Chiller
Types have been designed
to cater to the diversified requirement of industrial chillers.Water chillers
have evolved due to the variety in availability of type of energy at industry
or small level.Types of chillers have made the use of the chillers
flexible in terms of energy input.
Water chillers
Industrial research in the field of chiller
has eased the selection of chillers in an economical and robust manner.There is
no need of worry whatever type of energy you have available with you.
Each type
of chiller targets the specific users classified on the basis of type of
energy they have.
HVAC systems employ different types of chillers
within the available resources of clients or users.
A chiller may be the
best option for particular application while the same chiller may costly
or less feasible for another application or plant.
Hvacwater chillers are classified primarily
on the basis of cycle on which they work.
Vapor
compression chillers or compression chillers or electric chillers
Vapor
absorption chillers or absorption chillers or ammonia chillers or lithium
bromide chillers
HVAC chiller types
Vapor compression
chillers are further classified on following basis.
Type
of compressor i.e reciprocating compressor,rotory compressor or screw
compressor type vapor compressor chillers
Type
of condenser i.e air cooled condenser or air cooled chillers OR water cooled
condensers or water cooled chillers
Initial cost of air cooled chillers is lower than
water cooled chillers as air cooled systems don’t require cooling
towers,condenser water pumps and associated HVAC components.
Small systems are almost always air-cooled since
the extra requirements of water treatment and maintenance are capital intensive
and will not be offset quickly by the lower operating costs. Larger HVACwater chillers are almost always water cooled .HVAC engineers and technicians
feel easy with maintenance free or less maintenance equipment.
Water cooled chillers may be preferred because
ofbetter heat transfer properties of
water as compared to air cooled chillers.The primary advantage of water is that
the film coefficient is 10-100 times better with water than air.Film
coefficient controls heat transfer with given constant area and temperature
difference.
Water-cooled HVAC units make less noise and give more
cooling per square foot.While air cooled chillers have comparatively less
cooling per square foot and are more noisy HVAC machines.Noisy machines are
annoying especially for occupants of nearest offices or zones.
While comparing air and water cooled chillers, an overall
system cost (both running and installed) has to be worked out. Size of
HVAC unit matters when selecting the water cooled chiller or air cooled
chiller.SmallerHVAC units are usually air cooled, larger are water cooled.
HVAC engineers have to evaluate air cooled and
water cooled chillers keeping in mind various factors.Refrigeration engineer
has to consider the ambient conditions (wet bulb and dry bulb), number of hours
of operation, part load pattern and cost of maintenance etc.
HVAC experts have no single answer to when asked “water
cooled chiller is better or air cooled chiller?”. The fact is that “It all depends”.
Water-cooled chillers will normally condense at an average lower pressure than
air-cooled chillers. This is because the water temperature will
usually be less than the air temperature.If your condenser is operating at
lower pressure then it is directly reflected and translated into lower
operating costs or KW /TR(Ton of refrigeration).
The difference between the wet-bulb temperature
and the dry-bulb temperature gives the available difference between the
condensing temperatures of water-cooled and air-cooled condensers
respectively.
Cooling towers and/or evaporative condensers have
main disadvantage of maintenance and water treatment costs. This running cost
is not present in air cooled chillers.
Despite the advantages of lower IKW/TR of a
water-cooled system or water cooled chillers, sometimes, air-cooled systems are
preferred for various applications.
For example in 24 hour application of HVAC chillers,the
night time dry bulb temperature are generally much lower than the day time dry
bulb temperature, as compared to the difference in wet bulb temperature between
day and night. This advantage is best capitalized by an air-cooled
system or air cooled chillers.
Other examples could be where the wet bulb
temperatures are very high, especially coastal areas.
When comparing water cooled and air cooled chiller,scarcity of water and hence cost of water is
another issue which is a point to be considered while deciding on the
system. This happens in many countries .
Space is also of importance in some applications.
Air cooled chillers can utilize open space, such as a parking lot, roof or
ground level area. Although smaller in size but Water cooled chillers require a
mechanical room and cooling towers.
Air recirculation is very important for air
cooled chillers.Any obstruction in the circulation of air near water chillers
would adversely affect the performance of air cooled chillers.Air cooled
chillers can have problems with recirculation if there are walls higher than
the unit or too many units to close together. If there is a situation where
outside walls are required for building code and the unit will not be receiving
enough fresh air, then a water cooled application may be the best option.
Water cooled chillers have longer life,higher efficiency,large capacity and also refrigerant containment.While looking on disadvantatges of air cooled chillers,they have lower efficiency,comparatively short life span and available in
low capacity range.
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 hvacabsorption 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 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 .
