WO1988008108A1

WO1988008108A1 - Refrigerated tank container

Info

Publication number: WO1988008108A1
Application number: PCT/GB1988/000295
Authority: WO
Inventors: Robin I'anson
Original assignee: Sea Containers Ltd
Priority date: 1987-04-15
Filing date: 1988-04-15
Publication date: 1988-10-20
Also published as: FI894850A0; JPH02503465A; GB8709096D0; AU1574288A; AU601840B2; EP0380478A1; ZA882672B; DK509389A; BR8807468A; DK509389D0; KR890700794A

Abstract

A control system for controlling the temperature of a cargo in a refrigerated tank container (18) has coupled primary (1) and secondary (2) refrigerant circuits. The first circuit (1) has a compressor (4), an expansion valve (11) and a hot gas injection valve (14) connected between the outlet of the compressor (4) and the outlet of the expansion valve (11) so as to control the temperature of gas emitted from the expansion valve (11). A liquid coolant is provided in the secondary refrigerated circuit (2) to control the temperature of the cargo and an electrical circuit is provided with first and second controllers (23, 24), the first (23) of which is operable in dependence upon the temperature of the cargo to control the temperature of the liquid coolant in the second refrigerant circuit (2) directly and/or by controlling the temperature of the primary refrigerant. The second electrical controller (24) is responsive to the temperature of the liquid in the secondary refrigerant circuit to operate the hot gas injection valve (14). The temperature of the primary and secondary refrigerants is therefore controlled and hence the temperature of the cargo is controlled. Preferably the gas injection valve (14) operates by varying the ON time thereof or alternatively by varying the size of opening of the valve to control the flow of hot gas with the outlet of the expansion valve (11). A digital display of the temperature of the liquid coolant entering the heat exchanger (17) can be fitted to the tank container (18) which operates at a stable temperature within the range of -20°C to +30°C. The control system described conveniently controls the temperature of the cooling fluid in the secondary circuit to an accuracy of plus or minus 0.5°C of the selected operating temperature, whilst the external ambient temperature range can be -25°C to +40°C.

Description

REFRIGERATED TANK CONTAINER

This invention relates to a refrigerated tank contain er.

Refrigerated tank containers have been developed for transporting temperature sensitive cargoes at constant temperatures between -20 °C an +30 ºC. Generally control systems for controlling the temperature of such sensitive cargoes are adjustable both in a deep frozen mode or for close temperature control at temperature above freezing point. For cargo carriage temperatures above 10°C a heater could be fitted. The correct selection of optimum temperatures within a temperature range for a particular cargo ensures the minimum of chemical degradation or bacterial spoilage during transit.

The refrigeration system fitted to the refrigerated tank container is drivable from an external electrical power source and further consists of a primary direct expansion refrigeration circuit operating on refrigerant gas, for example, Freon Gas (R12), (R502) or (R22) linked to a secondary circuit where a coolant mixture of Glycol and water is pumped through heat exhanger panels fitted to the outside of the tank. The two separate modes of operation are controlled electrically by controlling the operation of the liquid coolant through the secondary circuit and an electro mechanical valves, etc, through which the Freon gas passes in the primary circuit.

Refrigerated tank containers have been used to transport temperature sensitive cargoes and the temperature control systems for those containers have proven to be unsatisfactory. Particularly it has been difficult to maintain cargoes at a constant temperature whilst external ambient temperatures are changing. In many cases, the temperature control systems have been unable to cope with change in ambient conditions and the temperature sensitive goods have deteriorated to an extent which has allowed sufficient chemical degradation and bacterial growth to render the cargo unusable.

It is an object of this invention to provide a control system for a refrigerated container by which the temperature within the container is maintained at a substantially constant level.

According to the present invention there is provided a control system for controlling the temperature of a cargo in a refrigerated tank container, the system including coupled primary and secondary refrigerant circuits, the first refrigerant circuit having a compressor, an expansion valve and hot gas injection valve connected between the outlet of the compressor and the outlet of the expansion valve to control the temperature of gas from the expansion valve, the secondary refrigerant circuit having a liquid coolant to control the temperature of the cargo in the container and electrical circuit means comprising first and second controllers, the first controller being operable in dependence upon the temperature of the cargo to control the temperature of the liquid coolant in the secondary refrigerant circuit directly and/or by controlling the temperature of the primary refrigerant, and the second electrical controller is responsive to the temperature of the liquid in the secondary refrigerant circuit to operate the hot gas injection valve and thereby control the temperature of the primary and secondary refrigerants and hence the temperature of the cargo.

In one preferred embodiment of a control system according to the present invention the hot gas injection valve is arranged to operate by varying the ON time of the hot gas injection valve. Alternatively, the hot gas injection valve may be arranged to vary the size of opening of the valve to control the flow of hot gas with the outlet of the expansion valve.

In either embodiment the compressor is conveniently switchable between its ON and OFF states in dependence upon the temperature of the liquid coolant in the secondary refrigerant circuit and/or the temperature of the cargo.

Preferably, the first electrical controller comprises mechanical means arranged to operate first switch contacts when a set minimum temperature of the cargo is reached to effect switching off of the said compressor. The mechanical means is also arranged to close second switch contacts when the cargo reaches a preset minimum temperature below the required cargo temperature to switch on a heater to heat up the liquid coolant in the secondary refrigerant circuit.

The first electrical controller preferably comprises a mechanical chart recorder into which is set the said preset minimum temperature of the cargo. When the temperature of the cargo and liquid coolant have risen by a predetermined amount the heater is switched off and a compressor is started. Conveniently, the second electrical controller comprises a electronic circuit arranged to receive electrical signals from a temperature sensitive probe located in the liquid coolant of the secondary refrigerant circuit. Preferably, a minimum operating temperature is preset in the second controller by operating digital temperature selection switches of the second controller. This preset minimum operating temperature is less than the cargo carriage temperature selected in the first electrical controller. Conveniently, the electronic controller calculates the rate of change of the temperature of the liquid coolant relative to the set point and instructs the not gas valve accordingly. The controller operates the hot gas valve to achieve optimum injection of hot gas to ensure the liquid coolant temperature reaches the set point in the minimum amount of time and with a minimum change in tolerances (plus or minus 0.5°C). Preferably, the second controller is provided with switch contacts to disconnect the compressor from the mains power supply if the temperature of the liquid falls below a present minimum temperature below the set point. Once the temperature of the coolant rises the switch contacts "close" and the compressor restarts. Conveniently, the second controller includes a digital display of the temperature of liquid coolant entering a heat exchanger fitted to the tank container.

Preferably, the refrigerated tank container operates at a stable temperature within the range -20 °C to +30 °C. The external ambient temperature range being -25 ºC to +40 ºC. Conveniently, in the chilled mode of operation the refrigerated tank container operates in the range of -4 ºC to +30ºC and the temperature of the cooling fluid in the secondary circuit is controlled to an accuracy of plus or minus 0.5 ºC of the selected operating temperature.

Conveniently, the gas used in the primary circuit is Freon (R12) whilst the coolant in the secondary circuit is a solution of Glycol and water.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram illustrating primary and secondary circuits of a cooling/heating system;

Fig. 2 is a schematic diagram of electrical circuitry for controlling the cooling/heating system of Fig.1;

Fig. 3 is a schematic view of a practical installation of a refrigeration unit; Fig. 4 is a schematic illustration of part of a temperature chart recorder including temperature controller adjustments.

Fig. 5 is a schematic illustration of a control box including various components of the electrical circuitry of Fig. 2, and

Fig. 6 is a graph illustrating temperature/time characteristic curves for a known control system and a control system according to the present invention.

Referring specifically to Fig. 1 there is shown schematically a diagram of a cooling/heating system having a primary circuit 1 and a secondary circuit 2 coupled together by a heat exchanger 3 in the form of a chiller unit. Primary circuit 1 uses Freon (R12), (R502) or (R22) as the refrigerant gas whilst the secondary circuit 2 utilises a mixture of Glycol and water having a ratio of 60:40, for example 80:20.

The refrigerant gas is pressurised within the primary circuit 1 by a reciprocating compressor 4 having stop valves 5 and 6 on either side thereof and a back pressure regulator 7. High pressure Freon leaving the compressor 4 is passed into a fan driven condenser coil where the gas is condensed to a liquid. The liquid is in turn collected in a receiver 9 which also acts as an accumulator.

The liquid collection in the receiver 9 is then passed through a filter dryer 10 and fed to an expansion valve 11 which is in practice mounted directly on the chiller 3. Conveniently, as the liquid passing to the expansion valve 11 from the filter dryer 10 is diverted through a sight glass 12 and an on load solenoid valve 13.

The primary circuit 1 is completed by a hot gas injection valve connected between the outlet of the compressor 4 and the expansion valve 11. With the valve 14 open hot gas is applied directly from the compressor 4 and injected that is ON/OFF rate, into the cold gas leaving the expansion valve thereby to accurately control the heat exhange temperature and hence the operating temperature of liquid coolant in the secondary circuit 2 as will hereinafter be described.

The secondary circuit 2 comprises a source of liquid coolant comprising a pressure header 15 which feeds a coolant pump 16 which in turn pumps the coolant through the chiller 3. The coolant at reduced temperature is fed from the chiller 3 to a heat exhanger 17 mounted on the refrigerated tank container 18 having a liquid cargo therein such as beer.

Within the chiller 3 the high pressure liquid is expanded to a low pressure and converts back to a gas. In expanding to a gas, the Freon drops in temperature to well below freezing point. At this point the low temperature gas mixed with the required amount of hot gas from the hot gas injection valve, passes through a heat exhanger coil within the chiller unit 3. The heat exchanger coil is surrounded by the coolant in the secondary circuit the temperature of this coolant drops to the required operating temperature. The Freon leaving the chiller unit returns to the compressor 4.

The cold liquid coolant leaving the chiller unit 3 is pumped to the heat exchanger 17. The Glycol/water mixture exhanges heat with the cargo inside the tank 18 to raise or lower the temperature thereof through the heat exchanger 17 and the tank shell. The coolant leaving the heat exchanger 17 is recycled through the chiller unit 3. A heater 19 is provided between the pump 16 and chiller unit 3 to raise the temperature of the liquid coolant should the cargo operating temperature fall below a preset operting temperature. Fig. 1 further illustrates electrical control circuits 20,21 in chain link line. The first control circuit 20 comprises a cargo temperature sensor probe 22 which monitors the temperature of the liquid cargo within the tank container 18 and feeds this signal to a controller 23 which incorporates a mechanical chart recorder. This controller has preset into it both the minimum operating temperature of the cargo and an affect temperature below the line of the required carriage temperature. The controller 23 is in turn connected with the heater 19 and the compressor 4. The signal received from the cargo temperature sensor 22 is arranged to operate a pair of cams (not shown) within the controller and when the required carriage temperature of the cargo is reached the first of these cams operates electrical contacts to effect disconnection of the power supply to the compressor 4. The second cam operates a further pair of electrical contacts when the temperature of the cargo reaches the offset temperature below the required cargo carriage temperature. When these further electrical contacts "close" the heater 19 is switched on to raise the temperature of the liquid coolant in the secondary refrigeration circuit.

When the temperature of both the cargo and liquid coolant has risen a preset amount, both pairs of electrical contacts will "open". The heater 19 is switched off and the compressor 4 restarts.

The second electrical control circuit 21 is provided to ensure that no part of the cargo falls below its minimum carriage temperature and for this purpose the electrical circuit 21 is provided with an electronic controller 24 including electronic circuitry. The controller 24 is connected to receive a signal on a temperature sensor probe 25 which monitors the temperature of the liquid coolant within the second refrigeration circuit 2. The electronic circuitry within the controller 24 calculates the rate of change of the liquid coolant temperature and feeds an appropriate control signal to the hot gas injection valve 14 to open this valve for a preset time. Whilst the valve 14 is open hot gas is injected into the gas expanding from the outlet of the expansion valve 11. The controller 24 thus operates the valve to achieve optimum injection of hot gas to ensure the liquid coolant temperature reaches the set temperature in the minimum amount of time with a minimum variation in tolerances (plus or minus 0.5ºC). If the temperature of the liquid coolant falls to a preset temperature below the minimum operating temperature of the cargo, electrical contacts within the controller 24 "open" to effectively switch off the power supply to the compressor 4. Once the temperature of the liquid coolant rises a preset amount the electrical contacts "close" and the compressor 4 is restarted. The controller 24 conveniently incorporates a digital display showing the temperature of the liquid coolant entering the heat exchanger 17 which is fitted to the tank 18.

The intricacies of the more detailed electrical circuitry, such as the electronic circuit of the controller 24 are well known in the art and since such details forms no part of this invention no specific description of the same is included.

In Fig. 6 the curve shown as a continuous line represents the temperature characteristic curve of known control systems used to control the temperature of a cargo in a tank container. The broken line curve represents the temperature characteristic curve of the control system of the present invention. From these graphs it can be seen that in known systems the minimum operating temperature of the cargo fluctuates above and below the set temperature causing the cargo to partially solidify particularly on the walls of the container, such solidification having a detrimental effect on the heat transfer characteristics, and the quality of the cargo, for example beer.

The electronic controller calculates the rate of change of temperature relative to the set point using proportional, integral and derivative (PID) logic to control the hot gas injection value . The temperature characteristic is a generally hyperbolic curve which after a few cycles settles to a fluctuation which is within the range plus minus 0.5ºC of the set minimum operating temperature of the cargo thus maintaining the optimum heat transfer and quality of the cargo.

A more general electrical circuit nor use in operating the refrigeration system is shown. Such electrical circuitry is operable from any convenient 50/60 Hertz three phase external power supply operating in the range 200 - 260V AC or 400 - 500V AC. With the supply selectro switch 30 in the mode, current flows to the voltage detection board 32 via transformer T2. The voltage selection circuitry on the board 32 selects relevant relay C1 or C2 to either energise the voltage transformer T1 to connect the main current directly to the electronic ciruitry via the closed contacts of relay C2. The voltage detection board detects the voltage of the incoming supply and should this voltage drop below 280V AC a relay on the voltage detector board operates to close low voltage contacts C1 and the low mains voltage is passed to transformer T1 where it is stepped up to 440V AC. The output from the transformer is then fed to phase reversal switch contacts 33 on phase reversal board 34.

When the voltage is greater than 360V AC, the voltage detector board energises dormant and the contactor C1C closes feading the power direct to the phase reversal switch contacts 33.

Simultaneously, the supply voltage is fed to two control transformers T3 and T4 having their primary windings connected in parallel and their secondary windings connected in series. Since the control transformer outlet voltage is directly proportional to the mains supply voltage, the auxiliary contacts operated by the low voltage contactor or high voltage contactor are selected to ensure that 24V AC is supplied to switch contacts 35.

The output from the switch contact 35 is connected to a phase detector board, phase reversal auxiliary locking switches, overload switches connected to the heater, a circulating pump and a condensor fan motor. The overload auxiliary contacts are connected in series and should any overload occur, the control voltage is switched and a red warning light is illuminated. The output from the overload switch is connected to an electronic gas pulse controller, microswitches within a chart recorder, and a coolant circulating pump contact coil.

Phase correction of the three phase electrical supply is effected by a phase correction system comprising an electronic phase rotation/phase loss detector and two mechanically interlocked 25 amp 3- phase contactors C3 and C4. The electronic detector is supplied from the output from the low or high voltage contactors in dependence upon the supply voltage. The phase rotation of the supply is sensed by a resistive/capacitive circuit and in dependence upon the rotation, one line (L1 with respect to L2 for L1, L2, L3 rotation) will have a higher potential than L3. This potential is operative to switch on an optical- diode which energises relay RLA, after suitable modification, and supplies 24V AC to a coil of contactor C4. Contactor BC1 closes allowing the passage of a 3-phase current to the main contactors of the system and simultaneously closes a mechanical interlock preventing contactor C3 from operating.

If the incoming 3-phase supply is connected in the order L1, L3, L2, relay RLB on the phase detector board 33 operates engaging contactor C3 which corrects the phase order and ensures that the fans and circulating pump operate in the correct direction. The phase sensing unit will also respond to any individual phase loss preventing component damage resulting from single phasing.

The refrigerated tank container to which the present invention is applied comprises an elongate cylindrical tank some 20ft (6.09 m) in length and approximately 8ft (2.44 m) in external diameter having rectangular end frames, 8'6" (2.59 m) square welded to it for supporting the tank and for providing standard ISO connectors at each corner of the rectangular frame so that the refrigerated container can be moved from one location to another by standard ISO transportation systems. The refrigeration system including the primary and secondary circuits and the control circuitry thereof is mounted on a platform fixed to the container side along the top of the container. Such refrigeration unit is illustrated schematically in Fig. 3 .

The temperature of the cargo is monitored on a mechanical chart recorder, Fig. 4, which preferably records on a recording disc 40 over a 31 day period and includes a temperature control override adjusting screw 41 for setting the operating temperature of the container and a calibration screw 42 for pointer adjustment. The refrigerated container as indicated above is operative in two modes namely the chilled cargo mode (-4°C to +30°C) in which the system is designed to accurately control the temperature of the Glycol coolant circulating in the secondry circuit. To assist control of the temperature an electronic sensor is mounted to monitor the temperature of the coolant fluid going to the heat exchange panels on the container. As the temperature of the coolant falls and approaches the preselected temperature selected at the controller, hot gas is taken direct from the outlet from the compressor 4, bypassing the condensor coil 8, and injected into the chiller 3 through the electronically operated hot gas injection valve 14. In the chiller 3 the hot gas mixes with the cold gas leaving the expansion valve 11. By varying the rate of the injection of hot gas ,a temperature balance is achieved at the selected temperature. The system is effective to control the temperature of the Glycol coolant to plus or minus 0.5ºC.

When operating in the chilled mode both the primary and secondary circuits 1, 2, are operated continuously. However, in the event of a failure in the hot gas injection system, resulting in a continuing drop in temperature, the temperature of the cargo is measured by the sensor 22 mounted in the tank 8. If the temperature of the cargo drops below a preset value, the controller switches off the compressor 4 preventing damage to the cargo. When the temperature rises a set amount the compressor 4 restarts. Therefore, a safety system is built into the control system.

In the frozen cargo mode (-4°C to -20°C) there is no need for accurate control of the secondary circuit cooling fluid. In this mode the controller monitors the temperature of the cargo through the sensing probe and switches off the compressor 4 when the temperature reaches a preset. The compressor 4 is restarted when the temperature rises above the set point. The cargo temperature therefore cycles through a small temperature range.

When operating in either mode the liquid fluid is pumped continuously through the secondary circuit to ensure an even temperature balance throughout the tank. Turning to Fig. 5 there is illustrated in practical layout for a control box containing many of the standard printed circuit boards and component parts, circuit breakers, transformers, relays etc. generally shown in Fig. 2. The same reference numerals are used in both Figs. 2 and 5 to illustrate the same component parts. The hot gas injection valve 14 has its own independently mounted control box 27 which is programmable to ensure correct temperature operation of the cooling system and automatically adjusts operating time of the injection valve 14 in dependence upon the occurence of any unwanted temperature variation and to correct the cargo temperature accordingly.

Therefore, there has been described a refrigerated tank in which the temperature of temperature sensitive cargoes can be maintained accurately at constant value.

Whilst the invention has been described with reference to a modulated hot gas valve 14 which is operable in either an ON or an OFF mode with the time in each mode varied, the injection valve 14 can be operated by controlling the aperture thereof and hence the flow of hot gas through the valve.

The system may for example be used with ISO tank containers with a capacity of to 21 c.u.m. and to a specification suitable for any liquid cargo approved for carriage in an ISO tank container. Tank containers of either sizes may also be used.

Conveniently, a dual voltage system may be provided to enable use in any country throughout the world, or on ships fitted with a suitable power supply. A diesel generator may be used external to the ISO frame for supplying power when the tank container is transported by road or rail.

The 5 HP refrigeration unit incorporates proven components but maintains accuracy of temperature by the use of an advance monitoring system utilising electrical/electronic components. The electrical circuitry of the individual circuit boards is well known and readily available and therefore have not been described herein in detail. The refrigeration unit is arranged to operate on a range of main/generator supply voltages. The tank is fitted with an automatic voltage sensing system which removes the need to check the correct voltage having been selected before connecting the tank to the supply. The container is also fitted with an automatic selection circuit with ensures the correct rotation of the supply regardless of the power phase rotation.

One particular refrigerated tank container is 20ft long by 8ft 6in in diameter and has a carrying capacity of 20,000 litres at a working pressure of 30ρsi (2 bar). The tare is 3,220 kg. The tank is convenientlyinsulated with polyurethane foam to reduce the heat loss. BTU 4.5kg of Freon are used together with 5 gallons (30 litres) of the Glycol/water coolant mixture. The cooling rate capacity of the refrigeration unit when the unit operates at 60 Hertz at +15 °C is 32,500 Btu's (8188kcal/h) and at -20°C 12,000 Btu/h (3025kcal/h) .

The optional heating unit preferably comprises a 3KW heater operating on 440V AC to produce 12,000 Btu's (including the heat gain from the circulatory pump.)

The mechanical chart recorder preferably has a clock work mode drive and operates in the range -25ºC to +50 ºC. However, an electric motor driven chart recorder can be used.

Claims

CLAIMS :

1. A control system for controlling the temperature of a cargo in a refrigerated tank container, the system including coupled primary and secondary refrigerant circuits, the first refrigerant circuit having a compressor, an expension valve and a hot gas injection valve connected between the outlet of the compressor and the outlet of the expansion valve to control the temperature of the gas from the expansion valve, the secondary refrigerated circuit having a liquid coolant to control the temperature of the cargo in the container and electrical circuit means comprising first and second controllers, the first controller being operable in dependence upon the temperature of the cargo to control the temperature of the liquid coolant in the second refrigerant circuit directly and/or by controlling the temperature of the primary refrigerant, and the second electrical controller being responsive to the temperature of the liquid in the secondary refrigerant circuit to operate the hot gas injection valve and thereby control the temperature of the primary and secondary refrigerants and hence the temperature of the cargo.

2. A system as claimed in claim 1, wherein the hot gas injection valve is arranged to operate by varying the ON time of the hot gas injection valve.

3. A system as claimed in claim 1, wherein the hot gas injection valve is arranged to vary the size of opening of the valve to control the flew of hot gas with the outlet of the expansion valve.

4. A system as claimed in any one of the preceding claims, wherein the compressor is switchable between its ON and OFF states in dependence upon the temperature of the liquid coolant in the secondary refrigerant circuit and/or the temperature of the cargo.

5. A system as claimed in any one of the preceding claims, wherein the first electrical controller comprises mechanical means arranged to operate first switch contacts when a set minimum temperature of the cargo is reached to effect switching off of the said compressor.

6. A system as claimed in claim 5, wherein the mechanical means includes second switch contacts which are arranged to close when the cargo reaches a preset minimum temperature below the required cargo temperature to switch on a heater to heat up the liquid coolant in the secondary refrigerant circuit.

7. A system as claimed in any one of the preceding claims, wherein the electronic controller is arranged to calculate the rate of change of the temperature of the liquid coolant relative to the set point and instructs the hot gas valve accordingly.

8. A system as claimed in any one of the preceding claims, wherein the second controller has switch contacts arranged to disconnect the compressor from the main power supply if the temperature of the liquid falls below a preset minimum temperature below the set point.

9. A system as claimed in claim 8, wherein the second controller includes a digital display of the temperature liquid coolant entering the heat exchanger fitted to the tank container.

10. A system as claimed in any one of the preceding claims, wherein the refrigerated tank container operates at a stable temperature within the range -20°C to +30°C.

11. A system as claimed in any one of the preceding claims, wherein the temperature of the cooling fluid in the secondary circuit is controlled to an accuracy of + or - 0.5ºC of the selected operating temperature.