Reduced refrigerant charge HP system = 100% 100 90 80 70 60 50 40 30 20 10 0 HP system

LP System


Indirect system / air cooled condenser

Indirect system / water cooled condenser 7

Reduced refrigerant charge Condenser design •  Tube and shell condensers have the largest charge – but they can be cleaned easily •  Plate and shell condensers contain only 10 – 20% charge compered to tube and shell condensers – to clean them they need to disassembled (gasketed version) or back-flushed (fully welded) •  Microchannel condensers can reduce the charge by 25 – 50% compared to plate and shell


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Reduced refrigerant charge Comparison of condenser types Tube & Shell condenser = 100% 100 90 80 70 60 50 40 30 20 10 0 Tube & shell


Plate and shell

Microchannel

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Typical refrigeration charges

Source: Courtecy of Nils Vestergaard, Danfoss Denmark TH. WITT Kältemaschinenfabrik GmbH

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LP refrigerant systems •  The liquid refrigerant from the condenser is completely expanded to the low pressure side Æ no high pressure receiver •  The charge is mainly on the low pressure side Æ increased safety •  The liquid is normally drained with high side float regulators at ambient temperature Æ reduced energy consumption


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LP refrigerant systems Additional aspects •  During stand-still the pressure is equilized Æ the compressor can start unloaded •  Stable pressure situation Æ no pressure fluctuations due to regulating-/ automatic control valves •  Easy mechanical design Æ failsafe, no no need for settings/adjustments •  Maintenance-free TH. WITT Kältemaschinenfabrik GmbH

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High pressure float regulator system •  In Europe the technical common solution •  Used since 1900 to automate the refrigeration system


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High pressure float regulators Two stage plant design •  avoid high gas temperatures for the second stage Æ improved efficiency


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Principle of the high side control Simple mechanical solution of expanding liquid refrigerant from the high pressure to the low pressure side, without gas passing Æ no wiring or controls required Æ low condensing temperatures are utilized Æ the refrigerant charge is stored at low pressure Æ liquid hammer is avoided


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Operation of a high side float regulator •  Refrigerant enters at top inlet valve •  Liquid level in the housing raises •  Float ball starts to lift •  Lever system moves slide stone on an orifice opening to release refrigerant to the bottom outlet valve •  Refrigerant is expanded in the orifice opening, releasing low pressure liquid/gas mixture to the LP surge drum TH. WITT Kältemaschinenfabrik GmbH

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Design of a standard float regulator Service valve for venting Low pressure nozzle

Inlet valve

Protective cap for hand lever control

Control unit (float ball with lever

Outlet valve Connection for safety valve TH. WITT Kältemaschinenfabrik GmbH

arm and orifice) Service valve for draining 17

Installation of high pressure float systems •  A high side float regulator should be installed right behind the condenser •  Avoid heat intake in the liquid refrigerant supply line to avoid too much gas formation •  Float regulators without internal low pressure nozzzle must be located with a slope to the regulator housing to allow for free draining •  One float regulator for each condenser is recommended TH. WITT Kältemaschinenfabrik GmbH

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Function of a low pressure nozzle §  a small tube is connected to the outlet connection, with a defined orifice opening – gas is purged to the LP side Æ The pressure in the float housing is reduced §  No need to place the regulator underneath the condenser – possible above or up to 30 m away TH. WITT Kältemaschinenfabrik GmbH

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Considerations of high pressure float systems •  High pressure receivers are normally not allowed, because the will prevent the self recovery effect (exemption are systems with oil coolers, but then good system design is required) •  Non-condensables are not allowed in the system, beacuse the float ball will be blocked (also the efficiency of heat-exchanger will be decreased) •  Behind the float regulator is a two phase flow – risers must be designed accordingly TH. WITT Kältemaschinenfabrik GmbH

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Self recovery effect of float regulators •  When the float regulator was selected too small, refrigerant will back up in the condenser •  The effective condenser surface is decreased •  Condensing pressure will raise until float regulator is capable to drain the condensate


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Correct connection of high side float regulators •  Gas intake should be avoided, because this will block the float ball A and B not correct!!! → Gas intake


C and D correct!!! → no gas intake

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Parallel condensers with one float regulator •  It is possible to connect one float regulator to parallel condensers – provided they are the same type and have similar operating conditions •  To compensate for different pressure losses sufficient static head of the liquid supply line is necessary


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Parallel condensers System design

Courtesy of Evapco: to be supplied later


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Examples


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Indirect refrigeration systems Indirect systems use the refrigerant to cool down a brine •  the refrigerant is limited to the machinery room/condenser •  A heatexchanger cools down the brine •  A brine is pumped to the coolers •  Using CO2 as a brine allows for stable temperature in the cooler, because the brine is evaporating Consideration The additional heatexchange will increase the energy consumption TH. WITT Kältemaschinenfabrik GmbH

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Indirect refrigeration systems


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Reduced refrigerant charge Considerations Refrigerant charge reduction as reasonable as possible – not as low as possible! Attention Reduced HX surface will decrease the component size and reduce charge, but Reduced HX surface will increase ΔT and therefore increase energy consumption


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Advantages / Disadvantages Indirect refrigeration system Advantages •  Low refrigerant charge •  Refrigerant charge limited to the machinery room (safety increased) •  Easy handling of the brine Disadvantages •  Additional heat exchanger required •  Less efficient than direct systems (e.g. brine pumps use more energy than refrigerant pumps)


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Principle of flooded system


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Principle of flooded system

In the return line is a mixture of liquid and gas „F“ Overfeed rate should be 1.2 – 1.5 TH. WITT Kältemaschinenfabrik GmbH

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Examples of flooded systems


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Compact flooded systems Fully welded plate heatexchanger


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Compact flooded systems Example welded Plate HX


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Compact flooded systems Example gasketed Plate HX


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Compact flooded systems considerations Plate heatexchangers are sensitive towards liquid hammer or vibrations •  Uncouple compresor and HX frame •  Brine pumps should not push into the heatexchanger, but be installed behind the HX •  Use slow acting valves on the brine side •  Make sure the buffer reservoir on the brine side is sized sufficiently large enough TH. WITT Kältemaschinenfabrik GmbH

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Refrigeration systems direct refrigeration system


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Refrigeration systems direct refrigeration system •  refrigerant pumps supply evaporators •  Direct evaporation of refrigerant in the coolers •  Due to overfeed of refrigerant the coolers operate as flooded heat exchangers


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Advantages / Disadvantages direct refrigeration system Advantages •  Easy control of the refrigerant due to overfeed •  Energy efficiency maximised Disadvantages •  Larger refrigerant charge than indirect systems


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Why refrigerant pumps? •  Easy distribution of refrigerant to evaporators – advantages of flooded operation •  Due to the available delivery head even remote evaporators are supplied evenly Simple control •  Surge drum separation of liquid / gas – increased safety for the compressor •  The entire surface is used for evaporation due to overfeed – efficient and energy saving


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Types of refrigerant pumps Open refrigerant pumps •  With flanged motor •  Double Shaft seals with oil barrier •  Common shaft for pump and motor or coupling, which enables use of standard motors


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Types of refrigerant pumps hermetic refrigerant pumps •  With integrated, canned motor •  one shaft for pump and motor


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Pro / Contras open pumps PRO •  Robust, simple design •  Easy to maintain •  Trouble-free operation even in contaminated systems or dry-run conditions (for seconds) •  Use of standard motors possible •  Lower investment cost than hermetic pumps CONTRA •  Maintenance required TH. WITT Kältemaschinenfabrik GmbH

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Pro / Contras hermetic pumps PRO •  No maintenance required •  No leackages

CONTRA •  More sensitive (always liquid refrigerant needed, no dirt or oil contamination allowed) •  More complex work when repair is needed


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Required suction head NPSH for refrigerant pumps? •  Low density of ammonia does not provide much subcooling •  Refrigerant in a boiling condition (no subcooled liquid) •  Gas bubbles will form during pull-down conditions and varying loads Always provide refrigerant pumps with liquid refrigerant : 1 – 1,5 m suction head is normally sufficient Refrigerant pumps should be able to handle liquid/ vapour mixtures! TH. WITT Kältemaschinenfabrik GmbH

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Sub-cooling at 10m suction head 30

subcooling [K]

25 -40°C

20

-10°C

15

0°C +35°C

10

5

0 R134a

R404A


Ammoniak

CO2

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Velocity in the suction line

downleg

Liquid droplets

boiling

Gas bubbles

Sideglass

v < 0,3 m/s

v = 0,3 m/s v > 0,3 m/s TH. WITT Kältemaschinenfabrik GmbH

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Velocity in the suction line Velocity in the downleg of ammonia systems should always be < 0,3 m/s so gas bubbles can rise back to the surge drum


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Pump arrangement •  allow for sufficient space around the pump for normal ice accumulation around the pump and to clean filters and execute maintenance •  Straight runs will not improve pump operation when using refrigerants = boiling liquids (compared to e.g. operation with water) •  Top connection of the pump is recommended to allow gas bubbles can rise •  Piping and fittings should avoid gas accumulation TH. WITT Kältemaschinenfabrik GmbH

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Pump arrangement Two pumps in operation


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Pump arrangement One pump in stand-by


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Pumping vessel design •  Horizontal low pressure surge drums will give more surface area for settlement of oil compared to vertical surge drum •  Never connect pumps to an oil dome (except as shown bevor with the down leg well above the bottom of the surge drum •  Protude the downleg into the vessel and use vortex breakers


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Downleg design •  Protude the downleg about 30 – 40 mm into the surge drum •  Use vortex breakers


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Valve installation Liquid trapped

gas trapped

Valve spindles in liquid lines should always be installed in a horizontal poistion! TH. WITT Kältemaschinenfabrik GmbH

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Safety procurements


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Safety procurements Refrigerant pumps must be supplied with liquid at all times! •  Install a low level switch (LLS) that will shut off the pump if the liquid level above the vortex breaker is less than 15 mm


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Safety procurements Protect the pump against contamination •  Use shot-blasted pipes and vessels •  Install a start-up filter when contamination is likely (no shot-blasted pipes available) •  Clean the start-up filter frequently during the first weeks of operation until clean


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Start-up filters •  Use diameter of the downleg for the filter


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Safety procurements Protect hermetic pumps against contamination with oil •  Frequently drain oil from the system, because oil avoids proper cooling of the internal motor and bearings with liquid refrigerant •  Use automatic oil draining systems wherever possible (this will also increase system safety and avoid waste of lubricants) •  Change over of pumps in operation and stand-still, so pump in stand-still will not fill up with oil over time TH. WITT Kältemaschinenfabrik GmbH

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Safety procurements Never operate refrigerant pumps against closed discharge! •  Install a differential pressure operated overflow valve and set the pressure lower than the max. permitted delivery pressure


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Safety procurements Never operate refrigerant pumps against closed discharge! Use a diffential pressure switch with a delay of 30 – 60 sec. If the pump has not built up pressure during that time the pump is switched off


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Safety procurements - summary •  Control the minimum level in the surge drum •  fit an overflow valve Æ protect the pump against closed discharge •  connect the temperature protection •  avoid multiple starts/ stops within short periods of time •  run the pump frequently Æ avoid oil/debris can collect •  fit a differential pressure control TH. WITT Kältemaschinenfabrik GmbH

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Required delivery head

Pressure losses

pressure losses during pump cycle


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Required flow more liquid refrigerant is pumped to the evaporators than needed for evaporation. That way the surface is always fully wetted. mass flow pump mpump Recirculation rate = ----------------------------- -------evaporated refrigerant m Q0 e.g. recirculation factor = 4 means: 1 part refrigerant is evaporated and 3 parts are returned as liquid


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Typical recirculation rates Recirculation factor Refrigerant

NH3

R22

CO2

Air Cooler

3 - 4

2 - 3

1,2 - 2,0

Plate Freezer

7 - 10

5 - 10

5 - 10

Liquid Chiller


1,2 – 1,5

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Pump discharge line •  Velocity in the discharge line typically 1,5 m/s •  When more than one pump is installed or evaporators are located above the surge drum a check valve is needed to avoid backwards flow •  Check valves shoulb be installed as close to the pump as possible to prevent a gas buffer (or an additional vent line is needed right before the check valve)


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Pump discharge line •  Behind the evaporator there is a two-phase flow •  Design the piping to avoid liquid hammer or slugging


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Vent line It is important the pump is vented properly before started Some hermetic types have an internal vent line to the suction side. For these types the suction downleg must be open during stand still or suffictly long enough before start. For pumps that vent to the suction side there is no additional piping required. Vent line must be individually piped for each pump, so there is no interference between pump in stand-still and pump in operation Install vent line with a constant slope (no traps) to avoid liquid hammer TH. WITT Kältemaschinenfabrik GmbH

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Typical vent line arrangement


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Minimum flow line Also known as Qmin line for hermetic pumps Since hermetic pumps require liquid refrigerant at all times a bypass line is recommended with overflow valve to provide sufficient flow when too many evaporators close Some manufacturers require a permanent open bypass with a defined orifice (Qmin orifice) to allow for a constant by-pass flow. The Qmin orifice is sized individually for the selected pump.


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Qmin flow Attention The Qmin flow must be added to the required system flow that is calculated by the design engineer. The pump curves normally do not consider by-pass flows. Differential set overflow valves only open when the pressure reaches the set differential pressure (normally the maximum permitted pump pressure). Therefore no additional flow has to be considered


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Motor cooling/ reverse circulation line


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Motor cooling/ reverse circulation line


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Motor cooling line Also known as reverse circulation piping To provide the required liquid refrigerant flow at all times it is also possible to install a bypass line at the end of the motor back to the surge drum As with the minimum flow line mentioned before, a defined flow is required. To adjust the flow it is possible to use a defined orifice or a flow meter with needle valve. The Qmin orifice is sized individually for the selected pump. TH. WITT Kältemaschinenfabrik GmbH

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Reverse circulation flow Attention Alway connect the reverse circulation line to the gas area! Avoid traps and install the line with a constant flow upwards! The reverse circulation flow must be added to the required system flow that is calculated by the design engineer. (The pump curves normally do not consider by-pass flows.) Make sure the reverse circulation piping is supported properly, so it cannot get ripped off TH. WITT Kältemaschinenfabrik GmbH

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Qmax orifices Most hermetic pumps are only allowed to operate in a defined range. The permitted range is defined by the manufacturer, depending on the design condition. In order to provide sufficient back pressure most manufaturers will require a Qmax orifice, that is calculated individually for the selected pump and operating conditions. It is also possible to use constant flow valves instead that will avoid the high pressure loss of Qmax orifices, TH. WITT Kältemaschinenfabrik GmbH

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Qmax orifices Attention: Qmax orifices create an additional pressure loss that must be considered by the design engineer. Qpump

The effective pump curve is lower than the pump curve given in the documentation


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Constant flow valve Attention:Constant flow valves will ensure a constant flow at all times, but this may not be good for the operating conditions

Flow and pressure no longer corresponding


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Control system Operating conditions normally vary due to evaporators being opened/closed


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Demand control Easiest way to control a liquid pumping system Requirement: Automatic valves that signal open/closed position Pump is switched on when one or more evaporators are open and stopped when all evaporators are closed Attention: Avoid frequent starts/stops (as this will reduce the pump lifetime) by programming a minimum stand-still period and run time


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Influence of by-pass flow Bypass with overflow valve (set for a differential pressure) opened only when the max. permitted pressure is exceeded


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Two pumps in operation Operating two pumps will not double the flow due to greater overall resistance!


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Pressure control •  Since delivery head and volume flow is normally corresponding it is possible to use pressure control for switching on/off a second pump •  Delivery head to switch off the second pump is with ammonia typically 0,7 bar higher than the minimum pressure •  It is recommended to program a minimum run time to avoid frequent starts/stops Attention: Too small programmed differences btween minimum and maximim pressure may result in instable operating conditions TH. WITT Kältemaschinenfabrik GmbH

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Frequency converters Frequency converters will keep te pressure fairly stable and safe energy. The evaporator with highest pressure loss should be supplied properly at minimum frequency The set point should be about 0.3 bar higher than the minimum required pressure TH. WITT Kältemaschinenfabrik GmbH

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Frequency converters •  Check minimum and maximum permitted frequencies with the manufacturer (normally between 40 Hz and 60 Hz). •  Maintain the pressure in the system stable (typically with a proportional-integral (PI) control system •  Ramping times should be short to allow the check valve behind the pump opens fast enough


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Minimum stand-still period •  To allow for proper venting after the pump has been switched off and •  avoid frequent starts/stops (which will reduce the lifetime of any pump) More than 6 starts/stops per hour should not occur! Stand-still periods of 5 minutes are normally sufficient to vent an ammonia pump properly (with long Qmin or reverse circulation lines this may be longer)


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Minimum operating time The highest impact takes place when the starting torque of the pump consumes the highest current. A minimum run-time of 2 minutes - better 5 minutes – is recommended! Contiunous operation will increase the life-time of the pump.


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Evaltuation ammonia systems •  Direct ammonia systems have a very good record in Europe – very few accidents (similar to systems using synthetic refrigerants) •  Most cases of accidents occur during maintenance – good education of service personnel will avoid incidents •  Cascade systems with evaporating CO2 offer an alternative when no ammonia is accepted in populated areas


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Balance between refrigerant charge, efficiency and safety •  Pumped refrigerant systems offer highest efficiency •  Refrigerant charge about 3 l per KW capacity in Europe •  Safety should be in the focus during design (risk analysis), installation and operation by following codes of practice and regulations •  Use of prefabricated units will increase reliability TH. WITT Kältemaschinenfabrik GmbH

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Refrigerant charge reduction as reasonable as possible – not as low as possible!


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4 - Refrigerant Pumping - Monika Witt Presentation PDF

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