CCAT cooling requirements

CCAT-TM-100

Version Date Author Revisions 1 8/4/12 S. Padin 1st draft

1. SUMMARY

This note gives preliminary estimates of cooling water requirements for CCAT. For the first few years of operation, the estimated cold water requirement for electronics is 450 l/min (120 gpm) of 12◦C water, with 200 kW total heat load. During the lifetime of CCAT, this will increase to 1300 l/min (350 gpm), with 900 kW heat load. In addition, the forcers and hydrostatic bearings require 300 l/min (80 gpm) of water at ambient temperature, with a total heat load of 100 kW. The peak temperature recorded at the site since May 2006 was 9◦C, so it may be possible to meet all the cooling requirements using only outside water to air heat exchangers.

2. COOLING SYSTEM CONFIGURATION

CCAT will have 2 water cooling circuits to cool electronics and drives. One circuit will provide water at ambient temperature to cool the forcers and hydrostatic bearing oil. The second circuit will provide 12◦C water for cooling electronics and helium compressors. Most of the electronics will be on the telescope structure, but there will be a few water-cooled racks of computer equipment in the control room.

3. LOAD ESTIMATES

The spreadsheet at the end of this note gives cooling capacity and flow estimates, and the following sections provide some explanation. All the estimates assume the coolant is pure water. Roughly 20% higher flow rates will be needed for a water and antifreeze mixture with a freezing point of −30◦C.

3.1. Electronics

Instruments will have readout electronics and helium compressors that need cooling. The electronics will be located in water-cooled server racks on the Nasmyth platforms, while helium compressors will be at the bottom of the telescope on the azimuth structure. Helium compressors fail if the cooling water is too cold, e.g., at ambient temperature during winter, so slightly warmed water from the electronics racks will be used to cool the compressors. This approach requires some care to ensure the correct flow through the compressors, but it improves the overall efficiency of the cooling system. Each instrument will have its own cooling sub-circuit, so that a configuration that was used in the laboratory can easily be reproduced on the telescope. The water temperature for instrument cooling will be ∼ 12◦C at the electronics rack inputs, ∼ 18◦C at the rack outputs, and ∼ 31◦C at the compressor outputs. There will be several racks of computer equipment in the control room. These will be identical to the water-cooled racks on the Nasmyth platforms. The telescope drive amplifiers and power supplies will be water cooled (a standard option for Siemens direct drive components [1]). The ∼ 12◦C water used for cooling instrument electronics will also serve for the drive amplifiers and power supplies. The motion control cards for the drive will be air-cooled.

3.2. Forcers and bearings

The drive forcers and hydrostatic bearing oil require cooling to reduce thermal deformation of the telescope structure and to prevent overheating of the forcer coils and oil pumps. To keep pointing changes <0.1”, changes in point heat loads must be . 2 kW hr. The maximum total heat loss in the azimuth forcers on one side of the telescope is ∼ 10 kW, and 85-90% of this is dumped into the coolant loop [2], so any changes in heat load are <1.5 kW. The forcers can therefore be cooled with water at ambient temperature, in which case the return temperature will be ∼ 4 K above

1 ambient. Note that this approach might not be suitable for an optical telescope where seeing degradation due to local heating of the air is more important than pointing errors due to thermal deformation of the structure. The oil supplied to the hydrostatic bearing pads will be ∼ 6 K warmer than the return oil [3]. If the return tank is at ambient temperature, the potential heat load on the mount is ∼ 40 kW, so the oil in the tank will have to be cooled ∼ 6 K below ambient. This will require a refrigeration system, but it can dump the heat into a coolant circuit at ambient temperature.

4. COOLING OPTIONS

The electronics racks require coolant at ∼ 12◦C, which is warmer than the 9◦C peak outside temperature recorded at the CCAT site since May 2006 [4]. This raises the possibility of using heat exchangers rather than chillers to provide cooling water. Heat exchangers have an enormous advantage because they require much less power, but the potentially small temperature difference between the air and the coolant requires a large heat exchanger. Liebert EST400 is a 2.5 × 9 m heat exchanger with 400 kW cooling capacity at 1200 l/min flow, 10 K difference between air and water in, and 5 K difference between air and water out. A 1 MW load at 2000 l/min and 18◦C coolant return temperature would require 3 of these units at 8◦C outside temperature and maybe 4 units at 9◦C outside temperature.

5. COOLANT DISTRIBUTION

Many of the components that require cooling are located on the Nasmyth platforms, so the coolant pumps must provide the required flow 13 m above the telescope foundation. The heat exchangers must be located a few hundred meters from the telescope, so the heat plume will not cause pointing and wavefront errors (due to temperature gradients in the air or heat drifting back into the enclosure and deforming the telescope structure.) Cooling systems sometimes have dual circuits, where users collect heat in a small secondary circuit that is coupled to the primary circuit via a water to water heat exchanger. This makes maintenance easier, and reduces the impact of leaks in racks, but it lowers the efficiency of the cooling system, and increases the cost. The higher cost probably makes a dual circuit system impractical for CCAT.

6. FUTURE EXPANSION

Future expansion of the coolant distribution system will be inconvenient and expensive, so the following key com- ponents should be sized to handle the expected future loads: 1. Buried pipes from the cold water plant to the telescope, and coolant hoses running through the telescope azimuth wrap. 2. Cold water plant building. 3. Buried power lines to the cold water plant.

[1] Sinamics S120 booksize power units, Manual, Jan 2012. [2] Sinamics S120 peak and continuous load motors in the 1FN3 product family, Configuration Manual, Aug 2009. [3] E. Chauvin, “CCAT Mount Engineering Preliminary Design Report,” 9 Apr 2012. [4] http://www.submm.caltech.edu/submm.org/site/weather/cc.html

2 CCAT cooling requirements S. Padin 8/4/12

first light future load power duty flow Tin ΔT ΔP qty power flow qty power flow notes kW cycle l/min °C °C kPa kW l/min kW l/min 1 l/min=0.2642 gpm max 100 kPa=14.5 psi A. COLD WATER (12°C) 1. Drive electronics (on azimuth structure) Siemens 6SL3125-1TE32-0AA3 drive amplifier 2 0.5 8 40 4 70 22 22 176 22 22 176 50% duty cycle Siemens 6SL3135-7TE31-2AA3 power supply 2.2 0.5 8 40 4 70 3 3.3 24 3 3.3 24 50% duty cycle drives total 25.3 200 25.3 200

2. Instruments electronics rack 25 1 62 12 6 58 2 50 124 14 350 868 Liebert 08.006.043 helium compressor 10 1 11.5 27 13 150 8 80 92 42 420 483 Cryomech CP1110 run coolant throught racks instruments total 130 124 770 868 then compressors

3. Computers (in control room) electronics rack 25 1 62 12 6 58 2 50 124 4 100 248 Liebert 08.006.043

TOTAL COLD WATER 205.3 448 895.3 1316 temp rise 6.5 K 9.7 K

B. AMBIENT WATER 1. Forcers & bearings forcer 1.4 0.5 4 35 5 37 66 46.2 264 66 46.2 264 Siemens 1FN3450-2N bearing oil cooler 40 1 50 12 1 40 50 1 40 50 guess

TOTAL AMBIENT WATER 86.2 314 86.2 314 temp rise 3.9 K 3.9 K Cold water cooling circuit

Pump

12°C Heat

exchanger Drive Instrument Instrument Electronics 1 n

19‐22°C 450‐1300 l/min 200‐900 kW

Electronics Electronics rack rack Ambient water cooling circuit

Pump

Ambient He He

Heat compressor compressor exchanger Drive Bearing oil forcers cooler

Ambient + 4°C 300 l/min = ball valve 100 kW = flow control valve