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Measurement of an air-cooler capacity in normal operating conditions
Measurement of an air-cooler capacity in normal operating conditions S. Cichocki and K. Wolek
Mesure de la puissance d'un refroidisseur d'air en r6gime de fonctionnement normal Dans ce rapport on pr#sente une nouve//e rn#thode de mesure indirecte de la puissance frigorifique d'un
refroidisseur d'air. On indique le principe de mesure de cette puissance suivant une nouvel/e m#thode ainsi que certains avantages par rapport aux m#thodes pr#c#dentes. Le principal avantage est que /'on peut #tudier les refroidisseurs d'air install#s dans des entrep6ts frigorifiques. Cette m#thode soutient favorablement la comparaison avec /es normes de I'ASRE.
In the paper a new method of indirect measurement of air-cooler refrigerating capacity is presented. The theoretical basis of measurement of air cooler capacity by the new method is given and some advantages
over previously used methods are described. The main advantage of the n e w method is that air-coolers installed in cold stores can be investigated. The method compared favourably w i t h ASRE Standards.
Nomenclature Ap
kp P PF Pw to tp .wp
area of heat transfer surface, m 2 overall coefficient of heat transfer, W m -2 K -1 power of the heater, W power of the fan, W power of the humidifier, W temperature of the refrigerant boiling in the evaporator, °C air temperature at the air-cooler inlet, °C air flow velocity measured at the air-cooler inlet, m s -1
This paper presents results of studies on a theoretically developed and experimentally verified new method of testing air-coolers which proved to be as accurate as the hitherto applied methods and free from their disadvantages. The method is being applied in practice 3.4 and continually improved.
Laboratory testing conditions Testing of the performance of forced circulation aircoolers is usually realized by measurements made in special balanced ambient, or calibrated room type calorimeters. The test conditions and procedures used in such calorimeters are fixed by standards to be theoretically identical to actual operating conditions of air-coolers working in the refrigerating
z~t Atpo q~o Aq~o q~p
temperature difference tp2--tpl, K superheating of the refrigerant vapour, K capacity of the air-cooler, W divergence of measured results compared with ASRE method results, W relative humidity of air at the air-cooler inlet
Subscripts 1 2
with the container open with the container closed
installations of various cold stores. In practice the performances of air-coolers determined on the basis of measurements in calorimeters may differ greatly from their performances in real operating conditions. They may differ even at the same thermal conditions in calorimeters. This is because the influence of the method of feeding of air-coolers, the defrosting arrangements, and the oil content of the refrigerant may prove more important in practice than is allowed for in laboratory conditions. These disadvantages can be avoided by using the method presented in this paper. The method has been developed, tested and patented in Poland.
The measurement system The authors are from the Research and Development Centre of Chemical Equipment Industry, Krak6w, Poland. Paper received 29 January 1981.
Measurements can be realized in a cooling chamber in which the air-cooler is operated in a similar
Volume 4 Number 6 November 1981
0140-7007/81/060345-0352.00 @1981 IPC Business Press Ltd and IIR
,345
The measurement system (Fig. 2) consists of the container, 2, with the hinged front flap, 8, located within the cold room, 1, and equipped with the compensation unit, 3, air-guide, 4, thermometer, 5, and evaporator coil, 10. The container can also be fitted with a humidifier, 6 and a bulb, 7, for measurement of humidity. Inside the cold room there are a number of thermometers, 9, for measurement of air temperure. The evaporator saturation pressures or temperatures are measured at 11, and air flow rate at 13. The compensation unit, 3, consists of an electric heater, 12, of power input P and the humidifier of power input Pw. The unit is simple to install and dismantle in normal operating conditions and the air flow through the air-cooler remains undisturbed with the front flap open or closed.
The principle of the method depends on the production of the so called adiabatic enclosure around the air-cooler being tested. This adiabatic enclosure is created by means of the insulated container, 2, air cooler and the compensating heater, The variables which are measured are tp, to, ep and Wp. Measurements are gathered in two stages; initially with container, 2, open and later with it closed. Secondary subscripts 1 or 2 indicate values for these respective conditions. The method requires that working parameters have reached steady values, in both stages. The air-cooler capacity with the container, 2, open is as follows: (1)
tol )
The air-cooler capacity with container 2 closed is ~o2---- kpAp(tp2 - to2)
.: ..] \.
i
12
3
6
4
13 7 5
9
:":"$: I:. !: :.I:-; :.L :: :: " L : #: : :.t.: :...t..:..l
..LI...:.I...I
.'
I,
f
ii
\\
II "
"
:"": ' " I " " : ' " :
"'
:":"'
. . . . . .......
/ 9
2
9
I0 II
Fig. 2
System for measurement of air-cooler capacity
Fig. 2 d'air
/nsta//ation de mesure de/a puissance d'un refroidisseur
(/)ol is unknown, ~Oo2 is found from power measurements P, PF and Pw. Applying of the temperature difference ( t p - t o ) in (1) and (2) is justified by Schroth's results. 3
The m e a s u r e m e n t procedure
Description of the m e t h o d
q) ol = k p A p ( tpl -
8
I
environment to a cold storage room or refrigerated vehicle. The tested air-cooler is screened with baffles of light structure and suitable insulating properties which form the measurement container (Fig, 1 ).
=P+Pw+PF
(2)
Before the beginning of the measurements, the power of the heater, 12, is regulated in such a way that the air temperature tp~ with the container open is equal to temperature tp for which the air-cooler capacity is to be determined. The measurements are usually carried out in such a way that the air temperatures with the container open and closed have the same values within +0.5 K. The air-cooler refrigeration capacity is directly determined, at the temperatures of its operation tp and to, by measurement of the power input from the compensating heaters. If the power from the compensating heaters, q~o2, after closing of the container, is not equal to the aircooler power with the container open, Col, then the air temperature /p2 will not equal tpl. Assuming that k~4p is constant, it is possible to derive from (1) and (2) the air-cooler capacity in its working conditions (.tp=tpl and to=to1) (t0ol=
(/pl -- /ol )
-
(tp2- - / o 2 )
(P+Pw+PF)
(3)
Comparison of the results of tests and conclusions
Fig. 1 Container details, 1 container of insulating material: 2 - flap; 3 - tested air-cooler Fig. 1 D@tails de I'encein~e. 1 - e n c e i n t e on m a t # n a u 2 - vo/et, 3 - r e f r o i d i s s e u r d ' a i r en essai
346
isolant,
In order to verify the present method, measurements were made of the capacity of an air-cooler with evaporator area 128 m 2 in the calorimeter in accordance with the requirements of ASRE Standard 25-56 and also using the new method. Conditions were adjusted to maintain the same temperatures of air supplied to the air-cooler and the same superheating of the refrigerant vapour. The results are presented in Table 1. Differences between variables which were determined by
Revue Internationale du Froid
T a b l e 1. R e s u l t s o f m e a s u r e m e n t s
by different
methods
Tableau 1. R~sultats de mesures suivant diff~rentes m~thodes Measured parameters
ASRE method
tp,
°C
-5.2+-0.1
- 1 5.0 +- 0.1
Atno,
K
10.64-0.7
10.2+-0.7 -27.5+-0.2 12.0+0.2
to,,
°C
-17.9+0.2
q~oAs~E.
kW
1 6.4+-0.2 New method
G,
°C
Atpo,
K
to,
°C
(tpl-----tp2)
-5.2+-0.1
- 15.0+ 0.1
10.44- 0.7
12.4+-0.7
- 17.8+-0.2
-27.5+-0.2
q~ol,
kW
1 6.3+-0.2
12.0+-0.2
A~ o,
kW
- 0.1
0.0
A~o,
x 100%
-0.6
0.0
~OASRE
New method with partial compensation of air-cooler power (tp1 =~tp2) tp2,
°C
-6.1 -t-0.1
-16.4+-0.1
to2,
°C
-17.4+-0.2
-26.9+-0.2
15.2+0.2 -5.1 +-0.1
10.8+-0.2 -15.0+-0.1
P+PF+Pw( = tp ( = tpl),
~o2),
kW °C
Atpo,
K
9.0+-0.7
11.9+-0.7
t o ( = to1 ),
°C
~ol, Aq~o,
kW kW
-17.3+-0.2 16.4
-26.6+0.2 11.9
0.0
-0.1
0.0
-0.8
Aq~°"
x l O0
%
~OASRE
applying the t w o methods are w i t h i n the limits of errors in measurements and calculations. Savings obtained by using the described method, in comparison w i t h the ASRE methods, are as follows: one, the equipment is simpler and considerably cheaper; two, considerable reduction in the c o n s u m p t i o n of electric energy; and, three, reduction of labour costs for the measurements.
Volume 4 Num(~ro 6 Novembre 1 981
References
1 ASRE/ASHRAE/Standard method of rating air-coolers for refrigeration (American Society of Heating. Refrigerating and Air Conditioning Engineers Standard 25-56) 2 Schroth, H. H. Kennziffern in Luftbeaufschlagten Hochleistungsdampfern. Luft und Kaltetechnik 2 (1970) 93-95 3 Standard BN-76"2550-04 4 Reports issued at The Research and Development Centre of Chemical Equipment Industry, Krak6w
,347
Mesure de la puissance d'un refroidisseur d'air en r6gime de fonctionnement normal Dans ce rapport on pr#sente une nouve//e rn#thode de mesure indirecte de la puissance frigorifique d'un
refroidisseur d'air. On indique le principe de mesure de cette puissance suivant une nouvel/e m#thode ainsi que certains avantages par rapport aux m#thodes pr#c#dentes. Le principal avantage est que /'on peut #tudier les refroidisseurs d'air install#s dans des entrep6ts frigorifiques. Cette m#thode soutient favorablement la comparaison avec /es normes de I'ASRE.
In the paper a new method of indirect measurement of air-cooler refrigerating capacity is presented. The theoretical basis of measurement of air cooler capacity by the new method is given and some advantages
over previously used methods are described. The main advantage of the n e w method is that air-coolers installed in cold stores can be investigated. The method compared favourably w i t h ASRE Standards.
Nomenclature Ap
kp P PF Pw to tp .wp
area of heat transfer surface, m 2 overall coefficient of heat transfer, W m -2 K -1 power of the heater, W power of the fan, W power of the humidifier, W temperature of the refrigerant boiling in the evaporator, °C air temperature at the air-cooler inlet, °C air flow velocity measured at the air-cooler inlet, m s -1
This paper presents results of studies on a theoretically developed and experimentally verified new method of testing air-coolers which proved to be as accurate as the hitherto applied methods and free from their disadvantages. The method is being applied in practice 3.4 and continually improved.
Laboratory testing conditions Testing of the performance of forced circulation aircoolers is usually realized by measurements made in special balanced ambient, or calibrated room type calorimeters. The test conditions and procedures used in such calorimeters are fixed by standards to be theoretically identical to actual operating conditions of air-coolers working in the refrigerating
z~t Atpo q~o Aq~o q~p
temperature difference tp2--tpl, K superheating of the refrigerant vapour, K capacity of the air-cooler, W divergence of measured results compared with ASRE method results, W relative humidity of air at the air-cooler inlet
Subscripts 1 2
with the container open with the container closed
installations of various cold stores. In practice the performances of air-coolers determined on the basis of measurements in calorimeters may differ greatly from their performances in real operating conditions. They may differ even at the same thermal conditions in calorimeters. This is because the influence of the method of feeding of air-coolers, the defrosting arrangements, and the oil content of the refrigerant may prove more important in practice than is allowed for in laboratory conditions. These disadvantages can be avoided by using the method presented in this paper. The method has been developed, tested and patented in Poland.
The measurement system The authors are from the Research and Development Centre of Chemical Equipment Industry, Krak6w, Poland. Paper received 29 January 1981.
Measurements can be realized in a cooling chamber in which the air-cooler is operated in a similar
Volume 4 Number 6 November 1981
0140-7007/81/060345-0352.00 @1981 IPC Business Press Ltd and IIR
,345
The measurement system (Fig. 2) consists of the container, 2, with the hinged front flap, 8, located within the cold room, 1, and equipped with the compensation unit, 3, air-guide, 4, thermometer, 5, and evaporator coil, 10. The container can also be fitted with a humidifier, 6 and a bulb, 7, for measurement of humidity. Inside the cold room there are a number of thermometers, 9, for measurement of air temperure. The evaporator saturation pressures or temperatures are measured at 11, and air flow rate at 13. The compensation unit, 3, consists of an electric heater, 12, of power input P and the humidifier of power input Pw. The unit is simple to install and dismantle in normal operating conditions and the air flow through the air-cooler remains undisturbed with the front flap open or closed.
The principle of the method depends on the production of the so called adiabatic enclosure around the air-cooler being tested. This adiabatic enclosure is created by means of the insulated container, 2, air cooler and the compensating heater, The variables which are measured are tp, to, ep and Wp. Measurements are gathered in two stages; initially with container, 2, open and later with it closed. Secondary subscripts 1 or 2 indicate values for these respective conditions. The method requires that working parameters have reached steady values, in both stages. The air-cooler capacity with the container, 2, open is as follows: (1)
tol )
The air-cooler capacity with container 2 closed is ~o2---- kpAp(tp2 - to2)
.: ..] \.
i
12
3
6
4
13 7 5
9
:":"$: I:. !: :.I:-; :.L :: :: " L : #: : :.t.: :...t..:..l
..LI...:.I...I
.'
I,
f
ii
\\
II "
"
:"": ' " I " " : ' " :
"'
:":"'
. . . . . .......
/ 9
2
9
I0 II
Fig. 2
System for measurement of air-cooler capacity
Fig. 2 d'air
/nsta//ation de mesure de/a puissance d'un refroidisseur
(/)ol is unknown, ~Oo2 is found from power measurements P, PF and Pw. Applying of the temperature difference ( t p - t o ) in (1) and (2) is justified by Schroth's results. 3
The m e a s u r e m e n t procedure
Description of the m e t h o d
q) ol = k p A p ( tpl -
8
I
environment to a cold storage room or refrigerated vehicle. The tested air-cooler is screened with baffles of light structure and suitable insulating properties which form the measurement container (Fig, 1 ).
=P+Pw+PF
(2)
Before the beginning of the measurements, the power of the heater, 12, is regulated in such a way that the air temperature tp~ with the container open is equal to temperature tp for which the air-cooler capacity is to be determined. The measurements are usually carried out in such a way that the air temperatures with the container open and closed have the same values within +0.5 K. The air-cooler refrigeration capacity is directly determined, at the temperatures of its operation tp and to, by measurement of the power input from the compensating heaters. If the power from the compensating heaters, q~o2, after closing of the container, is not equal to the aircooler power with the container open, Col, then the air temperature /p2 will not equal tpl. Assuming that k~4p is constant, it is possible to derive from (1) and (2) the air-cooler capacity in its working conditions (.tp=tpl and to=to1) (t0ol=
(/pl -- /ol )
-
(tp2- - / o 2 )
(P+Pw+PF)
(3)
Comparison of the results of tests and conclusions
Fig. 1 Container details, 1 container of insulating material: 2 - flap; 3 - tested air-cooler Fig. 1 D@tails de I'encein~e. 1 - e n c e i n t e on m a t # n a u 2 - vo/et, 3 - r e f r o i d i s s e u r d ' a i r en essai
346
isolant,
In order to verify the present method, measurements were made of the capacity of an air-cooler with evaporator area 128 m 2 in the calorimeter in accordance with the requirements of ASRE Standard 25-56 and also using the new method. Conditions were adjusted to maintain the same temperatures of air supplied to the air-cooler and the same superheating of the refrigerant vapour. The results are presented in Table 1. Differences between variables which were determined by
Revue Internationale du Froid
T a b l e 1. R e s u l t s o f m e a s u r e m e n t s
by different
methods
Tableau 1. R~sultats de mesures suivant diff~rentes m~thodes Measured parameters
ASRE method
tp,
°C
-5.2+-0.1
- 1 5.0 +- 0.1
Atno,
K
10.64-0.7
10.2+-0.7 -27.5+-0.2 12.0+0.2
to,,
°C
-17.9+0.2
q~oAs~E.
kW
1 6.4+-0.2 New method
G,
°C
Atpo,
K
to,
°C
(tpl-----tp2)
-5.2+-0.1
- 15.0+ 0.1
10.44- 0.7
12.4+-0.7
- 17.8+-0.2
-27.5+-0.2
q~ol,
kW
1 6.3+-0.2
12.0+-0.2
A~ o,
kW
- 0.1
0.0
A~o,
x 100%
-0.6
0.0
~OASRE
New method with partial compensation of air-cooler power (tp1 =~tp2) tp2,
°C
-6.1 -t-0.1
-16.4+-0.1
to2,
°C
-17.4+-0.2
-26.9+-0.2
15.2+0.2 -5.1 +-0.1
10.8+-0.2 -15.0+-0.1
P+PF+Pw( = tp ( = tpl),
~o2),
kW °C
Atpo,
K
9.0+-0.7
11.9+-0.7
t o ( = to1 ),
°C
~ol, Aq~o,
kW kW
-17.3+-0.2 16.4
-26.6+0.2 11.9
0.0
-0.1
0.0
-0.8
Aq~°"
x l O0
%
~OASRE
applying the t w o methods are w i t h i n the limits of errors in measurements and calculations. Savings obtained by using the described method, in comparison w i t h the ASRE methods, are as follows: one, the equipment is simpler and considerably cheaper; two, considerable reduction in the c o n s u m p t i o n of electric energy; and, three, reduction of labour costs for the measurements.
Volume 4 Num(~ro 6 Novembre 1 981
References
1 ASRE/ASHRAE/Standard method of rating air-coolers for refrigeration (American Society of Heating. Refrigerating and Air Conditioning Engineers Standard 25-56) 2 Schroth, H. H. Kennziffern in Luftbeaufschlagten Hochleistungsdampfern. Luft und Kaltetechnik 2 (1970) 93-95 3 Standard BN-76"2550-04 4 Reports issued at The Research and Development Centre of Chemical Equipment Industry, Krak6w
,347