NH3 – H2O Absorption Systems Used for Research and Student ...

NH3 – H2O Absorption Systems

Used for Research and Student Activities

IOAN BOIAN, ALEXANDRU SERBAN, STAN FOTA, FLOREA CHIRIAC

Building Facilities Department

Transylvania University of Brasov

Turnului Street, No. 5, Brasov

ROMANIA

[email protected], [email protected], [email protected],

[email protected]; www.unitbv.ro

Abstract: - In the context of the sustainable development and of the future environment and energy concerns, a

new laboratory was developed based on absorption systems (a chiller-heater and a heat pump). The installation

together with the proposed experimental activity for this setup is hereby presented followed by a cycle

simulation illustrated by calculating the parameters of the ammonia absorption process. The student activity is

intended to familiarize the participants with the problems of energy efficiency, environmental development

and new facilities based on advanced technology. Research will be focused on the integration of such units in

the specific local features and on comparisons with vapor compression systems or traditional fuel-based

equipments.

Key-Words: - Environment, heat pump, absorption systems, education and research activity

1 Introduction The Transylvania University Installations

Department has completed and now operates a

complex HVAC system for cooling and heating of

indoor spaces through the interconnection of two

ammonia-water absorption systems manufactured by

ROBUR Company, shown in Figure 1.

Both equipments have been placed on a platform

outside the building. This way the chiller fan noise

(57 dB) and accidental ammonia contamination are

avoided. The chiller is operated as refrigeration

machine and the heat pump can be operated in a

reversible mode as a chiller-heater using heat energy

to provide cooling or heating. Both equipments are

interconnected supplying with chilled or warm water

the fan coils located into the faculty rooms

depending on the seasonal requirements. The

absorption heat pump is preparing warm water up to

60 oC recovering heat from the outside air. The

ammonia is the refrigerant being absorbed by the

water –the absorption fluid. For the balancing of the

water circuits a mose hydraulic separator is used.

The nominal temperature of the chilled water is

7.2°C returning back to the unit with 12.7°C for an

outside air temperature of 35°C. A 150 l storage

tank for hot water is installed in the underground

room next to the heater. The rest of the equipment,

circulation pumps, accumulators filters, and the

electrical wirring, direct measuring devices, and the

Fig.1. The heat pump and the chiller platform

Fig.2. The equipment installed the other side of the

wall next to the chiller and heat pump

Proceedings of the 8th WSEAS International Conference on SYSTEM SCIENCE and SIMULATION in ENGINEERING

ISSN: 1790-2769 131 ISBN: 978-960-474-131-1

loggers for the aquisition of data provided by the

sensor are located in the adjacent room presented in

Figure 2.

2 The Structure of the Installation

Two ROBUR absorption machines- single-effect gas

fired- one operating only as a chiller and the other,

reversible, working as a chiller or as a heat pump are

connected as shown in Figure 3.The schematic of

the ammonia absorption system and its components

are shown in Figure 4.

The main components of such a system are:

• The boiler is a direct-gas fired generator

supplied with heat from a direct-fired burner; the

stripping process takes place in its upper side

called analyzer .

• The absorber has two sections: the pre-

Fig.3. Hydraulic setup and component denomination

Fig.4. Schematic of the ammonia absorption system

Proceedings of the 8th WSEAS International Conference on SYSTEM SCIENCE and SIMULATION in ENGINEERING

ISSN: 1790-2769 132 ISBN: 978-960-474-131-1

absorption takes place in the solution-cooled

absorber, over the helical coil bearing weak

absorbent; the final absorber is cooled by means

of the atmospherically air.

• The reflux condenser (or the rectifier), being

cooled by the strong solution refluxes the

ammonia-condensate back to the generator

concentrating it in the vapor coming from the

generator.

• The solution pump is a pulse pump having a

reciprocating motion. It discharges strong

solution to the generator by means of a flexible

sealing diaphragm.

• The condenser is a finned-tube air-cooled

exchanger.

• The evaporator is a shell-and-tube heat

exchanger providing the maximum refrigeration

effect per unit mass of refrigerant.

• The sub-cooling economizer RHX is a tube-in-

tube heat exchanger.

3 Operating Principles

and Characteristics The chiller is an AYF 60-119/4 standard version

having a cooling capacity of 17.49 kW at a nominal

chilled water flow of 2735 l/h and a gas

consumption of 2.51 m3/h. The maximal sound

pressure level is 57 dB(A). The complementary

heating module supplying 2000 l/h of domestic hot

water can be operated in both heating and cooling

seasons. It has a capacity of 32.5 kW and is

provided with a storage tank installed in the inside

vicinity. To avoid the danger of possible freezing

during the heating season the circuit between the

heating module and the storage tank is filled with

antifreeze solution.

The chilled water at 7°C provided by the chiller and

by the heat pump operated as a chiller too is pumped

into the fan coils placed inside the rooms. As a result

the inside air is cooled and dried and the water

returns warmer at approx. 12°C to the refrigeration

units. Figure 5 shows the influence of the outside air

temperature on the cooling capacity for three values

of chilled water temperatures leaving the chiller. [8]

The GAHP-AR type heat pump has a heating

capacity of 35.2 kW for a thermal input of 25.2 kW.

Its cooling capacity (in the reversible operation

mode) is 16.9 kW. The unit recuperates 54 kW from

the ambient air for every 100 kW resulting from the

natural gas burned inside the equipment. The 144

kW heating capacity resulted is accompanied with a

flue loss of 10 kW.

During the heating season the heat pump supplies

the above mentioned fan-coils with warm water

having a maximum temperature of 60 oC even at

negative ambient-air temperatures, i.e. -20°C. But

the heating performance is affected by the outside

dry bulb temperature and also by the hot water

temperature leaving the unit, as shown in Figure 6.

4 Research and Student Activity This installation was realized to be used for different

research studies at doctoral and MSc level but it is

also useful for testing activities (AHU, cooling

rooms, hydronic coils for heating/cooling, radiators).

The experimental activity will be focused on:

• The determination of the system

preformance for different exterior

atmospheric conditions depending on local

climat and season, as well as on operating

limitations too

• The indoor air quality study

• The efficiency comparison of the cooling

systems based on absorption versus vapor

compression.

• The comparative study of heating systems

based on absorption heat pumps and on

conventional boilers using fossil-fuels

respectively.

24

26

28

30

32

34

36

38

40

42

-25 -20 -15 -10 -5 0 5 10 15 20 25 30

Outside Dry Bulb Air Temperature, deg C

Heating Capacity, kW

30°C

45

50

60

Fig.6. The heating capacity of the heat pump

as a function of outside temperature

in case of four values of hot water leaving temperature

10

12

14

16

18

20

22

10 15 20 25 30 35 40 45 50

Outside Air Temperature, deg C

Cooling Capacity, kW 3

7

10

Fig.5. The affected cooling capacity by

the outside air temperature and by

the chilled water leaving temperature

Proceedings of the 8th WSEAS International Conference on SYSTEM SCIENCE and SIMULATION in ENGINEERING

ISSN: 1790-2769 133 ISBN: 978-960-474-131-1

4.1. Example: System performance

evaluation

The nominal value of the heating/cooling capacity is

increased or decreased by a factor ROC (Relative

Output Capacity) [7] depending on the conditions in

which the system is operating as shown in Figure 7.

The Coefficient of Performance (COP) is also

influenced by the evaporation temperature and by

the cooling water temperature [3] as illustrated in

Figure 8.

4.1.1 Parameters to be measured The necessary parameters for the calculation of the

performance of the absorption system are pressures,

flow rates, relative humidity, and temperatures in

different cooled rooms, as follows:

Table 1

Pressure, bar

Solution Entering solution pump pa Leaving pA

Flow rate

Flue

gases

Leaving generator GVɺ [ sm3

]

Chilled

water

smɺ [ skg ]

Temperature, °C

Ambient Entering refrigeration tc

air Leaving unit t i

Strong

solution

Entering solution pump tsb

Leaving reflux

condenser tsbR

Entering pre-absorber

tsbAi Leaving tsbAe

Entering generator tsbi

Weak

solution Leaving generator tss

Solution Entering

pre-absorber tpAi

Leaving tpAe

Liquid Leaving condenser tc Entering evaporator tcs

Vapor

Leaving

generator/reflux

condenser tC/R

evaporator to sub-cooling toS

Chilled

water

Entering refrigeration

unit

ts1

ts2

Entering

fan-coils

tsi1,

…n

Leaving tse1…

n

Cooled

rooms ti

For the relative humidity the wet bulb and the dry

bulb temperature are necessary to be used in the

psychometric chart.

The output capacity of the absorption heat pump

GAHP-AR depends on the ambient air temperature

The needed solution temperature to drive the

desorption process with ammonia-water is in the

range between 120°C to 130°C. Temperatures in this

range can be obtained using low cost non-tracking

solar collectors. At these temperatures, evacuated

tubular collectors may be more suitable than flat-

plate collectors as their efficiency is less sensitive to

operating temperature. But the evaporation

temperature is determinant for the source

temperature existing inside the generator as

illustrated in Figure 9 [3], [2].

.

Fig. 7. Average output capacity of water-to water

NH3/H20 absorption heat pumps

versus source and sink temperatures

Fig.8. The COP for ammonia/water absorption equipment in

refrigeration applications

Fig.9. Required resource temperatures for ammonia/water

absorption equipment

Proceedings of the 8th WSEAS International Conference on SYSTEM SCIENCE and SIMULATION in ENGINEERING

ISSN: 1790-2769 134 ISBN: 978-960-474-131-1

4.1.2 Cycle simulation and COP evaluation

The weak solution mass fraction ξw is fixed by the

temperature of the solution originated in the

generator as liquid T4 and the pressure existing in

the generator pgen this one being determined by the

temperature existing in the condenser, T8. Similarly,

the strong-solution mass fraction ξs is resulting from

the temperature existing in the absorber Tabs and the

pressure from the evaporator pevap (corresponding to

the saturation temperature Tevap). The following

temperatures will be considered for this example as

operational conditions

• evaporation 5.06°C (515 kPa)

• condensation 37.82°C (1461 kPa)

• boiling 95°C

• absorption 40.56°C

The resulting mass fraction for the weak- and the

strong-solution respectively from the Duhring plot

of the vapor pressure of ammonia–water solutions is

illustrated in Figure 10 [3].

The circulation factor is calculated with the

expression

ws

s

ξξ

ξλ

−

−=

1 (1)

For this case the circulation factor is 86.5=λ

The specific heat transfer for the evaporator, will be

evaluated per unit mass of ammonia using the

specific enthalpy at points 10 and 11, see Figure 11,

1011 hhqevap −= =1133.9 kJ/kg (2)

The enthalpy for the states at points 7…13

(saturated ammonia having a mass fraction very

close to unity) can be evaluated from Properties of

Saturated Liquid and Saturated Vapor Tables [6].

The mass flow rate through the evaporator results as

evap

evap

q

Qm

ɺ

ɺ = (3)

For a cooling capacity of kWQevap 49.17= a mass

flow rate of 0,925 kg/min is resulting.

The specific heat transfer for the condenser,

absorber, and generator and for reflux condenser,

sub-cooling economizer can be written as

78 hhqcond −= (4)

89 hhqRHX −= (5)

45 hhqSHX −= (6)

( )15612 hhhhqabs −⋅+−= λ (7)

( )2527 hhhhqgen −⋅+−= λ (8)

For the states at points 1…6 and 14 the enthalpy

corresponds to solutions of ammonia in water and

can be evaluated from the tabulated values or from

the enthalpy–concentration diagram presented in

Figure 12. [6].

The heat rate for every component of the system is

calculated by multiplying the specific heat transfer

values calculated as above through the mass flow

Fig.10. The mass fraction of weak- and strong ammonia

solution

Fig.11. Single-Effect Ammonia/Water Absorption Cycle

Proceedings of the 8th WSEAS International Conference on SYSTEM SCIENCE and SIMULATION in ENGINEERING

ISSN: 1790-2769 135 ISBN: 978-960-474-131-1

rate: kWQcond 08.18= , kWQRHX 48.1= ,

kWQabs 30.27= kWQSHX 2.13= ,

kWQgen 0.29= W=123 W.

Finally, the coefficient of performance is evaluated

as

60.0==cond

evap

Q

QCOP

In case of the heat pump the corresponding COP is

1.4.

5 Conclusion Absorption chillers were introduced in the 1950s

and found a relatively large market until the late

1970s when vapor compression systems have been

considered more economical. Being a heat-activated

equipment absorption heat pumps and chillers can

save considerably fuel by using environmental or

waste heat available at temperature that is low

enough. A reduction of the carbon dioxide emission

together with a global warming potential is

resulting. In fact absorption systems exchange heat

with three thermal reservoirs contributing to overall

energy efficiency. The use of an absorption chiller

during high summer-cooling demand periods or

even in normal operating hours is economically

beneficial especially in case of a favorable cost ratio

of electricity to natural gas. Absorption systems may

have a simple payback of several years although

they have a higher initial cost than centrifugal

compressors. In today’s deregulating energy

industry, market forces are converging to bring back

into the market the air-cooled ammonia-water

absorption-gas air conditioners for residential or

light commercial applications. For small cooling

loads and for applications where water cooling it is

not possible to use, an H2O/NH3 system is preferred

[1].

Ammonia is a highly energy-efficient refrigerant

being an alternative for new and existing

refrigerating and air-conditioning systems having

high latent heat of vaporization (9 times greater than

R-12), and has a low boiling point. Ammonia is not

a contributor to greenhouse effect or global

warming, to ozone depletion (zero ODP), and is

environmentally benign [5].

Beyond the advancement and the transfer of

technology, the education of future engineers is a

key point not only in energy efficiency issues but

also in the sustainable environmental development.

For reasons like these the laboratory presented

above was developed.

References:

[1] Balaras, A. C., Henning, H. M., Wiemken, E.,

Grossman, G., Podesser, E., Ferreira, C. A. I

Solar Cooling. An Overview of European

Applications & Design Guidelines. ASHRAE

Journal, Vol. 48, 2006, pp.14-21. [2] Boian, I., Tzachanis, A. The Response of Water-

LiBr Solution Working Parameters at

Temperature Changings. 2nd Conference on

Sustainable Energy Transilvania University of

Brasov July 3-5, 2008

[3] Gosney, W. B:. Principles of Refrigeration.

Cambridge University Press, 1982.

[4] Hirai, W. A. Feasibility Study of an Ice Making

and Cold Storage Facility Using Geothermal

Waste Heat," Geo-Heat Center, Klamath Falls,

OR. 1982

[5] *** Ammonia as a Refrigerant. Position

Document, reaffirmed by ASHRAE Board of

Directors, January 26, 2006

[6] ***ASHRAE Handbook of Fundamentals

p. 30.34, and p. 30 68. 2009

[7] *** CEN/TC 228 WI 024:2005 (E). Heating

systems in buildings — Method for calculation

of system energy requirements and system

efficiencies — Part 2-2.2: Space heating

generation systems, heat pump systems. p. 60,

2005.

[8] *** GA Range – AYF Series; AY Range - AY

Series: Installation Use and Maintenance Manual

pp.20-22. 2006.

Fig.12. Specific enthalpy versus mass fraction

Proceedings of the 8th WSEAS International Conference on SYSTEM SCIENCE and SIMULATION in ENGINEERING

ISSN: 1790-2769 136 ISBN: 978-960-474-131-1