Capacity Control of Absorption Chillers

Absorption chiller

Capacity of absorption chillers needs to be adjusted with respect to load on HVAC water chillers.Capacity control of HVAC chillers is very logical and required to save the input energy at low or part-load on chillers.

Chillers providing absorption cooling use different types of input energy.Absorption chillers are generally controlled from input energy being provided to  generator(single effect absorption chillers) or high pressure generator (double effect absorption chillers). 

How capacity of absorption Chillers is controlled ?

Capacity of absorption chillers is controlled by varying the input thermal energy to generator.While input thermal energy to absorption chillers is regulated with reference to load on chillers .Suppose load on absorption chillers changes then the outlet temperature of chilled water will also be changed little and this is generally sensed by chiller instrumentation.Chiller controller now regulates the valve of thermal energy with respect to the change in thermal load.

Capacity Modulation of Absorption Chiller as load decreases

As load decreases  chilled water temperature decreases .When chilled water temperature decreases below predetermined set point,gas valve (in case of direct fired chillers) will start closing accordingly . As burner fire rate or steam flow rate is modulated chilled water temperature will be changed accordingly.This decrease in load would cause closing of steam inlet valve in case of steam operated absorption chiller (indirect fired chillers).

This closing of valve will reduce the heat input to generator of absorption chillers and temperature in generator will be reduced. By this technique of capacity modulation,heat input to chiller is lowered  in generator so accordingly less vapors of refrigerant (water in lithium bromide chillers) will be produced .These vapor get condensed in condenser of chiller and finally reach the evaporator of chiller at lower pressure.Now because less vapors reach evaporator so less heat will be extracted from chilled water and refrigeration effect will be reduced and chilled water temperature will increase accordingly.

Mass flow rate of refrigerant is directly proportional to load and refrigeration effect.

Generally between 30 and 100 percent of the hvac system design refrigeration load, the burner fired rate is modulated. Below 30 percent of design hvac load, the burner is cycled on and off, and all refrigerant and solution pumps remain.

On the other hand ,concentration of solution reaching the absorber from generator will drop accordingly.In other words as less thermal energy is supplied to generator of chillers, solution reaching the absorber will be less strong as it would still contain some  refrigerant. This little less strong solution will have less capacity to absorb refrigerant vapors as compared to strong solution.So accordingly more vapors will remain in evaporator and would increase the evaporating temperature and pressure in evaporator of absorption chiller. Finally a new balance will be established between gas inlet valve and load to achieve the preset temperature at outlet of evaporator.

Capacity Modulation of Absorption Chiller as load Increases

 
Similarly if load on hvac absorption chiller increases then chilled water temperature will also be increased accordingly.Chiller need to be adjusted in accordance with this new load value.Gas valve or steam inlet valve will be opened more to adjust the refrigeration effect and load on chiller.Rest of effect will be inverse of load decrease.

Water Chillers

Water chillers are backbone of HVAC cooling systems.Water chiller is a refrigeration machine which produces chilled water ( Approx 7 Degree C ~ 12 Degree C).The primary function of a chiller is to lower the temperature of water to such value such that it can be used for producing cooling effect in integration with other HVAC components. A water chiller is just like a other refrigerator machine but instead of keeping your food cool or making ice,it produces chilled water.The output of chiller is a water at lower temperature usually around about 10 o C depending upon the Chiller design.

HVAC water chillers play vital role in any HVAC systems along with other basic components. Chillers have got much attention in recent years for its new designs and performance improvement in existing designs. New designs have resulted in hvac chiller with better efficiency and less operation and maintenance costs. Another desirable features is energy saving and precise load or capacity control  for new chillers. We see the chillers with above features in market but as engineering professionals we hope to see more with design improvements with less O & M cost. It is essential part of HVACsystem to have a chiller. Chiller manufacturers have also expanded their production as well design capabilities to cater to the diversified demands of  HVAC chillers.

HVAC Chiller Basics

Working Principle of Vapor Compression Chiller

An HVAC chiller works on vapor compression or vapor absorption cycle.In vapor compression cycle, refrigerant circulates in evaporator ,compressor, condenser and expansion valve of a chiller. A thermodynamic process occurs in each of above chillers.Theoretical working of Vapor compression or compression chillers is explained below.

Heat addition to refrigerant in evaporator ( 1-2):

Refrigerant gets vaporized by taking heat from chilled water in evaporator thus serving its prime purpose. Refrigerant comes out of evaporator as vapors but on other side chilled water is produced. Thus heat is added to refrigerant at constant pressure but is extracted from chilled water. Both refrigerant and chilled water don’t get mixed and are separated by some solid wall in between them in evaporator like shell and tube design.

Compression of refrigerant vapors in compressor (2 - 3 ):

Working Principle of Water Chillers

Refrigerant vapors come out of evaporator and then compressed by chiller compressor to high pressure and temperature. Compressor requires energy input for its working and hence electric energy is supplied to it.

Heat rejection by refrigerant in condenser ( 3 - 4 ) :

Similar to evaporator ,but reverse is happening here. Refrigerant rejects its heat to outside cooling liquid or air .In this way, refrigerant gets condensed and outside media is heated. outside media e.g. cooling water may be cooled by cooling tower and recycled again into condenser.

Expansion of refrigerant in expansion valve ( 4 -1 ) :

Refrigerant in condensed form coming out of condenser is expanded in expansion valve and its pressure and temperature is reduced to level of evaporator so that above cycle is repeated again.

In above four components of hvac chiller ,evaporator is the part where we get chilled water after exchanging heat with refrigerant .

Must Read : Absorption Chillers vs Electric (Compression) chillers