nh3 – h2o absorption systems used for research and student
TRANSCRIPT
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