Section A Water Chillers: Fundamentals Application, and Operation PART 2 C hiller Fundamentals

Section A

 Water Chillers: Fundamentals
Application, and Operation
PART 2 

C hiller Fundamentals

Water Chillers

Refrigeration Machines:

However, a new class of refrigerants called hydrofluoro olefins (HFOs) is now
becoming available. The first of these refrigerants is Dupont’s R-1234yf, designed
to be a direct replacement for R-134a. While all research to date has focused on
applying R-1234yf to automobile compressors, Dupont anticipates that this refrigerant,
or blends of this refrigerant with R-744 (carbon dioxide), can reduce GWP
by 50% over R-134a. Already, retrofits of existing R-134a compressors with
R-1234yf have been implemented and the results are promising, with no oil problems
or capacity loss and even small efficiency improvements.
ASHRAE Standard 34-2010 classifies refrigerants according to their toxicity
(A = nontoxic and B = evidence of toxicity identified) and flammability (1 = no
flame propagation, 2 = low flammability, and 3 = high flammability). Thus, all
refrigerants fall within one of the “safety groups,” A1, A2, A2L, A3, B1, B2, B2L,
or B3. The “L” designation indicates that a refrigerant has lower flammability
than the range established for the “2” rating, but is not a “1.”
Table 1.3 lists the safety group classifications for common refrigerants.

Implementation of HCFC Refrigerant Phaseout in the United States

ABSORPTION REFRIGERATION:

 >
Absorption Refrigeration Cycle:

 
The absorption refrigeration cycle is a relatively old technology. The concept
dates back to the late 1700s and the first absorption refrigeration machine was
built in the 1850s. However, by World War I, the technology and use of reciprocating
compressors had advanced to the point where interest in and development of

HVAC Refrigerant Safety Groups

absorption cooling essentially stagnated until the 1950s. During this period, the
two-stage, indirect-fired absorption refrigeration machine was developed in the
United States, while the direct-fired, two-stage concept was perfected in Japan
and other Pacific-rim countries. The direct-fired option was developed primarily
in response to government energy policies around the Pacific rim.
The fundamental “single-stage” absorption cycle is represented in Figure 1.4.
The absorption chiller has no compressor; heat, directly or indirectly, provides
the motive force for refrigerant phase change. The evaporator consists of a heat
exchanger, held at low pressure, with a separate refrigerant (typically, water)
pump. The pump sprays the refrigerant over the tubes containing the chilled
water, absorbs heat from the water, and evaporates as a low-pressure gas. The lowpressure
gas flows to the absorber, due to the pressure differential. The absorber
is at a pressure lower than the evaporator because the concentrated absorbent
solution exerts a molecular attraction for the refrigerant. The absorbent solution is
sprayed into contact with the refrigerant vapor. Condensing of the refrigerant

Refrigeration Machines: 

Single-stage steam absorption chiller schematic.

occurs because the heat is absorbed by the absorbent. The absorbent, then, is
cooled by condenser water.
The absorbent now consists of a dilute solution, due to its having absorbed
water vapor refrigerant. The dilute solution is pumped to the concentrator, where
heat is applied to reevaporate the refrigerant. The concentrated solution of the
absorbent is then returned to the absorber. The refrigerant vapor goes to the condenser,
where it is condensed by the condenser water. To improve efficiency, a
heat exchanger is used to preheat the dilute solution, with the heat contained in the
concentrated solution of the absorbent.
Leaks allow air to enter the refrigerant system, introducing noncondensable
gases. These gases must be removed, or purged, to prevent pressure in the absorber
increasing to the point where refrigerant flow from the evaporator will stop. The
solution in the bottom of the absorber is relatively quiet and these gases tend to get
collected at this point. They can be removed through the use of a vacuum pump,
typically called a purge pump.

Refrigerants:

>
Today, there are two basic refrigerants used in absorption refrigeration chillers:
water/lithium bromide and water/ammonia. Larger absorption units utilize water/
lithium bromide solutions, while small units more commonly utilize water/
ammonia solutions.
Lithium bromide is a corrosive, inorganic compound that has a very high
absorption rate for water (hydroscopic). Thus, it makes an ideal “carrier” for the
water refrigerant in absorption cycle chillers. However, the corrosion issues make
lithium bromide solutions, especially at the higher temperatures associated with
direct-fired chillers, which are difficult to address. 

Since the water/lithium bromide solution in an absorption chiller is a corrosive
salt solution, the primary potential for corrosion in these chillers is the
ferrous
metals
used in them. The generator (or “concentrator”) is the most critical
location for potential ferrous corrosion since the highest salt concentration
and highest temperatures
are present in this heat exchanger, along with the
potential impact of erosion corrosion as the refrigerant vapor is driven off by
surface boiling.
Basic ferrous corrosion occurs when iron reacts with water to produce an iron
oxide called magnetite and hydrogen. Under acidic conditions, this reaction is
greatly accelerated. Also, in an absorption chiller, hydrogen is a “noncondensable”
gas, that is, it does not act as a refrigerant, and the performance of the chiller
is negatively impacted on.
There are basically two approaches to corrosion protection: (1) choose a metal
compatible with the chemical environment in which it has to survive, or (2) modify
the chemical environment so that it is less corrosive to carbon steel. The first
approach, which requires the use of high-quality stainless steel for heat exchanger
components, can add significantly to the cost of an absorption chiller and most
manufacturers and owners have been unwilling to pay the premium involved.
Since chemical modification is much cheaper, this is the approach most commonly
taken by chiller manufacturers.
Two types of chemical modifications are generally made:

 1. To reduce the acidity of the water/lithium bromide solution, a compatible
alkaline, lithium hydroxide, is added. To reduce ferrous corrosion, it is
desirable to maintain the solution as alkaline. But, since copper corrodes
readily at higher alkaline levels, it is necessary to maintain the level high
enough to help protect the steel, without being too high and accelerating
copper corrosion.
2. To additionally protect the steel in a corrosive environment, a compatible
corrosion inhibitor is added. In this case, this inhibitor is lithium
chromate. Lithium chromate “passivates” the iron with which it comes
into contact (i.e., makes it less reactive) by forming a protective molecular
film on the surface. Lithium chromate has the advantage of working
well at low-alkalinity levels, allowing the solution to be maintained at
levels more suitable to the copper in the chiller.
This chemistry balance is complex and requires routine adjustments to maintain
correctly. If the alkalinity is not adjusted properly, the solution will become
acidic and accelerate ferrous metal corrosion. If alkalinity is too high, the copper
in the chiller will corrode. If the lithium chromate level is too low, it offers poor
protection to the ferrous metals in the chiller. But, if the level is too high, it can
initiate pitting in the ferrous metals due to scaling and the resulting localized
corrosion,
while also increasing copper corrosion rates.
Water/ammonia solutions have much lower corrosion issues, but suffer from the
safety concerns associated with the use of ammonia. Thus, the use of ammonia as

Refrigeration Machines 15
the refrigerant in absorption cooling systems is typically limited to industrial
applications. 

 
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