Analysis
Climaveneta S.p.A

Full load efficiency of FOCS and FOCS-CA units

Part load efficiency of FOCS and FOCS-CA units – Analysis performed by comparing the IPLV and ESEER indices

Part load efficiency of FOCS and FOCS-CA units – Analysis performed by estimating annual power consumption according to the climate in the place of installation

Part load efficiency of FOCS and FOCS-CA units – Analysis performed by estimating annual power consumption according to the climate in different places of installation



Full load efficiency of FOCS and FOCS-CA units

The FOCS and FOCS-CA units use the new screw compressors optimised for R134A.
The outstanding efficiency of these compressors, together with the accurate design of the condensing and evaporating section, boost unit performance and obtain elevated EER values.



Fig. 1 Total EER of FOCS-CA and FOCS units according to cooling capacity

As of today, FOCS-CA, with EER>3.1 for all sizes, is the only air-chilled liquid chiller to belong to energy efficiency “class A”, according to the standards proposed by EECCAC, the “Energy Efficiency and Certification of Central Air Conditioners” work programme promoted by the European Community (see Tab. 1).
For the FOCS units, energy efficiency always remains between 2.7 and 3.1.

Tab.1 Energy labelling (EECCAC proposal - Energy Efficiency and Certification of Central Air Conditioners) for air-cooled chillers according to EER values.

Efficiency class

Limit EER values
A
EER>3,1
B
2,9<EER<3,1
C
2,7<EER<2,9
D
2,5<EER<2,7
E
2,3<EER<2,5
F
2,1<EER<2,3
G
EER<2,1



Part load efficiency of FOCS and FOCS-CA units – Analysis performed by comparing the IPLV and ESEER indices


Increasingly closer attention is being paid towards the power consumption of air-conditioning equipment.

The EER energy indices, calculated at rated full load conditions, are certainly an essential initial parameter for valuating the efficiency of the unit and, in particular, for making a preliminary comparison between different chillers.
Given that they refer to rated full load conditions, the EER indices do not consider the behaviour of the unit when works at part loads.
Chillers for air-conditioning, in fact, are mainly used with external air temperatures that are lower than rated values as well as at reduced load conditions when the separation stages of the cooling compressors are used.
This takes into account the fact that, in most conditioning systems, the lower the external air temperature is, the lower the ambient load gets. This is due both to the reduced amount of heat transmitted through the walls of buildings and to the reduced amount of cooling power required to treat the intake air.


These considerations lead to a more attentive choice of the unit and closer attention to part load efficiency, with evident economic advantages that will effect the initial purchase price, the cost of the system and running costs, and therefore the Total Cost of Ownership which is the sum of the costs over the entire lifetime of the product.

The market tends to consider large semi-hermetic compressors as unsuitable for working at part load conditions.
For this reason, great attention has been focused on multi-circuit units with more than one scroll compressor per circuit.
Scroll compressors in themselves are considered to be very efficient; the reduction in the cooling capacity obtained by turning individual compressors on and off allows system requirements to be tracked very precisely as one or more compressors working at full load are available for each step.
With this construction typology, it is essential to use leading-edge control units in order to optimise compressor operation by preventing them from stopping and starting too often and balancing their operating hours (FIFO function).
In part load and separated operating conditions, the reduction in power draw is greater than the reduction in power output, thanks to the oversized exchangers (condenser and evaporator) which are large enough to handle maximum refrigerant flow. The increase in the relationship between exchange surface and quantity of refrigerant boosts energy efficiency.

In the presentation of the new screw compressors optimised for R134a we saw how their full load efficiency is greater than that of R407C scroll compressors.
As regards behaviour at part loads, we also pointed out that these new screw compressors are fitted with an extremely efficient power adjuster working in either the infinite or four step modes.
The units fitted with screw compressors that can modulate power according to real load conditions obtain results that are higher than any system based on switching on and off individual compressors. Of course, this does not change the fact that the separation of screw compressors using a slide valve causes inefficiency but, as we will see later, these will never get near to neutralising the increased efficiency of the system.
Some considerations must also be made as regards construction and reliability.
In a hypothetical mono-circuit system with two compressors for each circuit, when one of the two compressors is switched off, power is reduced by 50%. This leads to an marked decrease in condensation temperature and an increase in evaporation temperature, with evident energy benefits. In units featuring these characteristics, great attention must be paid to variations: a sharp decrease in condensation temperature causes the liquid refrigerant, already at a higher temperature, to partially evaporate (formation of flash-gas in the liquid line). As a consequence, adjustment disturbances in the expansion valve feeding the evaporator occur. Similarly, switching on a compressor (rapid doubling of power) leads to strong oscillations in the adjustment circuit. This is often associated with insufficient overheating.
These effects must be carefully valuated and controlled. For these reasons, Climaveneta has always paid the greatest attention to the accurate design and testing of multiscroll units.

To compare the real part load efficiency of a unit fitted with R134a screw compressors against that of a multiscroll R407C unit, we considered two types of chiller that were configured in order to give approximately the same cooling power in rated conditions: about 660 kW with air at 35°C and water at 12-7°C.
The first chiller is a FOCS-CA 2712/B. At rated conditions, the unit provides a cooling capacity of 666 kW, with a compressor power draw of 190 kW and a total power draw of 208 kW. The unit is fitted with:

- 2 screw compressors designed for R134a applied to two separate cooling circuits;
- 1 direct expansion shell and tube evaporator with a dual cooling circuit and a single water circuit. Two asymmetrical passages on the coolant side optimise performance with R134a;
- 4 condenser coils;
- 12 six-pole fans with a diameter of 800 mm;
- R134a refrigerant.

For the second unit we took a chiller fitted with:

- 10 rated 25 Hp scroll compressors designed for R407C fitted to 4 cooling circuits; two circuits each with 3 compressors in parallel and two circuits each with 2 compressors in parallel;
- 2 large-size plate exchangers for R407C with an overall heat exchange efficiency superior to that of a shell and tube exchanger (T evap. Scroll > T evap. Screw);
- 8 generously sized condensing coils with an identical overall exchange surface to that of the chiller with two screw compressors;
- 12 six-pole fans with a diameter of 800 mm;
- R407C refrigerant.

Calculations gave the following results:

Unit with screw compressor FOCS-CA 2712/B:
Cooling capacity 666 kW
Total power draw 208 kW
EER 3.20

Unit with 10 scroll compressors
Cooling capacity 662 kW
Total power draw 264 kW
EER 2.51

At full load, the FOCS-CA 2712/B unit therefore features a total EER efficiency of 3.20 against 2.51 for the R407C multiscroll unit.

To compare the efficiency of the two systems, we can calculate the European IPLV index (called IPLVe for short) and the ESEER index. For a detailed description of the meaning of these indices and how to calculate them, please consult the energy index presentation.
These indices are calculated by evaluating the EER of the two units in 75%, 50% and 25% part load conditions, assuming that the external air temperature at these conditions is 30°C, 25°C and 20°C respectively.

Load
%
100%
75%
50%
25%
Air temp.
°C
 35
30 
25 
20 
Type of chiller
 
Scroll
FOCS-CA
2712/B
D%
Scroll
FOCS-CA
2712/B
D%
Scroll
FOCS-CA
2712/B
D%
Scroll
FOCS-CA
2712/B
D%
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Cooling Capacity
kW
662
666
+0,006%
497
 500
+0,006%
331
333 
+0,006%
166
167 
+0,006%

Compr. power draw

kW
229
190
-17,0%
136
118 
-13,2%
70,7
66 
-0,07%
28,8
31,1 
+7,99%
Total power draw
kW
264
208
-21,2%
155
131 
-15,5%
83,3
74,7 
-10,3%
35,1
35,5 
+0,01%

Total EER

kW
2,51
3,20
+27,5%
3,21
3,81 
+18,7%
3,98
4,46 
+12,1%
4,69
4,69 
-0,006%

Tab.2: chiller performance at part loads, FOCS-CA 2712/B with screw compressors optimised for R134a and multiscroll with ten R407C compressors:

At full load conditions, the FOCS-CA 2712/B unit with two screw compressors optimised for R134a is much more efficient than the same unit with 10 scroll compressors optimised for R407C (+27.5%).
As the load decreases, the efficiency of the multiscroll unit tends to get closer to that of the FOCS-CA unit optimised for R134a , approx. arriving at the same value for 25% part load condition.

Using the data from Tab.2 in the formulas for calculating IPLVe and ESEER, we obtain the values shown in Tab.3.
It can be seen that the percentage variations between the ESEER and IPLVe indices are not particular high, +10.6 % and +11.7 % respectively in favour of the chiller with two screw compressors optimised for R134a.

CHILLER TYPE
SCROLLR407C
FOCS-CA
SCREW/SCROLL
ESEER
3,85
4,26
10,6%
IPLVe
3,75
4,19
11,7%

Tab.3: IPLVe and ESEER comparison between a FOCS-CA 2712/B optimised for R134a and a chiller with 10 scroll compressors optimised for R407C:

If we suppose a seasonal demand for cooling power of 396,288 kWh (see following paragraph for calculation method), we can estimate the total power draw of the two chillers, both when using the IPLVe indicate and when using the ESEER index (tab. 4)
The total power draw is calculated as follows:

Power draw = Power demand (summer) / Index of efficiency at integrated part load

CHILLER TYPE
SCROLLR407C
FOCS-CA R134a
SCREW/SCROLL
Seasonal cooling power
396.288
396.288
 
ESEER
3,85
4,26
 
Total power draw (ESEER)
102.932 kWh
93.025 kWh
-9,62 %
IPLV
3,75
4,19
 
Total power draw (IPLVe)
105.677 kWh
94.579 kWh
-10,5 %

Tab.4: comparison of total power draw of a FOCS-CA 2712/B with R134a and a 10 scroll chiller with R407C assuming a seasonal demand for cooling power of 396,288 kWh.

The difference in annual power consumption is 9,907 kWh, a reduction of 9.62% in favour of the FOCS-CA chiller optimised for R134a using the ESEER index as the valuation parameter, while the difference is 11,098 kWh, equal to 10.5%, if the IPLVe index is used.
Supposing electricity costs 0.13 euros/kWh, the annual saving is 0.13 X 9,907 = 1,288 euros/year if calculated according to the ESEER index and 0.13 X 11,098 = 1,442 euros/year if calculated according to the IPLVe index.




Part load efficiency of FOCS and FOCS-CA units – Analysis performed by estimating annual power consumption according to the climate in the place of installation

We will now estimate annual power consumption for the two chillers, assuming:


- installation in a typical central Italian town. We will take Perugia as an example;
- evaporator inlet/outlet water temperature: 12 / 7°C;
- a linearly variable load demand starting from a maximum of slightly below the cooling capacity of each of the two chillers with an external air temperature of 35°C (640 kW) to a minimum of 0 kW with an external air temperature of 15°C.

This is an acceptable estimate for residential air-conditioning installations in the Mediterranean area, where a total load is supposed to be proportional to external air temperature and therefore to the transmission loads for the introduction of fresh air.

The values of this power consumption analysis for with the external climate of Perugia are shown in tables 5) and 6).

Tab.5 FOCS-CA 2712/B – time zone 8-18 – Perugia.

External air
temperature
Demand
External air temperature distribution
tot EER
Total power draw
Electricity draw
Cooling power delivered
°C
KW
hours/year
kW/kW
KW
kWh
KWh
A
B
C
D
E=B/D
F=E*C
G=B*C
15
0
325
0
0
0
0
17
64
288
5,39
11,9
3420
18432
19
128
269
5,14
24,9
6695
34432
21
192
292
4,90
39,2
11445
56064
23
256
292
4,66
54,9
16031
74752
25
320
258
4,46
71,7
18511
82560
27
384
129
4,15
92,4
11924
49536
29
448
80
3,88
115,4
9235
35840
31
512
44
3,61
141,9
6244
22528
33
576
34
3,34
172,2
5856
19584
35
640
4
3,20
200,0
800
2560
 
TOTALE
2015
 
90161
396288

Tab.6 10-scroll unit for R407C – time zone 8-18 – Perugia

External air temperature
Demand
External air temperature distribution
tot EER
Total power draw
Electricity draw
Cooling power delivered
°C
KW
ore/anno
kW/kW
KW
kWh
KWh
A
B
C
D
E=B/D
F=E*C
G=B*C
15
0
325
0
0
0
0
17
64
288
5,21
12,3
3540
18432
19
128
269
4,91
26.0
7007
34432
21
192
292
4,61
41,6
12157
56064
23
256
292
4,32
59,3
17309
74752
25
320
258
3,98
80,4
20744
82560
27
384
129
3,71
103,4
13338
49536
29
448
80
3,40
131,7
10535
35840
31
512
44
3,10
165,2
7268
22528
33
576
34
2,82
204,5
6954
19584
35
640
4
2,51
255,0
1020
2560
 
TOTALE
2015
 
99872
396288

The comparison between annual power consumption of the two different chillers is summarised in fig. 3


The comparison between annual power consumption of the two different chillers is summarised in fig. 3


From tables 5) and 6) we can deduce the annual power consumption of the two chillers:

- FOCS-CA 2712/B chiller with R134a: 90161 kWh


- chiller with ten scroll compressors optimised for R407C: 99872 kWh


The difference in annual power consumption is 9,711 kWh, amounting to 9.7%, in favour of the FOCS-CA 2712/B chiller with two screw compressors running on R134a.
Assuming the cost of electricity to be 0.13 euro/kWh, the annual saving amounts to = 0.13 X 9,711 = 1,262 euros/year.

It should be pointed out that, at the same seasonal cooling demand, the difference in the total power draw of the two chillers, calculated according to the climate in the place of installation, practically coincides with the value calculated using the IPLVe index and, in particular, the ESEER index.
This shows how the use of these indices offers an extremely rapid and reliable valuation of the power consumption of cooling units as it also considers part load operation.

CHILLER TYPE
SCROLLR407C
FOCS-CAR134a
Seasonal cooling power
396.288
396.288
Total power draw (calculated)
99872 kWh
90161 kWh
ESEER
3,85
4,26
Total power draw (ESEER)
102.932 kWh
93.025 kWh
IPLV
3,75
4,19
Total power draw (IPLVe)
105.677 kWh
94.579 kWh

The IPLVe or ESEER indices therefore allow us to calculate the seasonal electricity draw and compare various chillers, bearing in mind the differences in their part load performance.




Part load efficiency of FOCS and FOCS-CA units – Analysis performed by estimating annual power consumption according to the climate in different places of installation

Similar evaluations to the above have been made, considering the two different chillers and always using a linearly variable load, with climate figures relative to round-the-clock operation in various European cities.

 
 
Tipo Chiller
 
10 Scroll R407C
FOCS 2712/B R134a
 
Absorption kWh/year
Absorption kWh/year
Saving kWh/year
Saving
%
 
Perugia
152.801
138.967
13.834
-9,05%
Milano
138.738
126.580
12.158
-8,76%
Francoforte
86.420
79.306
7.114
-8,23%
Parigi
76.557
70.573
5.984
-7,82%
Londra
50.337
46.920
3.417
-6,79%

Table 7 Summary of annual power consumption in various European cities.

In this case, power consumption was not evaluated by applying the ESEER or IPLV indices, but considering the effective performance of the unit over an operating period of one year.

The advantage of the chiller with screw compressor running on R134a compared with the multiscroll varies according to the climate conditions relative to various European cities. The advantage, ranging between 6.79% and 9.05% decreases as we pass from the Italian climate to the central/northern European climate because, maintaining the same comparison criterion (maximum load at an external temperature of 35°C and no load with external air at 15°C), part load operation becomes more important. In this condition, the EER of the multiscroll tends to reduce the advantage of the screw compressors.
In all cases, power consumption of the chiller with screw compressors running on R134a is always significantly lower than that of the chiller with 10 scroll compressors running on R407C.



Climaveneta S.p.A