1. Mukesh Pratap Singh (mukesh.dtc@gmail.com)
Concept Note - Waste Heat Recovery from refrigeration cycle
Introduction
The basic refrigeration system or air-conditioning system consists of removal of
heat from one source and expelling this heat to atmosphere, treating
atmosphere as heat sink. The removal of heat in general is known as cooling
effect and represented in generic term as tons of refrigeration. The expelled
heat to atmosphere can be convert into useful heat. Advancement of
Technology has developed different application to use this rejected heat.
Simultaneous requirement of heating load when cooling is generated are most
efficient form of applications where recovered heat results in obvious savings in
operating costs. This also improves cooling performance of refrigeration cycle.
Heat Recovery Desuperheater in refrigeration cycle has obvious commercial
benefits that have been commercial exploited in recent past. A Desuperheater
is supplementary heat exchanger in refrigeration condensing section and is
able to recover heat at temperatures substantially above the condensing
temperature.

2. Mukesh Pratap Singh (mukesh.dtc@gmail.com)
Basic refrigeration cycle has four components evaporator coils, compressor,
condenser coils and throttling or expansion valve. The evaporator coils provides
us with the refrigerating effect. The compressor in the cycle necessarily handles
refrigerant in gaseous form and system design necessitates avoiding use of
liquid refrigerant reaching to compressor. Superheating the gas in evaporator
ensures this.
The compressor raises the pressure of the refrigerant by adiabatic
compression. By Ideal gas law the temperature of gas is also increased.
Condenser performs three fundamental operations
(a) Cooling the superheated gas to condensing temperature
(b) Condensing the gas by removing latent heat of the gas and then
(c) Sub-cooling the condensate.
Refrigerant is then passed through throttling valve where the pressure of
refrigerant is lowered to evaporating pressure. This low-pressure liquid is
passed through the evaporating coils that transform liquid back into vapour.
This evaporation of refrigerant is known as cooling effect. The cycle repeats in
continuous mode. The major effects of refrigeration cycle are latent heat of
condensing in condenser and latent heat of evaporation in chiller. These two
latent heat quantities define performance of refrigeration. By forcing the
refrigerant to condense from a gas to a liquid, latent heat is transferred from
the refrigerant to the other medium. Conversely, by forcing the liquid
refrigerant to change into a gas inside the evaporator, heat is removed. The
other forms of heat in refrigeration cycle such as superheat in compression and
sensible heat in condensation are necessary elements for working of cycle. The
suction line heat exchanger is optional accessory which increases the
performance of the refrigeration system

3. Mukesh Pratap Singh (mukesh.dtc@gmail.com)
WASTE HEAT AVAILABILITY:
Heat of condensation has three phases as explained above. One may be
tempted to say that recoverable heat is same as heat removed from refrigerated
space or cold rooms. A small correction is required to above statement that the
recoverable heat also has added element of heat of compression. This can be
mathematical written as,
Heat of rejection (E1) = Heat of evaporation (E2) + Heat of compression (E3)
Since condensing latent heat is at constant temperature ranging between 35 to
40°C. This heat has limited applications whereas; condensing heat above this
temperature that is equivalent of energy input to compressor has wider range
of energy applications. Hence, this heat recovery has wider importance.
It is tempted to say all heat rejected to atmosphere is available for recovery.
This is partly true. Rejected heat also contains heat of compression in addition
to cooling heat removed. However major part of this heat is available at lower or
non-usable condensing temperature. Taking this in to account it is important
to assess the available / recoverable heat in each process and generation of hot
water in the system.
As seen in the figure, Desuperheater is piped in series between the compressor
gas discharge and the condenser. It is a special purpose heat-exchanger,
designed to transfer hot gas energy to hot water use. Every refrigeration system
has available heat energy, which can be recovered. This heat is comprised of
two components; “superheat” and “condensing” heat energy. The “superheat”
typically accounts for 15% of the total system heat of rejection, whereas the
“condensing” energy accounts for about 85%. The Desuperheater uses hot
refrigerant gas “superheat” energy to heat domestic hot water to 60 deg C.
ADVANTAGES TO THE REFRIGERATION CYCLE

4. Mukesh Pratap Singh (mukesh.dtc@gmail.com)
1. Improved System Operation
Because heat recovery is equivalent to adding 15% more condenser
capacity, it lowers the compressor head pressure. This helps prolong the
life of the air conditioning compressor and promotes improved system
operation.
2. Slight increase in capacity
The heat recovery decreases the gas passing through the throttling valve
thus there is small increase in amount of liquid refrigerant, thus there is
slight increase in refrigerating capacity.
3. Reduced refrigeration Costs
Desuperheater reduces the operating costs of the air
conditioning/refrigerating system by 3% to 5%. This means additional
energy efficiency for the overall system when the Heat Recovery is in use.
4. Reduction in power cost
Reduced operating hours of cooling tower fan and thereby reduced power
cost.
5. Lower maintenance cost
Improved condenser cooling and reduced condenser clogging results in
lower maintenance cost of the refrigeration is used
6. Reduction in cost of fuel
Desuperheater provides hot water for other needs such as cleaning,
pasteurization etc. equivalent, thermal heat being lower for overall
heating cycle; it saves fuel required for hot water heating.

5. Mukesh Pratap Singh (mukesh.dtc@gmail.com)
AMOUNT OF THE HEAT CONVERSION:
We have already seen the available heat but all the heat available is not
completely recovered. The amount heat recovered depends upon the heat
exchanger effectiveness. The heat exchanger generally has their effectiveness
between 60-80%.
Once the amount of heat that may be transferred to the water is determined, it
is then appropriate to view this energy recovery in terms of the energy savings
that may be achieved. One ton of heat removal is equivalent to 12600 kJ/hr.
Every liter of water requires 8.75kJ/hr of heat addition to raise its temperature
1 deg C. Thus we can have hot water per hour free for every ton. Free is relative
since the equipment has some costs attached. Assuming the alternative was to
heat the water electrically (or burning fuel), the heat recovery unit would
provide savings on electricity (or on burning fuel). To get overall savings, simply
multiply hourly savings by the hours/day of operation. Then consider the
number of days/year that cooling is required. Several other considerations are
important:
 Since installation of a heat recovery unit requires the addition of other
components in the refrigerant lines, your warranty or service agreement
may be affected.
 Heat recovery units recover heat only when the chiller is operating.
Therefore, savings will be reduced if the chiller operating hours are
reduced.
 Long runs of refrigerant or water lines can add to the cost, as well as
resulting in additional heat loss in the system.
 If the unit produces hot water faster than it can be used, the excess
capacity may be wasted, thereby reducing potential savings.
 All heat recovery units should be provided with bypass valves that allow
the unit to be isolated from the system in case of leaks or required
maintenance.

6. Mukesh Pratap Singh (mukesh.dtc@gmail.com)
 Heating water to temperatures higher than recommended results in
lower output in gallons per hour.
 Heat recovery systems can be adopted for all air conditioning units from
as small as 2 tons up to the largest chillers available
Dairy processing:
Dairy processing plants usually use energy efficiently. Heat transmitted to milk
products during pasteurization is normally rejected to incoming cold milk in
the regenerator. A large percentage of waste energy is in the heat rejected by
the refrigeration condensers. It can be used in generating hot water for use in
cleanup
CONCLUSION:
Heat recovery from refrigeration is effective and provides reliable source of heat
to fulfill major demand of hot water. System can preheat water up to 60°C
without affecting refrigeration cycle. Simultaneous generation of chilled water
and usable hot water results in
 Increased output of heating system
 Reduced fuel consumption and
 Enhanced overall system efficiency.