Digital Controls

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Digital control systems are available for all Lintern units

The traditional mechanical-electric control approach is to use a variable operated switch (pressure switch, temperature switch, etc.) on each control set point on which the unit or certain function is needed to be turned on or off. The number of switches will be equal to the number of control points. All the switches have to be physically adjusted individually.

The digital control approach uses a central processing unit controller (CPU) to process signal inputs from sensors such as pressure transducers and temperature sensors and control associated motors, heaters, valves etc. Unlike the mechanical-electric control approach, the digital controller first takes the signals from the sensors and processes them in the CPU according to the logic determined by the program before issuing the control output. The logic is programmed to suit the application. The digital control approach is easily programmed to respond to a calculated variable, e.g., reacting at certain superheat or subcool while the mechanical-electrical control can only respond to a directly measurable parameter.

As previously discussed, when using mechanical-electric controls, the control functions are accomplished by a series of switches. Thus for a complex control system, the hardware becomes very complicated and difficult to understand and troubleshoot. As standard Lintern units read Low-Pressure, High-Pressure, Return Air Temperature, Differential-Pressure and Overload, electric-mechanical control panels are large and contain many individual components and associated wiring. Conversely, when utilizing a digital control approach, functionality is defined by the program and the hardware contains far less individual components and wiring. This simplifies the unit structure, makes control options very flexible and minimizes the need to change the physical components for different control requirements. Generally speaking, the more complicated the control function required, the more advantage can be gained using digital control.

Another important application of digital control is for special requirements such as wide range capacity control utilizing Lintern’s Variable Frequency Drive technology where a central condenser is driving multi-evaporators for different control spaces etc. In those cases, the only choice is to utilize digital controls.

The majority of customer equipment Lintern cools today is computerized and automated. There are increasing demands for the communications between customers’ central controllers and air condition systems. The digital control method inherently possesses the capability to interface with the customer’s controllers so that the air conditioning unit can be monitored, set or controlled as a subsystem of our customer’s controller. This advantage is becoming an important reason for customers to select an air condition system with a digital controller.

Features of Digital Control Systems:

The digital control systems are available for all standard Lintern units.

To utilize the advantage of the digital control approach compared with mechanical-electric control, the following changes are made on the control sequences to improve unit performance.

  1. Two consecutive Oil Failures (OF) are required to cause a permanent OF trip. This is very helpful for avoiding OF tripping at start up.
  2. A permanent Low-Pressure (LP) trip will occur only if 14 LP trips occur within one hour. This will prevent the compressor from endless short cycling in the case of a low refrigerant charge.
  3. The Unload Control is controlled by both indoor temperature and low pressure. Before the room temperature gets low enough to shut down a compressor, it first unloads (low load control) the compressor. And before evaporator pressure reaches freezing pressure it also unloads (low ambient control) the compressor. Only when the above two conditions both get corrected does the compressor go to full load. This considerably reduces the number of times the air conditioner short cycles therefore, reducing the chance of OF and fuse trip.
  4. A reduction in the starting load is accomplished by the unload controls starting the system at 50%.
  5. To prevent short cycling, a two-minute timer must expire before the system will restart.

The digitally controlled Lintern air conditioning system is designed to operate independently from a computer. It is however, capable of being fully monitored, recorded, and modified remotely by a computer when appropriately interfaced.

Operation history is recorded by the controller. Up to 400 data points for each analog variable and 150 data points for each binary variables are stored in the built-in memory of the controller for later analysis. This data is very valuable for troubleshooting and maintenance planning.

Limitation of the Current Control System:

Due to the temperature limitation of the selected control board, if the OAT is higher than 120 F, the controller shall be located inside the controlled.