HVAC
How do you identify the location of a suspected water leak underfloor and how is it reset?
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Follow yellow cord, should be wet somewhere
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To reset, dry up and get rid of moisture
Explain the automatic system response to a failed electric room CRAC unit.
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Dampers open
The CRAC unit in UPS room next door will go 100% fan speed to support both rooms
If CRAC does not go to 100% fan speed, we need to lower the temperature set point.
Explain the difference between positive and negative pressure in the Data Halls. Which do we use and why?
Positive pressure – more pressure inside than outside
Negative pressure – more pressure outside than inside
We use Positive Pressure so that particulates and debris is removed from the data halls.
Define the function of an FPT.
FPT – Fan powered terminal
designed to regulate airflow and temperature in specific PBB zones by using dampers and a fan
Given that the exhaust fans in the UPS room do not run continuously, what makes them start and stop?
Exhaust fans are activated and controlled by hydrogen gas detectors
Hydrogen can be released from certain types of batteries, particularly lead-acid batteries during charging (and especially during overcharging)
BMS: Misc -> Suite Auxiliary Monitoring
Three operating modes:
Normal – all temps and pressures within setpoints, 80% of chilled water supply thru V-1 valve, 20% thru buffer tank
Discharge – when chilled water supply is over 3˚F from setpoint. The 80%-valve closes and 100% water flows through buffer tank (TES).
Recharge (only after 60 minutes of discharge) – 50 gallons per minute flow thru buffer tank until temperature within 0.5˚F of setpoint
Define Dewpoint and Static Pressure.
Static Pressure - the resistance air encounters as it moves through the ductwork, which affects how efficiently the system circulates air
Dew point – the temperature the air needs to be cooled to in order to achieve a relative humidity (RH) of 100% (May cause moisture to form on electrical equipment)
How many KW/h equate to 1-refigerant Ton? How many BTU’s equal to 1-Refrigerant Ton?
~3.5 kw/h to 1-refrigerant ton
12,000 BTUs to 1-refrigerant ton
What is an XDM unit and its function?
Vertiv XDM300 – a split indoor chiller located in our CRAC galleries, providing PG-25 coolant to the chiller system
a. System design (components, refrigerant, coolant, redundancy) - XDM Unit
R-410a circuit - Just like a CRAC unit, XDMs are equipped with 4 scroll compressors with the use of R-410a refrigerant and paired with condenser MCV430 with Econo-phase option
PG-25 circuit - Unlike CRACs, XDMs have 2 variable speed pumps to push the PG-25 coolant (propylene glycol) liquid to the customer CDU’s (Cooling Distribution Units.)
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Heat exchanger – these two circuits intersect and allow the XDMs to operate as a CHILLER for the distribution of coolant.
***Note there is NO evaporator coil for the R-410a circuit because of this heat exchanger.
***These pumps will be on UPS power to ensure no loss of power during utility disruptions.
N+4 Redundancy per suite
What is the purpose of an expansion tank in the CRAC gallery?
Prevents pressure buildup caused by water expanding as it heats
Explain Hot-Aisle Containment and Cold-Aisle Containment.
Hot-aisle containment isolates the hot exhaust air from servers within an enclosure, allowing the rest of the data center to be cold
Cold-aisle containment isolates the cold air supply within an enclosure
What is Cascading iCom control? What is the worst case scenario when shutting down an iCom unit?
Cascading iCOM control – CRACs are evenly distributed/positioned and networked to ensure balanced cooling operation within a data hall
Worst-case scenario is accidentally shutting down all units in a group when intending to only shut down one
Explain why an XDM has three disconnects.
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Power sources (primary and secondary)
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Pump
Define the function of a Humidifier,
a. How many per suite
b. Relative Humidity understanding
c. Relationship of Humidity and Temperature
Humidifier – function is to increase the moisture content (humidity) in the air
a. 4 PER SUITE, 2 IN EACH GALLERY.
B. Humidity is the measure of water vapor in the air.
Relative humidity means the humidity relative to the maximum amount of water that air could hold at that temperature (see video…)
C.
The hotter the air, the more total amount of moisture it can hold
The colder the air, the less amount of moisture it can hold
What is approach temperature and how is it used in the XDM's?
Approach temperature is the measure of the efficiency of the heat exchanger (ideally, a low number) between hot water going in (return) and cold water going out (supply).
This helps determine if the heat exchanger is getting dirty
What is an MAU unit and its function? Include:
MAU – Make-up Air Unit
Provides ventilation (100% fresh air), pressurization (slightly positive pressure within data halls), and dehumidification (removing moisture) for data halls
b. Sequence of Operation
Same as RTU, no return
Uses R-454b refrigerant
2 per suite, only 1 needed
How do you calculate the number of CRACs needed for a given IT-load?
Take the IT load and divide that by the capacity of a single CRAC unit (then round up)
For example, for M100…
3129 kw / 265 = 11.8
Round up to 12 CRACs needed
b. Sequence of Operation - RTU
Outdoor air goes thru dampers, filters, evaporator coil
Air moves via Supply fans, heating if necessary, to the FPTs (fan powered terminals) in the building
Describe the three operational modes of a CRAC unit
Compressor mode - Under normal conditions, the refrigerant goes through the compressor (typical refrigerant cycle)
PRE – Pumped economizer refrigerant
HVAC system takes advantage of outside cold air to perform cooling.
Rather than the compressor moving the refrigerant, a pump moves the liquid refrigerant and bypasses the compressor
b. Sequence of Operation - XDM Unit
Inside the XDM, PG-25 chilled water goes through our building loop to the customer CDU (cooling distribution unit, also with a heat exchanger), with a setpoint of 77˚F Supply
The valves before the customer CDU is the demarcation point, (so our job is simply to supply the chilled water to this point). The customer CDU distributes that chilled water through their customer loop and to the racks
Return water (high setpoint of 96˚F) goes back to XDM
As a redundant resource, buffer tanks (thermal storage tanks, 2 per gallery, 850 gallons) will allow for 80 seconds or run time if needed.
What is an RTU unit and its function?
RTU – Rooftop Unit
Self-contained packaged air-conditioning unit for comfort-cooling and heating in PBB spaces
Describe the pressure-temperature relationship.
When a gas is pressurized, the temperature rises (via compressor = pressure increaser)
a. System design (components, refrigerant) - FOR RTU
Air intake, supply and return dampers, filters, evaporator coil, fan, condenser coil
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Uses R-410a refrigerant
Define CRAC and explain its function.
a. System design (components, refrigerant, redundancy)
CRAC – Computer Room Air Conditioner
a specialized HVAC system designed to precisely control temperature, humidity, and air quality in data halls and electric rooms.
a.
Equipped with 4 scroll compressors with use of R-410a refrigerant across 2 circuits
Each unit is paired with a condenser unit on the roof with 6 fans (three per circuit)
Vertiv DSE265 (265kw) – in the CRAC galleries, serve the data hall
Paired with condenser MCV430
Vertiv DSE165 (165kw) – in the UPS room, serving the UPS room
Paired with condenser MCV330
N+2 Redundancy