S. Grohmann, ST/CV
M. Olcese, EP/HC
G. Hallewell, V. Vacek, EP/ATI

1. INTRODUCTION

The choice of using C4F10 as refrigerant for cooling in the ATLAS Pixel Detector has several advantages. However, the low-pressure operation leads to some limitations, which have to be considered in the circuit design.

The general pressure course in a C4F10 evaporative cooling system is shown in fig. 1. The saturation pressure at –20 °C evaporating temperature is 0.46 bar absolute. Furthermore, a minimum system pressure of around 0.25 bar should be reasonable, which corresponds to a saturation temperature of about –32 °C at the compressor inlet.

Fig. 1: Pressure pattern for a refrigerating circuit with C4F10 at operating temperatures t_C/t₀ = 30/-20°C

These figures illustrate that there is only a pressure drop of roughly 200 mbar available to assure favorable heat transfer and flow conditions in the evaporator and suction line. Consequently, the suction line has to be as short as possible and routing to a compressor installed in USA 15 or on the surface seems to be excluded.

We are therefore proposing a 'short-loop circulation system' using a refrigerant supply pump. The idea is basically derived from so-called 'flooded systems', which are especially used for commercial ammonia chillers. The adaptations in circuit design for the ATLAS Pixel Detector are described as general concept below. The proposal is to be recommended for the ATLAS SCT Detector and other refrigerants beside C4F10 as well.

 

2. FLOODED EVAPORATION

The general scheme of a flooded refrigerating circuit working with high-pressure float regulator is shown in fig. 2. In certain applications this configuration is preferred to dry expansion systems because:
- a uniform contact of evaporating liquid along the whole heat exchanger surface is achieved, which leads to an even temperature profile and
- smaller temperature differences lead to smaller operating costs (irrelevant for ATLAS).

Generally, the cooling systems in the ATLAS SCT and Pixel Detectors have to operate in the same mode, i.e. it has to be guaranteed that there is still boiling liquid at the outlet of each stave. Otherwise the considerable worse heat transfer coefficient of superheated refrigerant would cause a significant temperature rise within the affected detector section.

It is therefore the obvious thing to apply elements of this scheme in the circuit design for the SCT and Pixel Detectors looking at the similarity of cooling demands.

3. CIRCUIT DESIGN FOR THE ATLAS PIXEL DETECTOR
1. General Concept

The pure adoption of the scheme according to fig. 2 doesn't solve the problem of limited pressure reserves with C4F10. Any flow resistance higher than 200 mbar (stave + suction line) would increase the evaporating temperature in the detector. For those reason the circuit has to be split (fig. 3).

The distance between the experiment and the service cavern USA 15, which is about 100 m, can be overcome with a conventional refrigerating machine using e.g. R404A or R507 as refrigerant. Since these working fluids operate at over pressure the flow losses in the long suction line do not limit the operating scope as much as in case of C4F10 and tube dimensions can be reasonably designed. On the other hand, the low-pressure C4F10 circuit operates in the experimental area. Evaporated refrigerant is condensed by the evaporator of the R404A circuit. Boiling C4F10 is moved by a refrigerant supply pump (suction head >1 m).

fig. 3: Flooded system with separated refrigerant circuits

In fig. 3 the whole C4F10 circuit works at operating temperature of around –20 °C. However, since there is no space to insulate the supply and return tubes within the experiment the scheme has to be further modified.

 

2. Circuit Design of the 'Short-Loop Circulation System'

The circuit design of the 'Short-Loop Circulation System' for the ATLAS Pixel Detector cooling is shown in fig. 4 to 6. fig. 4: General arrangement of the ATLAS Pixel cooling circuits fig. 5: C4F10 'Short-Loop Circulation System' for ATLAS Pixel cooling

 

fig. 6: C4F10 lg p, h – diagram, operating conditions according to fig. 5

 

Boiling C4F10 (point 1) is pressurized by static pressure (DH > 1 m, point 2) and then sucked by the refrigerant supply pump. The pump pushes the sub cooled liquid on top of the experiment (point 4). The pressure at this state is equivalent to the condensation pressure of a conventional circuit (compare to chapter 3.4). Between point 4 and point 5 an electrical 'preheated' rises the liquid temperature above the dew point temperature (e.g. 10 °C, condition still sub cooled) before the supply tube enters the ATLAS experiment. Reaching the Inner Detector (respective Pixel detector) the refrigerant is expanded from point 6 to operating pressure/-temperature at point 7. Then the saturated refrigerant is partially evaporated in the detector staves to point 8. Before the refrigerant enters the suction line outside the Inner Detector area remaining liquid should be evaporated in an electrical 'drier evaporator' and superheated to point 9. By this means temperature respective pressure fluctuations caused by slugging liquid in the suction line and condensation or frost formation on the tube surface can be avoided. Point 9 to 10 represents the pressure drop in the suction line. From point 10 to point 1 the C4F10 is condensed by the evaporator of the R404A refrigerating machine.

The operating conditions in the lg p, h - diagram in fig. 6 are idealized without pressure drops in the heat exchangers (especially evaporator and drier evaporator) and thermal losses in the pump and the tubes. Figures can be taken from the following table:

Table 1 Operating Conditions of the C4F10 'Short-Loop Circulation System'

	Temperature [°C]	Pressure [bar]	Enthalpy [kJ/kg]	Remarks
Cooling capacity (1/8 of Pixel Detector): 2 kW
Refrigerant mass flow: 0.04 kg/s
Point 1	-32.2	0.25	167.6
Point 2	-32.2	0.42	167.6	DH = 1 m
Point 3	-32.2	6.44	167.6	DH = 20 m
Point 4	-32.2	3.14	167.6	tsat = 30 °C
Point 5	10	3.14	210.4	Q = 1.7 kW
Point 6	10	4.66	210.4	DH = 10 m
Point 7	-20	0.46	210.4	x = 0.30
Point 8	-20	0.46	260.4	x = 0.79
Point 9	10	0.46	305.2	Q = 1.8 kW
Point 10	9.6	0.25	305.2

 
3. Components of the Circuit

The R404A condensing unit consists of a compressor, a condenser and control equipment, which is installed in USA 15. The condenser is cooled by cooling tower water or cold water coming from the primary circuits. This kind of unit is commercially available.

The R404A evaporator / C4F10 condenser can be typically designed as shell-and-tube vessel. These heat exchangers are known from cascade systems and commercially available as well.

Flooded systems normally work with side channel pumps as refrigerant supply pump. These pumps have very low NPSH values and they are able to maintain performance under extreme sucking conditions, e.g. in case of pressure drop while starting. Some products are able to handle two-phase flow up to 50 % vapor. The application of this kind of pump makes an oil-free operation of the C4F10 circuit possible using only industrial standard devices (this is not the case for oil-free compressors). The pump has to be driven by a hydraulic motor since it operates in the magnetic environment of ATLAS.

The design of all the other components of the C4F10 circuit, i.e. detector staves, manifolds, injectors and piping, is only indirectly affected by the general circuit design in view of pressure levels and flow rates. However, these conditions are not different from the single circuit operation.

The C4F10 condenser, the pump and all the tubes between point 1 and 5 including pre-heater have to be insulated to avoid frost formation but this can be done since the are located outside the detector.

 

4. Circuit Control

The aim of the circuit control is to achieve and stabilize the operating temperature of the pixels, i.e. a certain flow rate and evaporating temperature has to be realized. Since active control device are undesirable inside the detector (capillaries or injectors are used for expansion) this aim is achieved by setting the high- and low-pressure conditions of the system.
 
The lowest pressure in the system exists in the C4F10 condenser. The value of around 0.25 bar absolute is controlled by the capacity of the R404A evaporator. In order to avoid pressure fluctuations and allow continuous operation e.g. for uniform cool down the R404A compressor should be speed controlled.
 
In combination with a certain flow rate the pressure drop in the suction line and the evaporators themselves lead to the desired medium boiling pressure/-temperature in the staves. The flow rate again is mainly determined by the pressure upstream the expansion device. Since height differences are given from the design the value can be converted to the outlet pressure of the pump. This pressure is controlled by a bypass valve around the pump. By this means a float control of condenser and pump can be avoided. The set point of the bypass valve could be fixed or remote controlled.
 
The electrical pre-heater is used to avoid tube temperatures below the dew point inside the detector. The capacity can be controlled by temperature measurement. The electrical drier evaporator additionally prevents from pressure fluctuations. Its capacity is controlled by temperature (and pressure) measurement as well.
 
4. CONCLUSION

The 'Short-Loop Circulation System' enables the usage of C4F10 as refrigerant for the ATLAS Pixel Detector cooling. The advantages in comparison to a single cooling circuit are:
- oil-free operation can be realized with industrial standard components,

- limited 'pressure drop stocks' of roughly 200 mbar can be reserved for the heat exchangers and return lines inside the detector.
The control system is simple and able to provide as well capacity control as starting and switch off operation in the magnetic environment.

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