Danfoss Automation of Commercial Refrigeration Plant

8/8/2019 Danfoss Automation of Commercial Refrigeration Plant

1/36


2/36


3/36


4/36


5/36

Dan oss A/S (RC-CMS / MWA), 03 - 2004 RG00A502 3

Manual Automation o Commercial Re rigeration Plant

Re rigeration plant withthermostatic expansionvalve and aircooledcondenser

In this plant an air-cooled unit has replaced thewater-cooled condenser. Air-cooled condensersare normally used where no cooling water isavailable or where the use o cooling water is

orbidden.

Replacing the manual valve ahead o theevaporator with a thermostatic expansion valve(pos. ) ensures that the evaporator iscontinuously supplied with the amount o re rigerant necessary to keep a constantsuperheat in proportion to the load.

This o course presupposes that the selectedexpansion valve suits the evaporator concerned.A actor here is that in conditions o maximumload the expansion valve supplies precisely theamount o re rigerant the evaporator is able toevaporate. In addition the superheat setting o the valve must match the evaporator.Superheat is generally understood as being thenumber o C the evaporator has minus theboiling point o the medium at the existingpressure and with all liquid evaporated.

Superheat in an evaporator is de ned ast p s = C superheat, where t is the temperaturemeasured at the point on the evaporator wherethe expansion valve sensor is placed, and p s is thepressure measured at the same point. (Therelevant pressure is converted to C).

For urther details on superheat, see page 7.

Fig. 2

Thermostatic expansion valve

Automatic expansion valve


6/36



Re rigeration with Finnedevaporator

Thermostat type KP 6 ( ) cuts the ans (2) in andout depending on the room temperature.

Thermostatic expansion valve type TE (3) withexternal pressure equalization regulates liquidinjection in the evaporator, dependent on thesuperheat but independent o the pressure dropacross the evaporator.

Liquid distributor type 69G (4) distribute re ri-gerant liquid equally to the individual evapo-rator sections.

The compressor is cut in and out on the low-pressure side o the combined high and lowpressure control type KP 5 (5) depending on thesuction pressure. In addition, the high-pressureside o this control gives protection against toohigh a condensing pressure by cutting out thecompressor i it becomes necessary (e.g. whenthe ventilator is de ected or the air ow is blocked(dirt)).

Sight glass type, SGN (6) indicates too highmoisture content in the re rigerant and too little

ow to the thermostatic expansion valve. Theindicator changes colour when the moisturecontent is too great. Vapour bubbles in the sightglass can mean insufcient charge, insufcientsub cooling or partial clogging o the strainer.

Fig. 3


7/36



Thermostatic expansionvalve

Thermostatic expansion valve type T 2, the bulbo which is placed immediately a ter theevaporator, opens on rising superheat. Pressureon the diaphragm ( ) increases as bulbtemperature increases and pressure under thediaphragm increases as the evaporatingtemperature increases. The pressure di erential, which corresponds tothe re rigerant superheat, mani ests itsel as a

orce, which tries to open the valve against the

opposite orce o the spring (2). I the di erential,i.e. superheat, exceeds the spring orce the valvewill open.

The ori ce assembly, with ori ce (3) and valvecone (4) can be changed. To suit capacityrequirements, there are eight di erent sizes tochoose rom.

Fig. 4

T 2

Thermostatic expansionvalve with distributor Fig. 5

TE 5 + 69G

Distributor type 69G ensures an equal distri-bution o re rigerant to the parallel sections o the evaporator.

The distributor can be installed either direct onthe thermostatic expansion valve as shown or inthe line immediately a ter it. A distributor oughtalways to be tted so that the liquid ow throughthe nozzle in the distributor pipes is vertical. Thisensures that the e ect o gravity on liquiddistribution is as little as possible. All distributionpipes must be exactly the same length.

For evaporators with a large pressure drop,thermostatic expansion valves with externalpressure equalization must always be used.Evaporators with liquid distributors will alwayshave a large pressure drop; there ore always useexternal pressure equalization.


8/36



Expansion valves

Upper diagram: The diagram shows an evaporator, which is ed by athermostatic expansion valve with internal pressureequalization.

The degree o opening o the valve is regulated by:Pressure p b in the bulb and capillary tube actingon the upper side o the diaphragm anddetermined by the bulb temperature.

Pressure p o in the valve discharge connectionacting under the diaphragm and determined bythe evaporating temperature.

Spring pressure p s acting under the diaphragmand manually adjustable.

In the example shown, the pressure drop in theevaporator p is measured in C re rigerant pressure 5 (20) = 5C. Provided that the valve spring hasbeen manually adjusted to a pressure p s corresponding to 4C, it ollows - in order to achieveequilibrium between the orces acting over andunder the diaphragm - thatp b = p o + p s ~ 5 + 4 = C. That is, the re rigerant has to be superheated by (20) = 9C be ore the valve begins to open.

Lower diagram: The same evaporator coil, but this time ed by athermostatic expansion valve with external Pressureequalization connected to the suction line a ter thebulb.

The degree o opening o the valve is now regulatedby:

Pressure p b in the bulb and capillary tube actingon the upper side o the diaphragm anddetermined by the bulb temperature.

Pressure p o - p in the evaporator outlet actingunder the diaphragm and determined by theevaporating temperature and the pressure dropin the evaporator.

Spring pressure p s acting under the diaphragmand manually adjustable.

Provided that, as stated above, pressure drop p inthe evaporator corresponds to 5C and springpressure p s in the valve to 4C re rigerant pressure, it

ollows that p b = p o p + p s ~ 5 5 + 4 = 6C. That is, the re rigerant now has to be superheatedby 6 (20) = 4C be ore the valve begins to open.

The amount o charge in the evaporator and henceits capacity become higher since a smaller portion o the evaporator sur ace is used or superheating.

Fig. 6

Conclusion: Thermostatic expansion valves with external pressure equalizing must always be used or evaporators with alarge pressure drop. Evaporators with a liquid distributor will always have a large pressure drop; there orealways use external pressure equalization.


9/36



Thermostatic expansionvalve, method o operation Fig. 7

The thermostatic expansion valve is controlled bythe di erence between bulb temperature t b andevaporating temperature t o. The valve openswhen the temperature di erential rises, t b to = t, i.e. when re rigerant superheat rises the valvewill have a larger opening rate. See g. 6.

Solid curve p o and dotted curve p b gives vapourpressure or the re rigerant and chargerespectively. Chain-dotted curve p o + p s represents the re rigerant vapour pressure curvep o o set in parallel with a constant springpressure p s, the actory setting or example.

At a given evaporating temperature, t o, a pressurep o + p s acts under the valve diaphragm and triesto close the valve. Pressure p b acts over thediaphragm and tries to open the valve.

The gure shows equilibrium between p o + p s and p b at evaporating temperature t o and bulbtemperature t b respectively. Practically speaking,di erential t b to, the static superheat, is the

same within the entire working range o the valverom to' to t o".

That is to say, irrespective o the evaporatingtemperature operated with inside the workingrange, the thermostatic expansion valve willregulate liquid injection so that re rigerantsuperheat a ter the evaporator is held to thevalue determined by spring pressure p s. I thedi erential between bulb temperature t b andevaporating temperature t o is less than the staticsuperheat t, the valve is closed (t

b t

o< t; p

b t; pb > p o + p s).

I the di erential between bulb temperature t b and evaporating temperature t o is equal to thestatic superheat t, the valve is just about toopen or just about to close (t b to = t; pb =p o + p s).


10/36



Thermostatic expansionvalve with MOP charge Fig. 8

It can sometimes be desirable to use athermostatic expansion valve with a limitedworking range - or example, in re rigerationplant with only one evaporator where cooling

rom a completely or partially temperatureequalized condition occurs only as an exception(a ter repair or de rosting).

For such plants it may be cheaper to use a smallercompressor motor dimensioned in accordancewith the load a ter cooling down. However,during cooling down such a motor will becomeoverloaded and cut-out on the thermal overloadprotection.

To eliminate this risk, a thermostatic expansionvalve with a MOP (Maximum Operating Pressure)charge can be used. This pressure-limited valvewill only begin to open at a low evaporatingtemperature, t MOP, since the charge is adapted toproduce a bend in the vapour pressure curve P b.

This means that static superheat t is very high atevaporating temperatures higher than t

MOP, i.e. in

practice the valve will remain closed until thecompressor has reduced the suction pressuresufciently to ensure that the electric motor isnot overloaded.


11/36



Combined high and lowpressure control

Lowpressure side (LP): The LP connector ( 0) is connected to the suctionside o the compressor. When pressure alls onthe low-pressure side the circuit betweenterminals 2 and 3 is broken. Turning the LPspindle ( ) clockwise adjusts the unit to cut out(break the circuit between terminals 2 and 3) at ahigher pressure. Turning the di erential spindle(2) clockwise adjusts the unit to cut in again(make the circuit between terminals 2 and 3) at asmaller di erential.Start pressure = stop pressure + di erential.LP signal unction between terminals A and B.

Highpressure side (HP): The HP connector ( ) is connected to thedischarge side o the compressor. When pressurerises, on the high-pressure side the circuitbetween terminals 2 and 3 is broken. Turning theHP spindle (5) clockwise adjusts the unit to cutout (break the circuit between terminals 2 and 3)at a higher pressure. The di erential is xed.Stop pressure = start pressure + di erential.

Lowpressure and Highpressure control

Fig. 9

KP 5

Combined high and low pressure controltype KP 5 has a single-pole changeoverswitch ( 2).

Fig. 0

Lowpressure control type KP 1Contains a single pole changeover switch (SPDT),which breaks the circuit between terminals 2 and3 when pressure in the bellows element (9) ails(on ailing suction pressure), i.e. the connector( 0) must be connected to the suction side o thecompressor.

Turning the range spindle ( ) clockwise adjuststhe unit to cut in - to make the circuit betweenterminals 2 and 3 at a higher pressure. Turningthe di erential spindle (2) clockwise adjusts theunit to cut out again - to break the circuitbetween terminals and 2 at a smallerdi erential.

Start pressure = stop pressure + di erential.

Highpressure control type KP 5Is built up in the same way. Bellows, spring andscale are o course suitable or the higherworking pressure. In this case, the switch breaksthe circuit between terminals and 2 whenpressure rises in the bellows element (9), i.e.when condensing pressure rises (the connectormust be connected to the discharge side o thecompressor ahead o the shut-o valve).

Turning the range spindle clockwise adjusts theunit to cut out - to break the circuit betweenterminals and 2 at a higher pressure. Turningthe di erential spindle (2) clockwise adjusts theunit to cut in again - to make the circuit between

terminals and 2 at a smaller di erential.Stop pressure = start pressure + di erential.


12/36



Highpressure control,method o operation

High-pressure control type KP 5 is connected tothe high-pressure side o the re rigeration plantand stops the compressor when the condensingpressure becomes too high. The control containsa pressure-controlled single- pole changeoverswitch (SPDT) where the contact positiondepends on the pressure in the bellows (9). See

g. , drawings A and B.

Via the adjusting spindle ( ) the main spring (7)can be set to exert a suitable counter- pressure tothe bellows pressure. The down-ward resultant o these two orces is trans erred by a lever (2 ) tothe main arm (3), one end o which is tted with atumbler ( 6).

The tumbler is held in position on the main armby a compressive orce, which can be adjusted byusing the spindle (2) to change the pull rom thedi erential spring (8).

The orces rom the bellows pressure, main springand di erential spring are thus trans erred to thetumbler ( 6), which will tilt when the orces comeout o equilibrium because o changes in thebellows pressure, i.e. the condensing pressure.

The main arm (3) can only take up two positions.In one position a orce is exerted on each end o the arm and creates opposite torques around itspivot (23). See drawing A. I the bellows pressuredecreases, the main spring exerts an increasing

orce on the main arm. Finally, when the countertorque rom the di erential spring is overcome,the main arm tilts and the tumbler ( 6)instantaneously change position so that thecompressive orce o the di erential spring lieson a line near the arm pivot point (23). Thecounter-torque rom the di erential spring thusbecomes almost zero. See g. , drawing B.

The bellows pressure must now rise to overcomethe orce rom the main spring because thespring orce torque around the pivot point (23)must also all to zero be ore the snap system canreturn to its initial position.

On alling bellows pressure (see g. , drawingA), the main arm moves instantaneously to theposition shown in g. , drawing B when thebellows pressure is reduced to the stop pressureminus the set di erential pressure.

Conversely, the main arm moves instantaneouslyrom the g. , drawing B position to the g. ,

drawing A position when the bellows pressurehas risen to the stop pressure = start pressure +di erential pressure. See also text or gs. 9 and

0 regarding adjustment o type KP.

The contact system is specially designed so thatthe make contact travels at the initial speed o the snap action until it reaches the xed contact,while the break contact separates rom the xedcontact at the maximum speed o the snapaction. The system has been made possible bythe use o a small striker ( 9) and accuratelymatched contact springs.

The contacts (20) make with a smaller orce thanthey break, which means that in practice bounceduring make is eliminated. The holding orceduring make is exceptionally high. At the sametime the system gives an instantaneous break

unction so that the holding orce is maintained00% right up to break. For these reasons the

system is able to operate with high currents andits unction is not impaired by shocks. Comparedwith traditional designs, the system has givenexceptionally good results in practice.

Fig.


13/36

Dan oss A/S (RC-CMS / MWA), 03 - 2004 RG00A502


ThermostatFig. 2

KP 6

Thermostat type KP 6 , which has a single polechangeover switch ( 2), makes the circuitbetween terminals 2 and 3 when bulbtemperature rises, i.e. when room temperaturerises.

Turning the range spindle ( ) clockwise, increasesthe cut-in and cut-out temperature o the unit. Turning the di erential spindle (2) clockwisedecreases the di erential between cut-in andcut-out temperatures.

Filter drierFig. 3

DML / DCL

Filter drier type DML / DCL has a sintered charge,a so-called solid core (3). This is pressed by thespring (2) against the polyester mad (4) andcorrugated per orated plate (5).

The charge or core in the lter drier consists o material which e ectively removes moisture,harm ull acids, oreign particles, sediment andthe products o oil breakdown.

Sight glassFig. 4

SGI

Sight glass type SGI / SGN has a colour indicator( ) that changes rom green to yellow when themoisture content o the re rigerant exceeds thecritical value. The colour indication is reversible,i.e. the colour changes back rom yellow to greenwhen the plant has been dried, e.g. by replacingthe lter drier.

Sight glass type SGI is or CFC, sight glass typeSGN is or HFC and HCFC (R 22).


14/36



Automatic water valveFig. 5

WVFX

Automatic water valve type WVFX opens onrising pressure in the bellows element ( ), i.e.when condensing pressure increases (theconnector on the bellows housing must beconnected to the re rigerant side o thecondenser).

Turning the hand wheel (2) counter clock-wisetightens the spring, which means that the valvewill open at a higher condensing pressure. I thehand wheel is turned clock-wise the valve willopen at a lower condensing pressure.


15/36



Finned evaporatorFig. 6

The nned evaporator is designed or orced aircirculation over the parallel evaporator coils. Theair circulation should always be on the counter

ow principle so that the evaporator coils areuni ormly loaded. There ore the relation betweenair ow and re rigerant ow ought always to be asshown in the upper gure.

In this way the largest temperature di erence(see right hand gure) is ensured between the airt l and the evaporator sur ace t at the re rigerantoutlet o the evaporator. That is to say, re rigerantsuperheat t will be rapidly a ected by a changein the temperature o the incoming air (the load)and will thereby rapidly give a signal to thethermostatic expansion valve to change theliquid injection.

It is important that the evaporator coils areuni ormly loaded. For example, with a downwardvertical air ow through the evaporator, theincoming air will load the rst evaporator coilsmore than subsequent coils. The rear coils will bethe least loaded and will there ore determine towhat degree the thermostatic expansion valveopens. I a small amount o re rigerant liquid romthe rear evaporator coils passes the point wherethe bulb is located, the valve will close despitethe act that the rst coils require a supply o re rigerant liquid because o a larger load, i.e.brisker evaporation.

The thermostatic expansion valve bulb must notbe in uenced by alse e ects; such as air owthrough the evaporator and the bulb mustthere ore be placed on the suction line outsidethis air ow. I this is not possible, the bulb has tobe isolated.

Note that a thermostatic expansionvalve with external pressureequalization is used.


16/36



Re rigeration plant withoil separator and heatexchanger

Fig. 7

In principle, in re rigeration plant the oil shouldremain in the compressor. Out in the system itwill do more harm than good because it willimpair the capacity o the evaporator and

condenser. Also, i the oil level in the crankcasebecomes too low, there will be a risk o insufcient compressor lubrication. The best protection against these disadvantagesis the installation o an efcient oil separator, typeOUB ( ).

Furthermore, a heat exchanger type HE (2) o ersthe ollowing advantages:

Superheating the suction gas providesgreater protection against liquid knock inthe compressor and counteracts ormation o condensate or rost on the sur ace o uninsulated suction lines.

Sub cooling the re rigerant liquid counter-actsthe ormation o vapour, which will reduce thecapacity o the thermostatic expansion valve.

Operating economy will o ten be improvedbecause sources o loss such as un-evaporated liquid drops in the suction gasand insufcient sub cooling o the re rigerantliquid are completely or partially eliminated.


17/36



Oil separator Fig. 8

OUB

Hot high-pressure gas is supplied to the oilseparator type OUB through the connector ( ). The gas then ows around the oil tank (2) and

through the lter (3) where the oil is separated. The vapour, now poor in oil, leaves the oilseparator through the upper connector (4).

Separated oil is collected in the bottom o the oiltank (2), which is kept heated by the incomingvapour. In this way the separated oil is stored in a

warm condition, i.e. with the lowest possiblere rigerant content.A oat valve (5) regulates oil return to thecompressor.

Heat exchangerFig. 9

HE

Heat exchanger type HE has been designed witha view to achieving maximum heat transmissionat minimum pressure drop. The outer spiral-

ormed chamber (4) leads hot re rigerant liquid ina ow counter to the ow o cold re rigerantliquid in the inner chamber (3). Built in to theinner chamber are o set n sections.

Heat exchanger type HE is manu actured in brassand copper and has very small dimensions inrelation to its heat transmission capacity. The

spiral ormed outer chamber (4) orces the hotre rigerant liquid over the entire heattransmission sur ace and prevents the ormationo condensate on the outer jacket. The built-ino set n sections in the inner chamber (3)produce turbulent ow in the re rigerant vapour.Heat transmission rom liquid to vapour is thusvery e ective. At the same time, pressure drop iskept down to a reasonable level.


18/36



Re rigeration plant or alarger cold store Fig. 20

Complete re rigeration plant or a larger cold store with temperature above reezing point

To ensure e ective shut-o o the liquid lineduring compressor standstill periods, solenoidvalve EVR ( ) has been installed since bulbtemperature may be expected to rise morerapidly than evaporating temperature and causethe thermostatic expansion valve to open.Protection against overcharging the evaporatorduring compressor standstill periods is providedby making the solenoid valve close at the sametime as the compressor is stopped.

The liquid line is equipped with type GBC (2) orBML manual shut-o valves to make replacemento the lter drier easy.

Pressure on the high and low-pressure sides o the compressor can be read on the pressuregauges shown. The pressure gauges can be shuto with the three-way valves type BMT (3).


19/36



Solenoid valve Fig. 2

EVR

Solenoid valve type EVR is a servo-controlledelectromagnetic shut-o valve. Throughequalizing holes (2) the upper side o thediaphragm ( ) is pressure-equalized with thevalve inlet pressure on the underside. Whencurrent energizes the coil (3) the pilot ori ce (4) isopened. This ori ce has a larger through- owarea than the total area o the equalizing holes.

Pressure over the diaphragm is reduced by theow through the pilot ori ce to the valve outletside and the larger inlet pressure on theunderside li ts the diaphragm. When the coil isde-energized, the pilot ori ce closes and thediaphragm is drawn onto the valve seat as thepressure over it increase through the equalizingholes.

Shuto valveFig. 22

BM

Shut-o valves types BM have a triple diaphragmseal ( ) o stainless steel. A thrust shoe (2)prevents direct contact with the spindle (3). Thespring (4) together with the pre-stresseddiaphragm is able to hold the valve open atoperating pressures down to P e = bar.

The counter seat in the cover (5) prevents theingress o moisture. The valves are available instraight, and /4" T versions. Flow through the sideport o the T version can be shut o leaving theend ports permanently open.


20/36



Key diagram, control currentor re rigeration plant, fg. 20 Fig. 23

The diagram must be read rom top to bottomand rom le t to right. The individual circuits aredrawn so that no leads cross. Power-consumingcomponents are shown at the bottom o thediagram. These include relay coils in the motorstarters, solenoids coils, regulation motors, etc.Motor starter thermal relays F are shown adjacentto the contacts between terminals 95 and 96.Manual reset S is also shown. Relay auxiliary

contacts K between terminals 3 and 4 areshown at the top o the diagram. Designations

3, 4, 95, 96, etc. correspond to those containedin Dan oss in ormation on contactors and motorstarters.

Relay coils K serve the auxiliary contactsbetween terminals 3 and 4. The auxiliarycontacts are drawn in their de-energized coilposition. Under the neutral wire and each relaycoil there is an indication o in, which circuit theassociated auxiliary contacts, can be ound.

Terminal designation 3- 4 is, by de nition,always a make contact (NO), while terminaldesignation - 2 is always a break contact (NC). The key diagram should be read as ollows: When,on rising cold store temperature, thermostat typeKP 6 cuts in (when switches S and S2 are made)between terminals 2 and 3, relays K and K2 inmotor starters type CIT pull in and start theevaporator ans. At the same time the associated

auxiliary contacts in circuits 3 and 4 are made.Relay K3 in compressor motor starter type CITpulls in i the combined high and low pressurecontrol type KP 5 is made between terminals 2and 3, and i switch S3 is made. The compressorstarts and at the same time the auxiliary contactin circuit 5 connects current to coil E in the EVRsolenoid valve in the liquid line. The solenoidvalve opens and re rigerant liquid is injected intothe evaporator, regulated by thermostaticexpansion valve type TE.


21/36



Motor startersFig. 24

The Dan oss motor starter range up to 420 A ismade up o modules. It consists o a basic module(contactor type Cl) onto which up to ourauxiliary contact blocks (type CB) can be clippedas necessary. There is also a range o thermalrelays (type TI). The le t-hand diagram shows a

motor starter with start- stop / reset unction. Thestart contact (type CB-S) carries the terminaldesignation 3- 4. The right-hand diagram showsa motor starter with stop/reset unction,controlled via a thermostat, pressure control, orsimilar.

The motor starters are equipped with a thermalrelay having three indirectly heated bimetals. Through a cut-out mechanism the bimetals break the bounce- ree switch between terminals 95and 96 in the event o overloading. Large currentasymmetry between the three motor phasesactivates a built-in di erential cut-out, whichensures an accelerated trip - as distinct rom whatoccurs under a normal symmetrical overload. Thecut-out is partly temperature-compensated; upto a temperature o 35C it compensates or anyrise in the ambient temperature not arising romoverloading.

The motor starters are available in severalversions. The examples shown are tted with amanually lockable stop and reset or the thermalrelay, i.e. the starters must be manually reset a terthermal cut-out.

The modules are based on thermoplastic (CI) andBakelite/thermoplastic (TI), and all main andauxiliary contacts are made o a special silveralloy. All steel parts are e ectively corrosionprotected.So t starter type MCII and circuit breaker type CTIare also available rom Dan oss.


22/36



Central re rigeration plantor cold store temperatures

above reezing point

Fig. 25

Temperature and relative humidity play asigni cant role in the keeping o oodstu s andthe various categories o ware must be stored in

the most avourable conditions. There is usethere ore or cold stores having di erenttemperatures and humidities; not only the roomtemperature but also the evaporatingtemperature must be under control.

In the example shown, the ollowingtemperatures might be considered:

The room temperature in all 3 cold stores arecontrolled with KP-62 thermostats opening andclosing the EVR solenoid valves.

Two evaporating temperature regulators typeKVP ( ) throttle the suction line a ter theevaporator in the +8C and +5C stores so that

the evaporating temperatures are maintained at+3C and 5C respectively.

Combined high and low pressure control type KP5 (2) cuts the compressor in and out at a suitably

low suction pressure to maintain evaporatingtemperature in the 0C store at 0C.

During compressor standstill, check valve typeNRV (3) prevents re rigerant rom the evaporatorsin the +8C and +5C stores condensing in thecoldest evaporator, i.e. the one in the 0C store.

Check valve type NRV (4) a ords protectionagainst re rigerant condensing in the oil

separator and compressor top cover i thesecomponents become colder than the evaporatorduring plant standstill periods.

Room temp. Evaporat ing temp.

Vegetable store +8C +3C

Sliced meat andsalad store

+5C 5C

Meat store 0C 0C


23/36



Evaporating pressureregulator Fig. 26

KVP

Evaporating pressure regulator type KVP openswhen pressure rises on its inlet side, i.e. whenpressure in the evaporator rises (increasing load). Turning the regulating screw ( ) clockwisecompresses the spring (5) and increases theopening pressure, i.e. evaporating temperaturerises. The regulator has a bellows ( 0) o the samediameter as the valve plate (2). This means thatpressure variations on the outlet side o theregulator have no e ect on the automaticregulation o the degree o opening since

pressure on the top o the valve plate is balancedby pressure on the bellows. The regulator alsoincorporates a damping device ( ) so thatpressure pulsations in the plant do not a ect the

unction o the regulator.

To make adjustment o the valve easier, it is ttedwith a special pressure gauge connection (9),which makes it possible to t or remove apressure gauge without rst having to empty thesuction line and evaporator.

Check valveFig. 27

NRV

Check valve type NRV is available in straight orangle versions with are as well as solderconnections. Solely the pressure drop controlsthe unction o the valve across it.

NRV straightway version: The valve plate is tted to a brake piston ( ),which is held against the valve seat by a weak

spring (2). When the valve opens, the volumebehind the brake piston becomes smaller. Anequalizing hole (slot) allows the re rigerant toslowly escape to the outlet side o the valve. Inthis way the movement o the piston is broken;an arrangement that makes the check valve wellsuited or lines where pressure pulsations canoccur.


24/36



Key diagram, control currentor re rigeration plant fg. 25 Fig. 28

Thermostat type KP 62 in the +8C room controlssolenoid valve E type EVR in the liquid line whilethe two other thermostats type KP 62 in the +5Cand 0C rooms respectively control motor startersK and K3 type CIT or the evaporator ans, andsolenoid valves K2 and K3 type EVR in the liquidlines.

Combined high and low pressure control type KP5 controls motor starter K4 type CIT or the

compressor motor.

A condition or this unction is that manualswitches S , S2, S3 and S4 must be made.

The compressor motor is thus only indirectlycontrolled by the room thermostats and is able;

or example, to run or some time a ter all thethermostats have cut out.

However, since it is unlikely that all the roomthermostats will cut out at the same time, this

orm o control will result in some a ter-eva-poration, which can be advantageous as regardsliquid hammer in the compressor butdisadvantageous as regards the end o are rigeration period. When a room thermostatcuts out, slight evaporation will still continue andthe charge in the evaporator concerned willbecome smaller. When the room thermostat cutsin again, the e ect o the smaller charge will beto make it more difcult or un-evaporatedre rigerant to enter the suction line during thesudden priming at the beginning o theevaporator-operating period.


25/36



Re rigeration plant orreezer display counter Fig. 29

As this plant operates most o the time at lowevaporating temperatures, interrupted only byautomatic de rosting once or twice every 24hours, it is advantageous to have an electriccompressor motor o a size corresponding tonormal operating conditions, i.e. relatively smallload at low suction pressure.

However, a ter a de rosting this small motorwould be overloaded and there would be a risk o motor burn-out. As a sa eguard against this risk acrankcase pressure regulator type KVL ( ) isinstalled which rst opens when suction pressurein ront o the compressor has been reducedsufciently to avoid overloading the motor.

Regulating system KVR (2) + NRD (3) is used tomaintain a constant and sufciently high

condensing pressure in the receiver on air-cooledcondensers at low ambient temperatures.

During winter operation the ambient tempe-rature ails and with it the condensing pressure o the air-cooled condenser. The KVR regulatesdependent on the inlet pressure and begins tothrottle when the pressure drops below the setvalue. As a consequence, the condenser becomespartly charged with liquid and its e ective area isreduced. In this way the required condensingpressure is re-established.

Since the actual regulating task during winteroperation is to maintain the receiver pressure at a

suitably high level, the KVR is combined with atype NRD di erential pressure valve installed inthe bypass line shown. The NRD begins to openat a di erential pressure o .4 bars. When the

condensing pressure ails, the KVR begins tothrottle. This increases the total pressure dropacross the condenser + KVR. When this pressuredrop reaches .4 bars, the NRD begins to openand thus ensures that the receiver pressure ismaintained.As a rule-o -thumb, it can be assumed that thepressure in the receiver is equal to the pressureset on the KVR minus bar.

During summer operation, when the KVR is ullyopen, the total pressure drop across thecondenser and KVR is less than .4 bars. There orethe NRD remains closed.

The charge can collect in the receiver duringsummer operation. There ore the plant must beequipped with a sufciently large receiver. The

KVR can also be used as a relie valve betweenthe high-pressure side and low pressure side toprotect the high pressure side against too high apressure (sa ety unction).

The pressure-lubricated compressor with oilpump is protected against oil ailure bydi erential pressure control type MP 55 (4). Thecontrol stops the compressor i the di erentialbetween the oil pressure and suction pressure inthe crankcase becomes too low.

A type 077B thermostat is installed in the counter,with its sensor located in the cold room. I thetemperature rises above the set value, a signal

lamp lights up.


26/36



Di erential pressure controlFig. 30

MP 55

Di erential pressure control type MP 55 is used asa sa ety pressure control on pressure-lubricatedre rigeration compressors. A ter a xed timedelay the control stops the compressor in theevent o oil ailure.

The oil pressure element OIL ( ) is connected tothe oil pump outlet and the low-pressureelement LP (2) is connected to the compressorcrankcase. I the di erential between oil pressureand pressure in the crankcase becomes less thanthe value set on the control, current to the timerelay is cut in (contact T - T2 made, see wiringdiagram).

I contact T - T2 remains made or a lengthyperiod because o a all in pressure in relation tothe pressure in the crankcase (suction pressure),the time relay cuts out the control current to thecompressor motor starter (time relay contactchanges over rom A to B, i.e. control current isbroken between L and M).

The minimum permissible di erential pressure,i.e. the minimum oil pressure at which undernormal operation the di erential pressure controlsustains current to the time relay cut o (contact T - T2 broken), can be set on the pressureadjustment disc (3). Clockwise rotation increasesthe di erential, i.e. increases the minimum oilpressure at which the compressor can still run.

The contact di erential is xed at 0.2 bars. There ore, current to the time relay will be rstcut o during start, when the oil pressure is 0.2bars higher than the minimum allowabledi erential pressure. This means that atcompressor start the oil pump must be capableo increasing the oil pressure to 0.2 bars morethan the set minimum permissible oil pressurebe ore the end o the time delay. Contact T - T2 must break so quickly a ter start that the timerelay never reaches its A to B changeover point(break between L and M). See also key diagram,

g. 35.


27/36



Crankcase pressureregulator Fig. 3

KVL

Crankcase pressure regulator type KVL opens onspindle ( ) clockwise tightens the spring (5) and

alling pressure on the outlet side, i.e. on alling

the regulator then begins to regulate at a higherpressure ahead o the compressor. Turning thepressure on the outlet side.

Condensing pressureregulator Fig. 32

KVR

Condensing pressure regulator type KVR openswhen pressure on its inlet side rises, i.e. whencondensing pressure rises. Turning the spindle ( )clockwise tightens the spring (5) and increases

the opening pressure so that the condensingpressure rises.

Like the previously mentioned evaporatingpressure regulator type KVP, all regulators are

tted with a pressure-equalizing bellows ( 0) toeliminate pressure variations on the inlet side o type KVL and the outlet side o type KVL Allregulators are also tted with a damping device

( ) so that pressure pulsations in the plant donot a ect regulator unction.


28/36



Di erential pressure valveFig. 33

NRD

Di erential pressure valve type NRD begins toopen at a pressure drop o .4 bars and is ullyopen at 3 bars. When the valve is installed as abypass, it ensures that the receiver pressure ismaintained.

The contact system in evaporator thermostattype 077B makes on rising temperatures. Turningthe range spindle clockwise increase the cutintemperature o the thermostat, i.e. thetemperature at which the signal lamp lights up.

Evaporator thermostatFig. 34

077B


29/36



Key diagram, re rigeration plant or reezer display counter, fg. 29

Time switch P controls changeover contact t,circuit 2, which makes or breaks control currentto contactors K and K2 type Cl or the respectiveelectric heating elements under the evaporators,and or the evaporator ans. When K2 is cut in, Kis cut out, i.e. the evaporator ans are stoppedduring de rosting. At the same time, motorstarter K3 type CIT or the condenser an is cutout via the auxiliary contact (brake contactbetween 2 and 22) in circuit 4. A signal lamp His switched on via the auxiliary contact (makecontact between 3 and 4) in circuit 6. Whenmotor starter K3 cuts out, the auxiliary contact(make contact between 3 and 4) in circuit 5breaks and motor starter K4 type CIT or thecompressor is cut out. Thus, the compressor alsoremains at a standstill.

Pressure control type KP ( ) is connected so thatit cuts out on rising pressure. This cuts outde rosting when the suction pressure hasincreased to such an extent that there is no more

rost on the evaporator. When contactor K2 is cutout, motor starter K3, and with it motor start K4are cut in via the auxiliary contacts (make contactbetween 2 and 22) in circuit 4 and in circuit 5(make contact between 3 and 4). Assumingswitches S and S2 are made. This starts the condenser an and the compressor.At the same time, signal lamp H is switched o via the make contact between 3 and 4 in circuit6 and signal lamp H2 is switched on via theauxiliary contact (make contact between 3 and

4) in circuit 7. The evaporator ans are starteda ter a period by time switch P cutting incontactor K . During this delay the compressor isable to remove the heat accumulated in the

evaporators while de rosting was taking place,be ore the evaporator ans are started.

Low-pressure control type KP (II) is connectedto control the re rigeration plant during normaloperation. High-pressure control type KP 5 stopsthe compressor but not the condenser an whencondensing pressure becomes excessive.

A thermostat type 077B switches on signal lampH3 i the temperature in the display counterexceeds 8C. The signal lamps are connected toa 2 V battery system so that lamp H3 is able to

unction even i a mains supply ailure occurs.

Fig. 35


30/36



Main wiring diagram orcontactors

Fig. 36

Wiring diagram or contactors K and K2 type Clor the display counter re rigeration plant, g. 29.

For key diagram see g. 35.

The changeover switch or time switch P controlsthe contactors so that one is cut in while theother is cut out. The main contacts -2 and 3-4 incontactor K2 are each connected to an electricheating element. Contactor K has our maincontacts, each o which is connected to a

single-phase an ( -2, 3-4, 5-6, 3- 4).


31/36



Re rigeration plant orventilation air Fig. 37

Continued overleaf...

An electronically controlled suction pressureregulator type KVS ( ) is installed in the suctionline. The electronic regulator receives signals

rom a central control unit, e.g. a PLC, which inturn receives signals rom a temperature sensorlocated in the return air ow rom the room inwhich the ventilation air is to be cooled.

The KVS valve opens i the temperature o thereturn air rises.

I the temperature registered by the sensor rises,the valve opens a little more, and suctionpressure rom the evaporator is increased. At thesame time, the pressure drop across the valve isreduced as a result o reduced evaporation

temperature and increased suction pressure. Thisincreases the capacity o the evaporator andcompressor.

I the temperature registered by the sensor alls,the valve closes a little more, and suctionpressure rom the evaporator is reduced. At thesame time, the pressure drop across the valve isincreased as a result o increased evaporationtemperature and reduced suction pressure. Thisreduces the capacity o the evaporator andcompressor.

As plant such as this must be capable o runningirrespective o load, compressor capacity must beadjustable.

A capacity regulator type KVC (2) is suitable orthis purpose as this regulator is able to preventsuction pressure rom dropping to such an extent

that the compressor is either shut o by the low-pressure cut-out or is subjected to suctionpressure below the acceptable minimum. This isachieved by the KVC valve being set to startopening in order to prevent the above-mentioned limits rom being crossed. This hot-gas bypass trans ers some high-pressure gas romthe pressure side o the plant to the suction side,thus reducing re rigeration capacity.

This type o capacity regulation results in acertain degree o suction gas superheating. As aresult, the temperature o the high-pressure gasincreases, thus increasing the risk that oil in thecompressor pressure valves will become coked. Inorder to prevent this, a thermostatic expansion

valve type T (3) is installed in a bypass betweenthe liquid line and the suction line. The valvesensor is installed in the suction line immediatelyahead o the compressor.

In case o excessive superheating in this region,the valve opens and some liquid is injected intothe suction line. When this liquid evaporates,superheating is reduced and thus also the high-pressure gas temperature.

A solenoid valve type EVR (4) is installedimmediately ahead o the thermostaticexpansion valve (3) in order to prevent liquidre rigerant rom entering the suction line whenthe re rigeration plant is shut down.


32/36



Electronically controlled suction pressureregulator

KVS ( ) is a suction pressure regulator, activatedby a stepper motor. It alters the degree o opening in response to signals rom the EKC 368regulator which transmits pulses that cause thevalve motor to rotate in one direction or theother depending on whether the valve is to beopened more or closed more.

Capacity regulator The capacity regulator type KVC opens in

response to alling pressure on the dischargeside, i.e. alling suction pressure ahead o thecompressor.

Fig. 38

KVS KVC

Fig. 39


33/36




34/36




35/36




36/36

The Dan oss product range or there rigeration and air conditioning industry

Compressors or re rigeration and air conditioning These products include hermetic reciprocating compressors, scroll compressors and an-cooled condensingunits. Typical applications are air conditioning units, water chillers and commercial re rigerationsystems.

Compressors and Condensing Units This part o the range includes hermetic compressors and an-cooled condensing units or householdre rigerators and reezers, and or commercial units such as bottle coolers and drinks dispensers . Wealso o er compressors or heat pumps, and 2 and 24 V compressors or re rigerators and reezers incommercial vehicles and boats.

Appliance controlsFor the regulation o re rigeration appliances and reezers Dan oss supplies a product range o electromechanical thermostats produced according to customer speci cations; electronic temperaturecontrols comprising models with and without displays; service thermostats or servicing on all re rigerating

and reezing appliances.

Re rigeration and air conditioning controlsOur ull product range covers all control, sa ety, system protection and monitoring requirements inmechanically and electronically controlled re rigeration and air conditioning systems. The products areused in countless applications within the commercial and industrial re rigeration and air conditioningsectors.

Danfoss Automation of Commercial Refrigeration Plant

Documents

Transcript of Danfoss Automation of Commercial Refrigeration Plant