Carrier 16JT Series Start Up & Operation Manual

Carrier 16JT Series Start Up & Operation Manual Manual pdf 7 page image

During high load operation, some abnormal conditions cancause the lithium bromide concentration to increase abovenormal, with the strong solution concentration close to crys-tallization (see Equilibrium Diagram and Chiller SolutionCycle.) If, for some reason, the machine controls do not pre-vent strong solution crystallization during abnormal operat-ing conditions and flow blockage does occur, the strong-solution overflow pipe will reverse or limit the crystallizationuntil the cause can be corrected. The overflow pipe is lo-cated between the low-temperature generator discharge boxand the absorber, bypassing the heat exchanger, as shown inFig. 5.If crystallization occurs, it generally takes place in the shellside of the low-temperature heat exchanger, blocking the flowof strong solution from the generator. The strong solutionthen backs up in the discharge box and spills over into theoverflow pipe, which returns it directly to the absorber sump.The solution pump then returns the hot solution through theheat exchanger tubes, automatically heating and decrystal-lizing the shell side.Equilibrium Diagram and Chiller SolutionCycle — The solution cycle can be illustrated by plottingit on a basic equilibrium diagram for lithium bromide in so-lution with water (Fig. 7). The diagram is also used for per-formance analyses and troubleshooting.The left scale on the diagram indicates solution and watervapor pressures at equilibrium conditions. The right scaleindicates the corresponding saturation (boiling or condens-ing) temperatures for both the refrigerant (water) and thesolution.The bottom scale represents solution concentration, ex-pressed as percentage of lithium bromide by weight in so-lution with water. For example, a lithium bromide concen-tration of 60% means 60% lithium bromide and 40% waterby weight.The curved lines running diagonally left to right are so-lution temperature lines (not to be confused with the hori-zontal saturation temperature lines). The single curved linebeginning at the lower right represents the crystallization line.The solution becomes saturated at any combination of tem-perature and concentration to the right of this line, and itwill begin to crystallize (solidify) and restrict flow.The slightly sloped lines extending from the bottom of thediagram are solution-specific gravity lines. The concentra-tion of a lithium bromide solution sample can be determinedby measuring its specific gravity with a hydrometer and read-ing its solution temperature. Then, plot the intersection pointfor these 2 values and read straight down to the percent lithiumbromide scale. The corresponding vapor pressure can alsobe determined by reading the scale straight to the left of thepoint, and its saturation temperature can be read on the scaleto the right.PLOTTING THE SOLUTION CYCLE — An absorption so-lution cycle at typical full load conditions is plotted inFig. 7 from Points 1 through 13. The corresponding valuesfor these typical points are listed in Table 2. Note thatthese values will vary with different loads and operatingconditions.Point 1 represents the strong solution in the absorber, as itbegins to absorb water vapor after being sprayed from theabsorber nozzles. This condition is internal and cannot bemeasured.Point 2 represents the diluted (weak) solution after it leavesthe absorber and before it enters the low-temperature heatexchanger. This includes its flow through the solution pump.This point can be measured with a solution sample from thepump discharge.Point 3 represents the weak solution leaving the low-temperature heat exchanger. It is at the same concentrationas Point 2, but at a higher temperature after gainingheat from the strong solution. This temperature can bemeasured.Point 4 represents the weak solution leaving the drain heatexchanger. It is at the same concentration as Point 3, but ata higher temperature after gaining heat from the steam con-densate. This temperature can be measured. At this point theweak solution first flows through the level control device (LCD)valve and then it is split, with approximately half going tothe low-stage generator, and the rest going on to the high-temperature heat exchanger.Point 5 represents the weak solution in the low-stage gen-erator after being preheated to the boiling temperature. Thesolution will boil at temperatures and concentrations corre-sponding to a saturation temperature established by the va-por condensing temperature in the condenser. This conditionis internal and cannot be measured.Point 6 represents the weak solution leaving the high-temperature heat exchanger and entering the high-stage gen-erator. It is at the same concentration as Point 4 but at a highertemperature after gaining heat from the strong solution. Thistemperature can be measured.Point 7 represents the weak solution in the high-stage gen-erator after being preheated to the boiling temperature. Thesolution will boil at temperatures and concentrations corre-sponding to a saturation temperature established by the va-por condensing temperature in the low-stage generator tubes.This condition is internal and cannot be measured.Point 8 represents the strong solution leaving the high-stagegenerator and entering the high-temperature heat exchangerafter being reconcentrated by boiling out refrigerant. It canbe plotted approximately by measuring the temperatures ofthe leaving strong solution and the condensed vapor leavingthe low-stage generator tubes (saturation temperature). Thiscondition cannot be measured accurately.Point 9 represents the strong solution from the high-temperatureheat exchanger as it flows between the two heat exchangers.It is the same concentration as Point 8 but at a cooler tem-perature after giving up heat to the weak solution. The tem-perature can be measured on those models which have sepa-rate solution heat exchangers.LCD — Level Control DeviceTC — Temperature Control (Capacity Control)Fig. 6 — Typical Flow Circuits, (Simplified)Arrangement Shown for 16JT810-8807