Jim Grant Forum Moderator 120 Posts
Re: Hot glycol circulation15 January 2007 at 1:29pmThe best approach to this problem is to evaluate the hydraulics and mechanical refrigeration systems independently through the use of pressure gauges, thermocouples and flow meters.
If the mechanical refrigeration system can be analyzed with a thermodynamic diagram, strategically placed thermocouples and a compressor curve then any problems will be identified. A common problem with these units is failure to achieve the desired duty at the design temperature. A 200-ton refrigeration unit may achieve 200 tons at a temperature 5ºC above design basis, but only 180 tons at design temperature and is indicative of a capacity limitation. The hydraulics of the glycol system should be analyzed by evaluating the pumping capacity, line sizes and equivalent length of the system.
The effect of an increase in circulation of hot glycol can only be determined by doing the specific calculations for the heat transfer in the exchanger of concern. Much of the effect will depend on the ratio of hot-to-cold specific heats and flow rates as well as the distribution of heat transfer resistances for the exchanger. Increasing the turbulence (i.e. Reynolds number) will increase the film heat transfer coefficient on the glycol side only if it is in transitional or turbulent flow already. If the glycol side represents most of the heat transfer resistance then this will have a significant effect on the performance of the unit. If, however, the glycol side represents a small portion of the heat transfer resistance then there will be very little benefit achieved by the higher film heat transfer coefficient.
The ethylene glycol refrigeration system seems to be a bottleneck to production. What characteristics would indicate that the problem is the hydraulic system versus the mechanical refrigeration system? Also how would I determine what incremental benefit I would gain by greater hot glycol circulation, increasing turbulence but lowering the LMDT?
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