Precise media temperature control

Sunday, December 19, 2010

Precise refrigeration and air conditioning is essential for many applications and demand is constantly increasing. One example is the requirement to keep a temperature of exactly 20°C in a measuring room where machine components are measured to fractions of a millimetre. Here it is important to eliminate even the smallest temperature changes so that the measuring results are not affected. However, applications in the food industry are also possible. In applications such as this, constant nozzle air temperature from evaporators is the key to success. This can be easily achieved with electronic suction throttle regulation or media temperature control.

Suction line installation

In the case of a media temperature control for refrigerating plants with a pipe size of up to 54 mm, an electrically controlled valve (e.g. type "KVS") is fitted in the suction line. This valve is equipped with a stepper motor and is controlled by an electronic controller (e.g. "EKC 368").

A fast-reacting temperature sensor (e.g. "AKS 11" or "21") is fitted as the probe. This sensor measures the actual value of the outlet air temperature of the evaporator and feeds this back to the controller. The controller now compares its temperature set point with the actual value and decides whether the stepper motor valve should be opened further or closed. A completely open valve ensures maximum refrigeration and therefore the lowest possible outlet temperature in the actual load situation. A valve that is almost closed creates a high outlet temperature. Therefore, the control gives the valve the command "open valve" in situations where the reference value is far lower than the actual value. For example, this would be the case if the reference value were 0°C and the actual value 15°C. Even with a temperature difference of just 1 K, the control forces a small opening to prevent the control from overshooting the set point. With a system such as this, temperature fluctuations of maximum +/- 0.5 K are to be expected.

Different transient processes in the start-up phase

Special attention must be paid to the start-up phase of regulation. The "EKC 368" control has three different transient processes available. The first option is the fastest possible refrigeration. In the case of this setting, a very clear undershooting of the outlet temperature of the evaporator is to be expected to facilitate fast refrigeration and ensure the reference temperature is quickly achieved. The second option only allows slight undershooting. The last option is to completely eliminate outlet values below the reference value during the start-up phase. The advantage of this is very gentle cooling of unpackaged goods, in which any unnecessary dehumidification is avoided. However, in this case, more time must be planned for achieving the required reference value.

Operation

The controller is operated using two push buttons. The controller can be completely programmed using these two buttons combined with a three-digit display where all important data is displayed. This means that any fitter at the plant can intervene in the control circuit and view relevant data. In the control menu, not only are basic adjustable values displayed, but it is also possible to precisely influence certain events by adjusting stability and amplification factors. 

Service

The service menu of the electronic controller is of particular interest to the fitter upon commissioning or servicing of the plant. All parameter values that begin with "u" display actual plant values which are important for all types of troubleshooting and for evaluating the state of the plant. These values provide information about the state of the plant. On the one hand, they can be read quickly, which saves the effort of measuring with the temperature gauge. And on the other hand, you can see immediately which values the control takes for granted. It is therefore standard procedure for an experienced fitter to check the control sensor and, if necessary, the defrost sensor in electronic systems before the actual commissioning (this is very easy to do for standard resistance sensors using an Ohm measuring device. At 0°C a "PT1000" sensor has a resistance of 1000 Ohm) to prevent lengthy troubleshooting of actual values incorrectly measured by the sensor. This procedure can be avoided by looking at the service menu; here it can be seen immediately (in case of doubt, double check with the thermometer) whether the value is realistic. 

Continuous electronic valves

The "KVS" is a continuous valve with which even the smallest fluctuations in evaporation pressure can be avoided. The "EKC 368" can be implemented as P, PI or PID control. P control involves standard regulation according to the deviation (example: if the outlet air temperature is too high, the degree of opening of the valve is always increased at the same speed). 

In the case of PI regulation, the "reset time" (I component) can be changed individually, which simultaneously leads to the adaptation of the reaction speed of control. In other words: the regulation either becomes faster or slower. Either can be necessary. The "D component" in PID control also optimises the control characteristics in the case of a sudden change to the reference value. This control mode is particularly advisable if the system is operated with external reference value changes, e.g. by master control. 

"KVS" actuator

Humidity 

Refrigeration specialists know that the topic of "humidity" plays an important role in refrigeration, especially where unpackaged goods, meat, vegetables and fruit are concerned.

High air humidity 

The constantly high outlet temperature of the "EKC-KVS" system means any undesirable humidity can be avoided. However, there can also be situations in refrigeration plants where humidity is required. For this purpose, indirect measures are normally taken, such as changing the evaporator fan levels or the evaporator speed (i.e. slower fan speed = lower evaporator temperature = dehumidification and vice versa). 

This point can be directly influenced with an electronic suction throttle system: simply change the reference value for the air outlet temperature with an external signal from 0 to 10 V and suddenly at high evaporation temperature values no or almost no dehumidification can be achieved, or a high level of dehumidification with low evaporation. Of course, it is also the case here that the dew point for dehumidification must always be undershot. 

It is always easy to readjust a system like this through the eliminated condensate quantity at the evaporator. A system like this is suitable for ventilation and comfort air conditioning plants and conditioning cabinets, as well as for fruit and vegetable storage.

Defrosting

The "EKC 368" also offers the option of defrosting regulation by external initiation through an input contact. After defrosting has been initiated, its standard procedure must be defined by setting the control appropriately. In the case of hot gas defrosting, the "KVS" valve closes during the defrosting procedure. After defrosting, the "KVS" valve does not open abruptly - somewhat like a standard solenoid valve. This is an advantage, particularly where the release of high pressure (and, if necessary, liquid) in the evaporator are concerned. Furthermore, it is not necessary to fit an additional valve in the suction line since the "KVS" can also be used for this shut-off function. 

In the case of electric defrosting, the "KVS" is opened. A defrost sensor in the evaporator package ends the defrosting process as soon as the temperature of the evaporator package has reached the temperature to which the sensor is set. For safety, a maximum defrost duration can also be set in the refrigeration controller, which ends the defrosting process in the case of a defective defrost sensor or a similar fault. After defrosting, the drip-off time begins. Once these processes have finished, the refrigeration process is restarted. In simple terms, the purpose of the drip-off time is to drain off ice that has turned into water on the evaporator via the condensate drain. 

Solution for large plants

If connection sizes larger than 54 mm are required for the suction line, it is recommended that you implement a solution with a main valve. This system consists of a main valve "ICS", a screwed on pilot valve "CVQ" and the corresponding control "EKC 361" (without defrost function - the alternative "EKC 367" is equipped with practically the same defrost function as "EKC 368").

Here, the main valve with pilot is also fitted in the suction line. The control effect and the regulation function correspond to the "KVS-EKC 368" system. However, with "CVQ" an actuator is used to operate the valve instead of a stepper motor. In this actuator a "PTC" heating resistor and an "NTC" resistor are fitted. The "PTC" can be controlled by the control with 24 V AC so that the actuator is heated. This heating closes and opens the valve. The "NTC" resistor sends the control confirmation of the degree of opening of the valve via the inner actuator temperature. The actual value of this actuator temperature and the current reference value can be read in the control menu of "EKC 361" ("367"). This is a good troubleshooting aid. 

If the reference value (current actuator reference) is 100°C, for example, and the actual value is the same (reference value/actual value overlap), the control action appears to be fine at first glance. If the current reference value is 100°C and the actual value is e.g. 50°C, without increasing any further, there is an error in the control or the actuator. In this case, you must check whether the control output for the heating "PTC" is correctly connected to the "CVQ" and is outputting 24 V AC. 

Remote service 

Remote service is also possible for this control system. It is possible to equip the control with a LON module and record the appropriate data using a master unit and to intervene in the control system remotely using a modem connection. This can be done using special software (type "Danfoss AKM"). This means that this media temperature control can be integrated in complex Danfoss control system networks.