#120 ACRD Requirements for Selecting 3 rd Party Remote Air-Cooled Condenser and Flooded Receiver Abstract The InRow RD (ACRD600, ACRD601, ACRD602, ACRD600P, ACRD601P, ACRD602P) is an air-cooled DX air conditioner for data centers. All air-cooled air conditioners require the use of remote air-cooled condensers. This application note outlines the requirements for selecting third party remote condenser and flooded receiver for InRow RD air-cooled product. Introduction The InRow RD is an air-cooled DX air conditioner designed to be placed in-row, between IT equipment racks. The in-row design allows the InRow RD to draw in air from the rear, capturing heat from the IT equipment in the hot aisle, and neutralizing it before it mixes with the room air. Conditioned air is then discharged into the cold aisle, ready for immediate use by the equipment in the adjacent racks. An air-cooled condenser is required to reject the heat load as well as the power drawn by the compressor. The air-cooled condenser, installed outside, rejects the heat to outdoor ambient air. General requirements for Air-Cooled Condenser 1. Refrigerant: R410a, design pressure is 4.2MPa. Design temperature for air-cooled condenser is -20 46 o C (-4 115 o F). 2. To be compatible with piping, the condenser connection size should be inlet 7/8 and outlet 5/8. Need one service port near pressure transducer for service usage. 3. Use a header cover above header and protect electrical (if no header cover, electrical part should meet IP54) 4. Condenser should provide power input for electrical heater for receiver which is 105W at 204Vac. Performance Requirements for Air-Cooled Condenser The selected air-cooled condenser must meet the thermal heat rejection (THR) requirement as stated below in Table 1. Schneider Electric generally offers remote air-cooled condensers for two outdoor ambient air temperatures in order to keep the refrigerant condensing pressures at a desired level: 35 o C (95 o F) and 46 o C (115 o F). The difference between outdoor ambient temperature and condensing temperature is called design condenser temperature difference. Table 1 Remote air-cooled condenser THR requirement By Dennis Feng Model Region Design Ambient Temp Power input THR MBH/1 F TD 1 _
ACCD75228 35C (95F) 208-230V 3ph 60HZ 5.7 MBH/1 F TD 3 kw/1 C TD ACCD75229 46C (115F) 208-230V 3ph 60HZ 7.8 MBH/1 F TD 4.1 kw/1 C TD NAM ACCD75230 35C (95F) 460-480V 3ph 60HZ 5.7 MBH/1 F TD 3 kw/1 C TD ACCD75231 46C (115F) 460-480V 3ph 60HZ 7.8 MBH/1 F TD 4.1 kw/1 C TD ACCD75232 GCN 35-46C (95-115F) 220V 1ph 50Hz 8.3 MBH/1 F TD 4.4 kw/1 C TD ACCD75232-C EMEA 35-46C (95-115F) 230V 1ph 50Hz 8.3 MBH/1 F TD 4.4 kw/1 C TD ACCD75233-C 35-46C (95-115F) 230V 1ph 60Hz 8.3 MBH/1 F TD 4.4 kw/1 C TD Elevation above sea level is a factor that negatively affects air-cooled condenser performance. The density of the air is reduced with higher elevation from sea level, the lower air density means lower air mass flow rate drawn by condenser fans. The negative effect of elevation from sea level must be taken into consideration during the condenser selections for the InRow RD units. Remote air-cooled condenser selection procedure may change from vendor to vendor, but size and capacity of the condenser increases with the higher elevation from sea level. If the altitude is 4,000 ft (1,200 m), the altitude correction factor is 1.10 and the corrected design THR is calculated by 4.4 kw/ o C (8.3MBH/ o F) multiplied by 1.10 or 4.8 kw/ o C (9.1 MBH/ o F). Table 2 Altitude correction factor Altitude ft (m) Correction factor 0 1.00 1,000 (300) 1.02 2,000 (600) 1.05 3,000 (900) 1.07 4,000 (1,200) 1.10 5,000 (1,500) 1.12 6,000 (1,800) 1.15 7,000 (2,100) 1.17 For example; if the summer outdoor air design temperature of a region is 95 o F (35 o C), and the altitude is 4,000 ft (1,200 m), the design condenser temperature is 122 o F (50 o C), so design temperature difference is calculated by subtracting 95 o F (35 o C) from 122 o F (50 o C) or 27 o F (15 o C). The selected condenser must have the minimum capacity of corrected design THR which is InRow RD THR multiplied by altitude correction factor 3kW/ o C * 15 o C * 1.1 or 50 kw (170,720 Btu/hr) at 27 o F (15 o C) design condenser temperature difference. 2
Flooded Receiver Requirement for Low Ambient Protection Table 3 General requirements receiver Part Number Region Requirement ACAC75013 GCN GB - NB/T 47036-2013 ACAC75014 NAM ACAC75015 EMEA PED 97/23/EC ASME ASME Boiler and Pressure Vessel Code VIII Division 1 Working principle The remote air-cooled condensers selected for InRow units must be equipped with flooded receiver assembly for head pressure control at low outdoor ambient air temperatures, because the InRow RD unit has a variable speed compressor that changes unit capacity according to compressor speed. The condensing temperature in the InRow RD unit must be kept not too low. Since the pressure differential across the expansion valve port affects the rate of refrigerant flow, low head pressure generally causes low refrigerant flow then results in low opening of expansion valve. Electrical expansion valve used on InRow RD unit working at low opening causes operation unstable if the environment various, it may cause liquid refrigerant entering the compressor. The heat rejection from hot gas refrigerant to outdoor ambient air in the remote air-cooled condenser occurs via forced or natural convection. When the condenser fans are running, they will draw air through condenser coil and convection heat transfer occurs. Without any air flow, there still is natural convection heat transfer which may be enough to drive the condensing temperature below critical point at low outdoor ambient temperature s. An air conditioner unit design that can regulate refrigerant mass flow rate and cooling capacities like InRow RD units do, need a flooded receiver assembly at higher outdoor ambient air temperature than expected. A flooded receiver assembly is required for InRow RD units at outdoor air temperatures of 40 o F (4 o C) and below. A viable method used to reduce and regulate condensing temperature at low outdoor ambient air temperature is flooding the condenser internal volume with liquid refrigerant. This method requires additional refrigerant R410a charge as shown below. A receiver assembly, which contains a relief valve, receiver, and head pressure control valve, is required to control the additional refrigerant during warmer ambient air temperatures. The idea of flooding the condenser internal tube volume with liquid refrigerant to reduce its capacity comes from a basic heat transfer fact: the heat transfer coefficient of a single phase (liquid) fluid is much lower than heat transfer coefficient of a two phase (gas and liquid) fluid. Additional flooded charge and flooded receiver selection The charge table in the InRow RD operation manual shows the refrigerant charges needed to fill the liquid line between the condenser and InRow RD unit, the unit s standard charge, and additional flooded charge for the condensers. 3
Standard charges for the 3 rd party condensers can be obtained from the specific vendor technical bulletins. To calculate the additional flooded charge, one should first obtain the condenser inner tube volume. The 100% flooded charge of the condenser then can be calculated by multiplying the condenser inner volume ft 3 (m 3 ) with 60.9 lb/ft 3 (975 kg/m 3 ) density of liquid R410a at 105 o F (40.6 o C) and 400 psig (27.5 bar). Finally, the additional flooded charge is calculated by multiplying the 100% flooded charge of the condenser with outdoor ambient coefficients that can be seen below. Table 4 Flooded refrigerant charge outdoor air temperature coefficient Design winter outdoor air temperature [ o F ( o C)] Flooded charge outdoor air temperature coefficient 40 (4) 20 (-7) 0 (-18) 0.6 0.6 0.7 Caution!!!: The size of the flooded receiver must be capable of holding the entire flooded refrigerant charge or damage may occur to the refrigeration system or compressor. The flooded receiver assembly must contain a receiver, head pressure control valve, pressure relief valve, and flexible heater. A drawing of one of the flooded receiver assembly designed by Schneider Electric can be seen below. The receiver should have three sight glasses at one quarter, one half and three quarters of receiver diameter for horizontally installed receivers to observe the refrigerant liquid level. External surfaces of the receiver should also be insulated. 105 W flexible rubber heater with thermostat built in should be wrapped around receiver before insulating the receiver. The 68 o F (20 o C) thermostat built in the heater will open when it senses temperature is lower than 68 o F (20 o C). Flooded receiver assembly without a heater may cause compressor starting problems. Without a heated receiver, all refrigerant will migrate to receiver since it is the coldest portion of the system. Figure 2 shows the way of designing flooded receiver assembly. The pressure relief valve should have a pressure relief rating of 609 psig (42 bars). Head pressure control valve selection The head pressure control valve with 295 psig (20.3 bar) set point or above will sense the drop in refrigerant discharge pressure and restrict condenser liquid outlet. This will result in flooding the condenser with liquid refrigerant. The Sporlan valve is used in SE designed flooded receiver assemblies; however, there are other options: combination of ORD (open on rise of differential pressure) and ORI (open on rise of inlet pressure) valves can also be used. Take LAC as an example: Selection guidance is sy stem capacity in table 3 at minimum design temperature and pressure drop across v alv e no more than 5 psi. Select a LAC v alv e f or a 10 ton (35 kw), R-410A unit with a minimum design ambient temperature of -20 F (-28 C). The LAC-10 has a capacity of 12.8 tons (47.3 kw) at a 5 psi (0.35 bar) drop across the v alv e according to the Low Ambient Capacity table below. The LAC-10 also has a capacity of 11.4 tons (49.6 kw) at a 2 psi (0.21 bar) drop across the v alv e according to the High Ambient Capacity table below. The LAC -10 is the correct selection. There should be a check valve at the liquid inlet. The flooded receiver assembly should be installed below the condenser coil level so that refrigerant can drain into the flooded receiver during compressor off cycle. 4
Table 5 Head pressure control valve selection Variable Fan Speed and Proportional Fan Speed Control The condensers offered by Schneider Electric are equipped with variable fan speed control. An air-cooled condenser selected for an InRow RD unit must have variable fan speed control, because cooling capacity and thermal heat rejection of an InRow RD unit changes with the load in a data center, a condenser without variable fan speed control will result in unstable condensing pressure. The variable fan speed controller is built in the condenser; set pressure of 276psig (19 bar), throttling range of 73psi (5 bar), and other functions are programmed into the fan speed controller. A pressure transducer is installed to measure refrigerant discharge pressure. There is no low voltage signal wiring between the InRow RD unit and the remote air-cooled condenser. The fan speed controller will start to increase fan speed from minimum fan speed when the discharge pressure reaches 276psig (19 bar), the fans will run at maximum fan speed when discharge pressure reaches 348psig (24 bar). The 3 rd party remote air- cooled condenser must have a pressure transducer and a fan speed controller, and fan speed control method should follow figure 1. This will provide condenser fans to operate on refrigerant discharge pressure rise without signal wires between InRow RD and the remote air-cooled condenser. Figure 1 Condenser fan speed control method 5
Caution!!! The 3 rd party condenser and flooded receiver assembly must comply with local codes and standards. The 3 rd party condenser must be capable of starting the condenser fans on discharge pressure rise, since InRow RD unit does not support interconnect wiring between the unit and the condenser. Figure 2 Piping schematic, head pressure control with single valve 1. Compressor 2. Valve, head pressure control valve 3. Condenser 6
4. Flexible Heater with Built in Thermostat 5. Pressure Relief Valve 6. Check Valve 7. Receiver with Sigh Glasses About the Author: Dennis Feng is a Cooling System Engineer in the IT Division at Schneider Electric. He received a Bachelor degree in Thermal Energy and Power Engineering from Soochow University in Suzhou, China and a Masters in Refrigeration and Cryogenic Engineering from University of Shanghai for Science and Technology in Shanghai, China. 7