HVAC Absorption Chillers vs Electric Chillers

In any HVAC system,selection of chillers involve many factors to be considered judiciously.These factors simultaneously present many advantages and disadvantages to end users.For the selection of water chillers,HVAC engineers consider all these factors keeping in view all the resources they have at their facility.

Absorption chiller vs electric chiller

Comparison of Absorption Chillers and Electric (Compression) Chillers

Following is the comparison of absorption chiller and electric chiller is presented below with aim to help you to decide the type of chillers for HVAC  system.

1. Absorption chillers have COP (coefficient of performance )of only 0.54–1.1 and it competes poorly with electric chillers (rotary compressor chillers). Electric chillers on other hand have COP from 1.0 ~ 8.0. 
2. Absorption HVAC chillers occupy approximately 50% more floor area than the equivalent  electric chillers (vapor compression chillers). 
3. Additionally due to height of absorption chillers, mechanical equipment rooms must be 6–10 ft taller than rooms housing electric chillers. Finally, because the liquid solution is  contained in long, shallow trays within an absorption chiller, the floor must be as close to absolutely level as possible.
4. In HVAC system,Absorption chiller will weight at least twice as much the equivalent electric chiller  (vapor compression chiller). 
5. Due to greater  size, absorption chillers are sometimes shipped in several sections, requiring field welding for final assembly.This is not case with counterpart electric chillers or compression chillers. 
6. Most of electric water chillers are shipped from the factory with their refrigerant charge  installed.While the refrigerant and absorbent (including additives) must be field installed in absorption HVAC chillers. 
7. When we talk about noise and vibration, absorption chillers (unless direct-fired) are quiet and essentially vibration-free as compared to compression chillers (electric drive chillers). Noise and vibrations in HVAC system annoy more than anything else . 
8. Due to the potential for crystallization of the lithium bromide in the chiller if it becomes too cool, the condenser water temperature must be kept above 75–808F.There is no problem of crystallization in electric chillers (vapor compression chillers). Forget about the crystallization and stay cooled if you have installed compression chillers . 
9. Absorption water chillers require sometimes an emergency power source if lengthy power outages are more often expected. Without power and heat input, the chiller begins to cool and the lithium bromide solution may crystallize. However, as absorption chillers consume very little electric energy so a small dedicated back-up generator can serve this purpose well. 
10. HVAC system also gives importance to the amount of heat to be rejected in condenser by water cooling or air-cooling media.The heat rejection rate from the condenser of lithium bromide chiller is 20–50% greater than for the equivalent electric chiller, requiring higher condenser water flow rates and larger cooling towers and condenser water pumps. 
11. Finally, an indirect-fired absorption water chiller will be at least 50% more expensive to purchase than the equivalent electric-drive chiller. Direct-fired absorption chillers will cost almost twice as much as an electric machine, and have the added costs associated with providing combustion air and venting (stack). 
12. Absorption water chiller or lithium bromide chiller uses natural refrigerants such water and eliminate the need of CFC or HCFC refrigerants which have high global warming potential. 
13. If you have plenty of waste heat or direct fire available ,and want to reduce your electric load ,then probably absorption chillers are the best solution.But obviously all above factors are to be considered for the selection of HVAC water chillers.

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.