HVAC Water Chillers Quiz # 3

Following MCQs practice Quiz is related to ,HVAC ,absorption cooling chillers (single effect and double effect) and Lithium Bromide Chillers.

(See here HVAC quiz # 1   HVAC Quiz # 2 )

 

(Should you  require following hvac quiz in pdf /ms word form,write down your email address in the comments below) 

1. Temperature ______when water is absorbed in lithium bromide solution

Does not change
reduces
increases
none of these

2. Which of following component shares common pressure with low pressure generator

generator
condenser
absorber
evaporator

3. Absorbent capacity increases when temperature of solution

Increases
decreases
remains unchanged
All of above

4. In ammonia-water system, ammonia is_____ and water is ______.

refrigerant,absorbent
absorbent,refrigerant
both refrigerant
none of these

5. In lithium bromide-water system, lithium bromide is_____ and water is ______.

refrigerant,absorbent
absorbent,refrigerant
both refrigerant
none of these

6. Crystallization in lithium bromide chillers is more likey

at lower concenrtation
at higher concentartion
in condenser
none of these

7. When crystallization occurs in lithium bromide chillers

chilled water temp decreases further
chilled water temp increases
no effect on working
none of above

8. In lithium bromide chillers ,crystallization is most likely to occur in

evaporator
absorber
LT hX
HT Hx

9. In lithium bromide chillers when crystallization occurs,

crystalls of refrigerant are formed
crystalls of absorbent are formed
refrigerant becomes ice
None of these

10. 4 components of absorption chiller are

evaporator,condenser,generator and absorber
evaporator,compressor,condenser,expansion valve
evaporator,condenser,generator and compressor
More than one statement is true

HVAC Quiz # 2

Following MCQs practice Quiz is related to ,HVAC ,absorption cooling chillers (single effect and double effect)and Lithium Bromide Chillers.

 

(Should you  require following hvac quiz in pdf /ms word form,write down your email address in the comments below)  

HVAC Water Chillers Quiz # 2      (See here HVAC quiz # 1)

HVAC Quiz

1. A generator in absorption system heats the solution to

separate the refrigerant
extract strong solution for recycle
vaporize absorbent
Both a & b


2. Absorbent (Lithium bromide) cycles through ____.

Evaporator and absorber
Absorber and generator
Condenser & evaporator
All of above


3. At the low pressure evaporator condition, the solution can absorb additional refrigerant if it

Compressed
Cooled
De-sorbed
Heated


4. In an absorption system, refrigerant

Cycles only between condenser and evaporator
is all time absorbed in the solution throughout the cycle
is most diluted on leaving the absorber
provides cooling in the evaporator


5. When leaving the generator, refrigerant vapors travel to_____and solution travels to_____.

Condenser, absorber
Condenser, evaporator
evaporator,absorber
None of above


6. One of main uses of absorption chillers-heaters is to provide ______

Chilled water only
Chilled & hot water
hot water only
none of above


7. Air ingress in absorption chillers,

increases the vacuum
increases the pressure
has no effect
all of above


8. Absorption chillers operate at

High vacuum
Normal temp & Press
Positive press
All of above


9. Double effect absorption machines have

one generator & one evaporator
two condensers
two generators & one evaporator
none of


10. Non-condensable gases in the system ____ the system pressure and _______system capacity.

decrease, increase
Increase, decrease
do not affect
none of these

Types of Air Conditioning Systems

Various types of air conditioning systems (also called types of HVAC systems) have been designed to cater to the demands of HVAC in various applications in the best economical and energy efficient way.

• In institutional, commercial, and residential buildings, air-conditioning systems are mainly for the Occupants’ health and comfort. They are often called comfort air conditioning systems.    

 

• In manufacturing buildings, air conditioning systems are provided for product processing, or for the health and comfort of workers as well as processing, and are called processing air conditioning systems.

Based on their size, construction, and operating characteristics, air-conditioning systems can be classified as the following.

Individual Air conditioning System (Window or split ac)

An individual room or individual air conditioning system normally employs either a single, self-contained, packaged room air conditioner (installed in a window or through a wall) or separate indoor and outdoor units to serve an individual room (split air conditioner)you can see a split air conditioner in the figure shown above.“Self contained, packaged” means factory assembled in one package and ready for use.

Space Air conditioning System

These air conditioning systems have their air-conditioning — cooling, heating and filtration—performed predominantly in or above the conditioned space. Outdoor air is supplied by a separate outdoor ventilation system (as a part of HVAC system). 

Packaged Air conditioning System   

These air conditioning  systems are installed with a single self contained, factory-assembled packaged unit (PU) or two split units: an indoor air handler, normally with duct work, and an outdoor condensing unit with refrigeration compressor(s) and condenser. In a packaged system, air is cooled mainly by direct expansion of refrigerant in coils called DX coils and heated by gas furnace, electric heating, or a heat pump effect, which is the reverse of a refrigeration cycle.

Central Air conditioning System

A central air conditioning system in hvac system uses chilled water or hot water from a central plant to cool and heat the air at the cooling or heating coils in an air handling unit (AHU). For energy transport, the heat capacity of water is about 3400 times greater than that of air. Central systems are built-up systems assembled and installed on the site.

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.

HVAC Water Chillers and Cooling Towers Fundamentals, Application, and Operation

HVAC Water Chillers and Cooling   Towers  


Fundamentals,Application,and Operation 


By Herbert W. Stanford, III


View       Free Download Link

Undesrtanding the basics of Double Effect Absorption Chiller

Double effect absorption chiller is an improvement in single effect lithium bromide chiller (read here all about Single effect absorption chiller ) . Double effect water chillers have more coefficient of performance and enhanced utilization of input energy.Basic working principle of these dual effect absorption chillers remains same as single effect but with addition of some components to vapor absorption cycle. Absorption cooling is enhanced using the system energy which would have been otherwise wasted as in case of single effect industrial chiller.In order to produce chilled water in these double effect chillers,water remains as a refrigerant and lithium bromide as absorbent in these HVAC water chillers. As compared to single effect  absorption chillers,double effect vapor absorption chiller has an additional heat exchanger and generator/concentrator.

HVAC double effect absorption chiller

Basics of Double effect Absorption Chiller Operation

HP Generator Process in Double effect Absorption Chiller Operation

In double effect absorption chiller,both in direct and indirect fired chiller operation, weak solution from the absorber is pumped into the HP generator through LT, condensate and HT heat exchangers. It is heated by operating steam and concentrated to the intermediate solution, and high temperature refrigerant vapor is produced.Instead of steam ,hot exhaust gases or waste heat may also be used as in case of direct fired absorption chillers.

LP Generator  and HT exchanger Process in Double effect Absorption Chiller Operation

In next step of chiller operation,Intermediate solution enters LP generator through HT heat exchanger in exchanging heat with weak solution which is passed through the tubes. Intermediate solution in LP generator is heated by the refrigerant vapor from HP generator, concentrated to the strong solution, releasing refrigerant vapor at the same time. 

LT Heat exchanger Process in Double effect Absorption Chiller Operation

The strong solution passes through the outside tube space of LT heat exchanger, enters absorber, transmitting heat to the weak solution from absorber. In absorber the strong solution absorbs refrigerant vapor again. 

Condenser Process in Double effect Absorption Chiller Operation

Refrigerant vapor from HP generator is condensed in LP generator to form condensate, which enters condenser through throttle. Refrigerant vapor formed in the LP generator flows to the condenser to form condensate also.

Evaporator ,Refrigerant Pump and Absorber Process in Double effect Absorption Chiller Operation

At normal temperature and pressure water boils at 212^oF but as pressure is lowered sufficiently below atmospheric pressure then boiling temperature is also decreased .Same thing is used in evaporator of absorption chillers(single and double effect ).
These two parts of refrigerant condensate flows into the flash chamber through U pipe. A part of refrigerant water is flashed to form vapor, which flows into the re-absorption chamber at the bottom of absorber, while another part of refrigerant water is cooled, and enters evaporator refrigerant pan. Refrigerant from evaporator refrigerant pan is pumped over the evaporator tubes for the refrigeration effect, and evaporates to form vapor by absorbing heat of chilled water flowing through tubes.

Produced refrigerant vapor enters absorber, and absorbed by strong solution in the absorber. Chilled water is cooled and return to the  HVAC system of customer. Strong solution is diluted by absorbing refrigerant vapor in absorber and absorbing flashed refrigerant vapor in the re-absorption chamber.

Solution Pump in Double effect Absorption Chiller Operation

Weak solution  is then transferred by solution pump to HP and LP generator for concentration. Heat generated is carried to atmosphere by cooling water. This process is continued, and refrigeration effect is repeated.