There are two main types of absorption machines: single effect and double effect. Single effect uses a single generator to regenerate the refrigerant and absorbent using a lower grade heat source, resulting in a COP of around 0.7. Double effect uses two generators and a medium grade heat source to regenerate the refrigerant and absorbent in two stages, improving efficiency with a COP of 1.36 to 1.43. Double effect machines are generally more feasible than single effect when medium grade heat sources are available due to their higher performance. It is difficult to design a single machine to operate effectively across both low and high-grade heat sources.

1. Single Effect VS Double Effect
QUESTIONS TO BE ANSWERED
• What do the terms mean?
• What is the basic difference between them?
• How do we choose between them?
• What would be the difference in performance?
• Can you compare the feasibility of both machines?
• Is it feasible to design a combination of both?
• Summary
• Abbreviations
• Contact Details

2. What does the terms mean?
SINGLE EFFECT:
 When the refrigerant and the absorbent are regenerated in a single
step using a lower grade of heat source, we term that as a single stage
absorption machine
DOUBLE EFFECT:
 When the refrigerant and the absorbent are regenerated in two steps
using a medium grade of heat source, we term that as a double stage
absorption machine
 The available medium-grade heat source generates vapors of refrigerant and an
absorbent solution with a intermediate concentration in the first step/stage.
 This generated vapors are used as a heat source in the second step/stage, where
the vapors from first step/stage heat the absorbent solution with a intermediate
concentration, to generate more vapors and increase the concentration of the
absorbent to the required level
 Hence, we claim this as a dual stage/double effect concept

3. What is the basic difference between them?
SINGLE EFFECT:
 We have only one generator to regenerate the refrigerant and absorber
 The heat exchanger/generator will be comparatively bigger in size, as
we have less energy available in the heat source(steam/hot water), in
turn, the flow will be huge.
 COP (Cooling Output / Heating Input (in KW or Kcal)) – 0.7 (no unit)
DOUBLE EFFECT:
 We have two generators to regenerate the refrigerant and
absorber, the HTG(High Temperature Generator) & LTG(Low
Temperature Generator).
 As we have split the generation process and also we have a better heat
source, the size of the heat exchangers/generators is pretty much less
when compared to a single effect machine.
 COP – 1.36 to 1.43 (based on the type of heat source)

4. How do we choose between them?
 Generally, we suggest a vapor absorption machine, if
 The power cost/ running cost of electrical chiller is very high and also if the power cuts is
for a longer duration, it may increase the size of DG and operating cost of the electrical
chiller
 Waste heat recovery is available – exhaust of a DG/LP steam @ turbine outlet
 The steam production cost is very low. E.g., In sugar plants, they have a co-generation by
using the baggasse, which is a by-product of the process
 The selection is mainly based on the heat source available
 Low grade heat source
 Low Temperature Hot Water – 75 to 150*C
E.g. – Jacket Water from a DG, condensate return
 Low Pressure Steam – less than 3.5 kg/scm
E.g. – Steam extracted from a Steam Turbine
 Medium grade heat source
 Medium Temperature Hot Water – 120 to 185*C
E.g. – Hot water from Solar Heaters
 Medium Pressure Steam – Between 3.5 to 10.5 kg/scm
E.g. – Steam from a boiler (maybe in excess or dedicated for VAM(if available @ low cost))
 Exhaust from a DG – more that 240*C – temperature of exhaust gas

5. What would be the difference in performance?
 The main difference would be, that, the single effect
machine requires almost twice that of a double effect
machine in the case of heat input, as the amount of
useful energy/enthalpy is less in a low grade energy;
hence, the COP of a Single effect is almost half of that
of a double-effect, but when we consider in terms of
performance, there will not be any difference, as the
machines are designed to provide the desired
output, whatever and however maybe the energy
input
 When the available energy for a single effect machine
is on the least extreme, then we will have a
considerable increase in the machine size

6. Can you compare the feasibility of both machines?

7. Is it feasible to design a combination of both?
 E.g., Let us consider a steam fired VAM which is
designed for a range of 2.5 to 10 kg/scm.
 Generally, whenever there is a drop in the inlet pressure of steam, we
have to increase the flow of steam, to achieve the required
performance. So, for the above requirement, we should select a pipe
size and a SCV, which suits the std. steam flow rate required at
2.5kg/scm pressure; but, if the inlet steam pressure rises, the
generator would be receiving more energy than the required and it’s
unable to design a SCV which could operate on the energy
requirement of the m/c for a particular steam pressure. Hence, we
cannot have a single machine which could operate for the above
given conditions.
 When the steam pressure drops below 3.5kg/scm, we do not require
the HTG, but, it is difficult to bye-pass the LiBr and steam flow, when
the steam pressure drops below 3.5kg/scm. Even, if it is achieved, it
is much more difficult to tune the m/c for operating at the above
mentioned condition.

8. Summary
We hope that,
 We had answered the questions which were asked
initially in the most easiest and simplest way
possible, so that, it would help you to take decision
which best suits your requirement and process, with
maximum heat recovery and minimum heat loss
 For anymore queries, please feel free to contact the
respective regional sales person or visit
www.thermaxindia.com and post your query
 The contact details are given below

9. Abbreviations
VAM Vapour Absorption Machine
m/c machine
SCV Steam Control Valve
HTG High Temperature Generator
LTG Low Temperature Generator
LiBr Lithium Bromide
COP Co-efficient Of Performance
Kg/scm kilogram/square centimeter
DG Diesel Generator
LP Low Pressure
E.g., Example
*C degree Celsius
KW kilo Watt
KCal kilo calories