Refrigeration, air-conditioning and cooling technology - 2007.pdf

PLH_KKK_32_85.QXP 25.05.2007 10:11 Uhr Seite 47Observation of constraints forpumps and refrigerating machinesThere are physical limits for all technical devices. The refrigeratingmachines require a volume throughput to prevent icing. If the watervolume which is required for the smallest control level of the vaporiserdoesn't flow, mechanical damage will result without a safetyshutdown.Minimum run-time of refrigeration generators and buffer modeWhen operating with constant water flow, safeoperation is possible when the switch-on/offcycles alternate as seldom as possible. Thismeans that the circulated volume must havesuch a high storage capacity that the minimumrunning time of the cooling generator is exceeded.From experience, one knows that 90 % ofsystems aren't suitable for this without takingadditional measures.Derivation of the specific factor 14.34Q w [kW]m·w =Cpw 4.2KJS· 3600shKJ· K [∆tw]kg · kBufferKJ · kg · K · 3600 s·mw= = 857s · 4.2 KJ · h · Kkgh860kghThe objective is to guarantee economic efficiencyand operating safety, long switch-on/off cycles,and therefore to get long running/idle timesfor the cold-water generator and the hydraulicdecoupling from the cold-water generator andconsumer system. This is possible with a hydraulicdecoupler.This efficiency is enhanced by nozzle pipes andlayering sheets in the tank. The size of the hydraulicdecoupler as a buffer is to be determinedas follows:L 860 kg hkg LFactor = = = 14.34 =min h · 60 min min minWhen there's a change in the specific heat capacity,the specific factor is also to be redetermined.Hydraulic decoupler circuit as bufferSi =kW · F tl · 14.34 · min∆twCapacitorsVaporiserThe minimum system content (Si) depends onkW Nominal cooling capacityF tl Partial load factor for multistagecooling generatorsmin Minimum running time∆tw Temperature differenceCpw Spec. heat capacityWilo Planning Guide - Refrigeration, air-conditioning and cooling technology 02/2007 47