Capacity Control in Absorption System

Capacity Control in Absorption System Points : capacity control in absorption system, write a brief note on capacity control in absorption system Capacity Control The true meaning of capacity control is capacity reduction, since operation with out control yield maximum refrigeration capacity. The need for capacity control irises when the refrigerant load drops off as Reflected in a reduction in chilled —water temperature returning to the absolute unit (assuming a constant rate flow of chilled water) with no Capacity control the temperature of the chilled water leaving the evaporator would decrease as would the pressure on the low pressure side of the absorption unit. The low pressure could reduce to the point where the refrigerant water would freeze.

Most control system on absorption unit to regulate constant temperature of chilled water leaving the evaporator. At less than full refrigeration loads, then the refrigeration capacity of the absorption unit must be decreased several methods available to achieve this reduction, but the. Net effect of them all is to reduce the flow rate refrigerant water at position 5, 6 and 7 is figure given below. The three methods for reducing the refrigerant water flow are

(1) Reducing the flow rate delivered by the pump 3 positions,
(2) Reducing the generator temperature.
(3) Increasing the condensing temperature.

METHOD (I) If the mass rate of flow delivered by the pump in the system given above were reduced form 0.6 to 0.4 kg/s. the rate of refrigerant flow through the condenser and evaporator would also be reduce by one- third resulting in a corresponding reduction in refrigerating capacity. This method is efficient since the same proportion as the refrigerating capacity reduces the rate of heat addition at the generator.

The statement that the mass flow that of refrigerant changes in the flow rate delivered by the pump is correct provided that the concentration of solution remain unchanged. The concentrations remain UN-change only if the, operating temperature in the component also remains fixed, and such is not the usual situation. In the condenser for example, if the supply temperature and flow rate of condenser cooling water remains constant, the reduction in flow rate of refrigerant being condensed causes the condensing temperature to drop similarly in the generator if the supply condition of steam or hot water remains constant, the generator temperature increases.

These Particular changes in temperatures result in higher LiBr concentrations leaving the generator at point which result in higher cop but they also are the conditions that could induce crystallization. On some commercial absorption units the adjustment flow, rate is the primary method of capacity control but is combined with method 2 or method 3 when is danger of crystallization. METHOD (2) A reduction in the generator temperature will reduce the refrigerant capacity and can be achieved by throttling the pressure ‘of the steam entering the generator or reducing the flow rate of hot water depending upon which is the heat source of the generator.

The change at one component is likely to affect the conditions at other components with change generator temperature, the flow rate of refrigerant water passing through the condenser and evaporator, drops, so with a constant flow rate and entering temperature of condenser cooling water the condensing temperature will drop. Similarly with a constants flow irate and entering temperature of water to be chilled, the evaporator temperature will increase. The changes in those-two temperatures will affect the high and low-side pressure, respectively and thus influence the concentrations leaving the absorber and generator. METHOD (3) A further method for reducing the refrigerating capacity of an absorption unit is to increase the Condensing temperature this can be done conveniently by increasing the temperature of cooling water supplied to the Condenser. Which in turn can be achieved by passing a fraction of the water around the cooling tower? The affect on the cycle performance of increasing the condensing temperature is the same as that of decreasing the generator temperature namely reducing the Liar. Concentration of solution returning from the generator to the absorber for a given rate of flow of solution handled by the pump and the flow rate of refrigerant circulation to the Condenser and evaporator decreases. Crystallization This ‘is an important phenomenon that needs to be understood If the lithium bromide solution becomes too concentrated it changes from a liquid to a solid (crystal) form. The maximum concentration possible is decreases as the solution temperature decreases Therefore, if a solution is already near its maximum concentrations and its temperature is then lowered it will crystallize (solidify). This is a serious problem in absorption system because if it occurs the crystallize LiBr blocks the piping and the machine stops working. There are three factors that can result in a drop in temperature of Solution.
(1) Power failure
(2) Condensing water temperature too low
(3) Air leakage in to the system 1. Power failure If the electric power is interrupted, the system ceases to operate. The temperature of the Concentrated solution in the heat exchanger starts to drop and may fall below the crystallization line. 2. Condenser water temperature too low. When the system is operated at full loads it’ the temperature of the cooling water is too low, the diluted solution temperature may fall low enough to reduce the temperature of the concentrated solution to the point of crystallization. 3. Air leakage into the system Air leaks into the system raise the evaporating temperature and the chilled water leaving temperature. A higher temperature increases the heat input and the solution concentration to the point of crystallization Manufactures have developed several devices to minimize the possibility of crystallization. One such device uses a by-bass valve to permit refrigerant to flow to the concentrated solution line when conditions that can causes crystallization are detected by the sensor: Procedure of De crystallization REMEDY Should crystallization occur, proceed as outlined below to de-crystallize the machine and put it in proper operating condition. Remove the chilled water control bulb, the lo refrigerant control bulb from the well to prevent damage, remove all thermostats with scale less than 225° F.

A. Stop the condensing water pump an tower fans. Keep the chilled water, solution and refrigerant pumps operating.
B. Open the refrigerant dump valve. This allows the refrigerant to dilute the solution when the refrigerant, pump -less suction, place service switch in the service position. This operation may need repeating in service cases.
C. In severe, cases of crystallization it will be necessary to apply external heat to the heat exchanger and / or the concentrated solution line. This heat may be from and external steam source or a gas torch.
When heat is applied externally by flame, case must be taken not to overheat any one area. Apply the heat evenly over a large area. Over heating results in blistering of the paint and indicates lack of solution inside.
D. Parts of the concentrated solution lines will undoubtedly be found to be crystallized. This is particularly true of the solution line for the generator to the heat exchanger and the solution line form tile heat exchanger to the educator.
E. Mark the position of the steam valve limit control. Place the service switch into service position to open the steam valve and operate the solution pump to gradually rise solution temperature to a maintained solution temperature of 200 to 215°F.
The hot solution will circulate between the absorber and generator through the heat exchanger tubes, and will return from the generator to the absorber.
To speed up de-crystallization utilize the solution pump as describe below.
When dumping the hot solution from the generator into the solution return line, stop the solution pump to that, this hot solution can be utilized in breaking up the crystals. After the solution has drained into the heat exchanger, start the solution pump. This procedure may have to be repeated several times.
F. Gradually cool machine by intermittent operation of condenser pump until solution temperature has reached 150°F then restart the cooling tower pump and fan Make certain that refrigerant dump valve is closed. Reset the steam valve to the original position. Replace control bulb and any thermometers that may have been removed.