Energy Optimisation in Industrial Refrigeration Systems
New measurement method solves the problem of a high pressure drop in risers and wet suction lines in refrigeration systems
In low-temperature ammonia systems the pressure loss in the return line to the compressor is significant for the capacity of the compressor and the efficiency of the system. The desigh of the return line whether wet or dry will have to be carried out give the minnimum pressure loss.
HB Products A/S, Danish Technological Institute (DTI), and Claus Sørensen in Vejle initiated a test to minimise the pressure drop in the riser pipes of a tunnel freezer by measuring the liquid / gas ratio at the exit of four evaporators. Riser pipes are found at the outlet of an evaporator where the pipeline rises up. Riser pipes usually run from a production location to the roof of the building and can vary in height from 2 to 8m. The refrigerant or liquid flow to the evaporator is controlled by a circulation pump, circulating more refrigerant than it evaporates.
The excess liquid must be lifted up by the gas velocity as a two-phase flow, up to 8m in height. In a riser, the exhaust gas velocity should be high in order to lift up the liquid. It is often associated with a large pressure drop that must be compensated by the compressor, creating increased energy consumption:
- Tests at DTI have shown that a pressure drop of 0.3bar, at an evaporation temperature of -40ᵒC, increases energy consumption by 25%.
A new sensor technology from HB Products makes it possible to measure the liquid gas in percent, and thereby control the pump or valve, in order to minimise the re-circulation and only supply the amount of refrigerant that evaporates.
Optimal efficiency is achieved by a circulation number of about 1.1, i.e. to add max 10% more refrigerant than evaporated.
Claus Sorensen A/S operates cold stores and freezer plants, which are strategically well-placed in relation to food production in Denmark as well as the country’s leading fishing ports. The capacity totals more than 800,000 cubic meters, with a freezing capacity of 1,800 tons a day.
The sensor measures the degree of dryness “X” of the gas in the gas pipes, and the value is converted into a 4-20 mA analogue signal corresponding to “X”. The sensor is available in two variants, "Rod Style” for installation in a pipe elbow and “In-line” for welding into the suction pipe.The sensor offers very precise measurements and provides instant read-outs of the gas quality, making it useable in most evaporators (plate, pipe, and air evaporators). The sensor’s microprocessor also functions as a controller, enabling the direct control of a modulating motor valve, thereby bypassing the external controller or the PLC. The controller can be set up with all the parameters that are necessary for regulating a modulating motor valve or a stepper motor valve. The sensor is available with a cable for direct supply to and control of the motor valve, or as a sensor where the signal is sent to the external controller/PLC.
HBDX sensors can be used to optimize and control a lot of processes in a refrigeration system, such as dry expansion (DX) control, draining of the evaporator during defrosting, control liquid circulation in overfeed evaporators, and draining and control of the condensation process in the condenser.