Controlling beyond the compressor In addition to monitoring the operating conditions of the compressor, intelligent modules may also be able to provide control of the compressor’s peripheral devices. For example, liquid injection can be applied when the discharge temperature is too high, unloaders can be cycled according to an analog input signal, compressor starting contactors can be staged as needed, the head cooling fan can be turned on only when needed, and the crankcase can be cycled with compressor operation without the need for a normally closed auxiliary contact on the compressor contactor.
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One approach is to establish five operating envelopes. One envelope represents the standard operating envelope for the compressor. Then there are two smaller envelopes that establish the warning and warning reset levels. Then, two larger envelopes establish the fault and the critical limit.
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In the fault and critical limit envelopes, the fault is followed by a timed shutdown. Therefore, if the condition persists, the compressor will be taken offline after the time delay has expired. 407C
PROVIDING INFORMATION TO A BIGGER SYSTEM As the module monitors the operation of the compressor, it may record the pressures, temperatures, load-steps and other information. This can prove very helpful when diagnosing system problems. Additionally, it may even be possible to share these conditions with a central control system for future use. Most parallel compressor systems installed in supermarkets already have this ability. However, it is likely that they are not able to protect individual compressors according to their intended operating envelope. In this sense, distributed compressor protection modules can provide important compressor-level information to the main control system.
427A
407A
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In order to facilitate the transfer of data, intelligent modules typically use a Modbus interface to connect with a master controller or two-wire RS485 communication port. M e c h a n i c a l
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