Current Affairs: Keeping Your Cool
Equipment RequirementsAlthough individual manufacturer's specifications vary, the majority of electrical distribution equipment is designed to operate properly and achieve normal life expectancy under ambient conditions of 32 degrees to 104 degrees F and 0% to 99% rh, noncondensing.
In most cases, only the upper limit on temperature represents a challenge. Unless serious extremes of temperature are anticipated, heating requirements are limited to maintaining equipment surfaces above the dewpoint to avoid condensation, and providing an acceptable work environment for service. Often the former can be accomplished using electrical resistance heaters installed inside the enclosures, making it possible to install even medium-voltage (over 600 V and less than 72,000 V) equipment in unheated space or outdoors.
Cooling IssuesThe effect of exceeding the 104-degree ambient varies with the type of equipment; power electronics in vfd's or uninterruptable power supply (UPS) systems may shut down via thermal sensors if the entering air temperature or internal temperature exceeds certain limits. Electronic devices such as relays and meters that are intended to be used in electrical switchgear may have ambient temperature specifications as high as 158 degrees.
In some cases, elevated temperatures require derating. Protective devices such as fuses and thermal-magnetic circuit breakers, which detect overloads through the heat produced by current flowing through them, will trip at lower-than-rated currents in an elevated ambient.
Less sophisticated equipment, such as motors, switchgear bus, and transformers, will continue to operate properly in an elevated ambient, but will develop higher internal temperatures that can cause insulation deterioration and reduce life expectancy.
An old rule of thumb in the electrical field is that each 18-degree increase in operating temperature decreases the life of electrical insulation by a factor of two. Keep in mind, however, that this effect is proportional, and that a few weeks per year of operation at elevated ambient does not have a major impact on the 10-year design life expectancy of a substation transformer, for example.
Outside AirIn my region (ASHRAE 1% summer design conditions of 92 degrees db/75 degrees wb) we often provide outside air ventilation as the only source of cooling for vaults or electrical rooms that contain power distribution equipment such as unit substations, switchgear, and panelboards with their associated metering and instrumentation.
Admittedly, on a 95-degree day with a design rise of 15 degrees, we can't maintain the 104-degree upper limit, but the frequency of such days is low and the impact on the equipment is negligible.
The impact of hot days can be further mitigated by ductwork design. If the intakes deposit outside air at floor level in the vicinity of the lower ventilation openings in the electrical equipment, and air is removed from the ceiling near the upper ventilation openings, a once-through flow pattern can be developed in which the equipment is not exposed to the higher temperature discharge air.
Designing these systems with a pressurized supply to keep the room positive with respect to the outdoors is helpful in keeping dust and other contaminants that are detrimental to cooling from building up in the electrical equipment, and it is mandatory if the surrounding environment contains corrosive or conductive contaminants. However, it often seems that the space layout discourages pressurized supply in favor of exhaust fans. In either case, the intake air must be effectively filtered.
Load CalculationsRegardless of whether you are providing mechanical cooling or pure ventilation, an accurate load calculation is important to ensure that adequate cooling is provided without excessive equipment costs or excessive air velocity.
In large substation installations and spaces such as UPS equipment rooms, the electrical equipment may occupy more than half of the cross-sectional area of the space. High volumes of cooling air flowing in the remaining space translate into high velocities that make it difficult to service the equipment. My next column will review the calculation of electrical equipment space cooling loads. ES