desuperheater

Use of Desuperheater in Refrigeration

1. 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.

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2. BASIC REFRIGERATION CYCLE:

Fig. 1 Basic Refrigeration cycle

As shown in figure 1 the basic refrigeration cycle has four components

evaporator coils, compressor, condenser coils and throttling valve. The

evaporator coils provides us with the refrigerating effect. The compressor in

the cycle necessarily handles refrigerant in gaseous form and system design

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Condenser

Compressor

Chilled Water

CT water

TEV

S L H E

From

Ev

apor

ato

r

To Evaporator


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.

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3 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 30 to 35 0C. 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.

Air conditioning unit generally removes heat from interior spaces

and this heat is rejected to atmosphere. 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. It

is also important to note that more recovered pre-heated water than actually

required, does not mean more savings. Availability of excess hot water,

should not be claimed as savings.

For example fuel fired (Diesel or Furnace oil) boilers are used to

generate hot water for domestic use, cleaning, or for process need. At the

same time refrigeration is used for process cooling, cold storage and

pasteurisation or ice making. Entire heat rejected by refrigeration is rejected

to atmosphere through air-cooled or water-cooled condensers. Many times

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this process of heat rejection and fresh heat generation is done across the

wall in the same plant and seldom noticed for complimenting each other.

While this energy is of low grade variety, it still represents waste energy.

Combining these two, waste heat and fresh heat generation makes clever

sense of energy conservation.

4 .THE CONCEPT:

Figure 2 Refrigeration cycle with the Desuperheater

As seen in the figure, Desuperheater is piped in series between the

compressor gas discharge and the condenser. It is a special purpose heat

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Desuperheater

Condenser

Compressor

Evaporator

Hot Water

TEV

S L H E

From

Ev

apo

rato

r


exchanger, designed to transfer hot gas energy to domestic hot water use.

Every air conditioning 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.

Many desuperheater sections combine both the cooling of the

superheated refrigerants and the part of saturated condensing requirement. A

typical selection will have a desuperheater that accomplishes approximately

one-third of the total heat rejection requirement.

Now we feel why can’t a single condenser system provide us with

same heat recovery and at same time be simple in construction?

Yes it can be used with some limitations that can be listed as follows

1. There is inability to economically provide water temperatures above 54

deg C.

2. Cooling tower winterization is required

3. In a system that circulates water from open cooling tower circuit to the

heating equipment, corrosion and particulate control are concerns.

4. In a system with heat exchanger to separate the open cooling tower

circuit from heating equipment the temperature of recovered heat is

reduced by 1-3deg C.

Let us see what advantages Desuperheater provides

5. ADVANTAGES TO THE REFRIGERATION CYCLE: 1. Improved System Operation

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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, pasteurisation etc. equivalent, thermal heat being lower for overall

heating cycle; it saves fuel required for hot water heating.

6. AMOUNT OF THE HEAT CONVERSION:

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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.

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

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7. ECONOMICS CONSIDERATIONS: The economics of this heat recovery is attractive. It is important to

ensure recovered hot water displaces prime energy (fuel or electricity) used

by facility. The economics can be adversely affected due to poor operating

hours of compressor. The system evaluation and total integration with facility

hot water system is essential step to ensure favorable economics of project.

Though there are other advantages of heat recovery system, over

empathizing of these benefits can create serious situation. While system is

conceptualized use of hot water should be properly estimated so that

recovered heat is not wasted. Providing metering of hot water flow and

recording the operating temperature will help to monitor and compare post

implementation performance of system. Generally 1litre fuel oil and 1kWh of

electricity delivers 270 & 28 liters of hot water respectively hot water at

55deg C. Thus with large requirement of hot water and refrigeration running

on one side, Desuperheater provides substantial savings in fuel.

8.USE OF CONVERTED HEAT:

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The heat recovered can be used effectively wherever both heating

and refrigerating effects are required. Typical applications include restaurants,

hotels, apartments, hospitals, office buildings and industrial processes...

anywhere hot water is needed

The amount of waste heat available in a plant varies widely from

sector to sector in the food industry owing to the different processes and

energy requirements involved. There are also substantial differences in waste

heat from plant to plant within each sector due to such factors as product

concentration, continuous or batch operation, plant size and location. The

actual amount of potentially useful waste heat can only be revealed by a

comprehensive audit of the plant's energy consumption. However, in each

sector there are general opportunities and applications for waste heat

recovery. These are briefly discussed below for various sectors of the food

industry.

8.1.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, in preheating boiler feed water, or in

heating culture tanks for some unit operations. Wastewater and exhaust from

spray dryers are other major sources of waste heat. Contaminants in the

wastewater may restrict its use for heat recovery, but dryer exhaust can be

recovered and used to preheat supply air for the dryer.

8.2.Poultry processing:

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In poultry processing, large quantities of energy are required for scalding,

cooling and freezing. The scalders and chillers have continuous overflow, and

thus large amounts of energy are lost. However, recovery of heat (or cooling

capacity) from scalders (or from chillers) is feasible, and the energy can be

returned to the scalders (or chillers). Heat can also be recovered from

refrigeration system condensers and used to preheat boiler makeup water or

washwater.

8.3.Fruit and vegetable processing (freezing and canning):Although the percentage of waste heat does not differ greatly between

freezing and canning plants, the sources of the waste streams do. In freezing

plants, the major source is the refrigeration system condensers. Heat is

available from the hot refrigerant and should be easily recoverable. A second

source is wastewater, however, waste heat from it is of low quality and solid

particles in the wastewater might cause fouling problems in heat exchangers,

Individual hot waste streams may offer some recovery potential if they are

intercepted prior to mixing with the main waste flow.

In canning plants, the major waste heat sources are retort vents and

wastewater. Heat from retort vents can be recovered, but no backpressure

should be applied to these vents, since free flow of steam is essential during

venting to ensure that adequate thermal processing of cans will be

accomplished.

Waste heat can be used at numerous points in the plant. Water heating for

can washing, blancher makeup water, and plant cleanup and boiler feedwater

are potential applications.

9. CASE STUDIES :

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Let us now evaluate how much savings are we seeing at

Case study 1:Hotel requires overall cooling of building and hot water for domestic use. Hot

water is required in room service as well as kitchen is supplied from hot water

that is maintained between 45 to 50 0C. Hotel has 50TR of refrigeration. The

desuperheater produces hot water and it is piped in existing hot water

system. Thus the required hot water is provided by desuperheater and

electricity /diesel firing makes up the short fall.

The assumed 15%heat recovery for 50ton we get 1575kJ/min

16 hrs operation per day provides 1.5 GJ /day.

Fuel heat gives 2751kJ/kg. (for HSD fuel)

(Assuming Density 0.84, efficiency .84)

Effective heat 1940 kJ/lit

Savings /day/TR /16 hr Operation of refrigeration system = 0.77 lit

Case study 2: Heat energy is removed from milk during cooling. The heat energy can be

'dumped' from an air-cooled condenser, or all/or part of it can be transferred

to water using a water-cooled condensing mechanism or milk precooler.

Water-cooled condensers and milk precoolers will reduce compressor

running- time. However, the greater energy savings usually come from

displacing water heating energy costs.

On most, dairy farms the milk heat available can preheat more water than the

required for nominal wash-up and sanitizing.

Milk Cooling

To maintain milk quality, milk must be cooled from about 39° C (cow body

temperature) to 3° C for safe storage. Milk is normally cooled by a

refrigeration unit removing heat from milk (source) to air or water (sink).

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27℃Evaporator Air

Hot water

As discussed hot water is required for sanitization and pasteurization. The

temperature of this water is around 70 0C.

Consider a refrigeration capacity of dairy as 30TR(2100kJ/min)

Assuming 15%heat recovery from it gives us 945kJ/min

20 hrs operation per day provides 1.13 GJ /day.

Fuel heat gives 2488kJ/kg. (for furnace oil)

(Assuming Density 0.94, efficiency .80)

Effective heat 1870 kJ/lit

Savings /day/TR /16 hr Operation of refrigeration system = 0.60 lit

Temperature controlled bath

Valve 24℃

Water pass

Refrigerant pass Compressor

35℃ Condenser Air

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Desuperheater

Hot water 65℃

Existing air-conditioning system


10. 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

600C without affecting refrigeration cycle. Simultaneous generation of chilled

water and usable hot water results in

1. Increased output of heating system

2. Reduced fuel consumption and

3. Enhanced overall system efficiency.

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Figure3. Circuit chart of experimental equipment


11.REFERENCE :1. HEAT CHILLER RECOVERY published by ASHRAE in 1999

2. A COURSE OF REFRIGERATION AND AIR-CONDITIONING by

S.C. Arora and S. Domkundwar

3. Best Energy Systems Inc. acrobat file BRAMPTON case study

4. www.agr.gc.ca

5. Internet.oit.edu

6. Fact sheet EES-26 published by UNIVERSITY OF FLORIDA

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http://www.agr.gc.ca/

desuperheater

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