Views: 0 Author: Site Editor Publish Time: 2026-09-10 Origin: Site
A transformer radiator removes heat from insulating oil and releases it to the surrounding air. The transformer core and windings generate heat during operation. The oil absorbs that heat, circulates from the transformer tank to the radiator, transfers heat through the radiator surface and returns to the tank at a lower temperature.
In the simplest arrangement, oil and air move by natural convection. In larger or more heavily loaded transformers, fans force air across the radiator, an oil pump forces oil through the cooling circuit, or a separate water circuit removes the heat. These arrangements are commonly described with the cooling designations ONAN, ONAF, OFAF and OFWF.
At CSTHEATEXCHANGER, we work with transformer oil coolers, air-cooled oil coolers, finned-tube heat exchangers and power-equipment cooling products. We design the radiator or cooler around the transformer’s oil circulation, heat duty, ambient conditions, pressure drop, materials, controls and maintenance requirements. A radiator is one part of the transformer cooling and protection system; it does not replace the transformer manufacturer’s thermal design or safety controls.
A transformer radiator is an external heat-transfer assembly connected to an oil-filled transformer tank. It provides more surface area than the tank wall alone, allowing hot insulating oil to release heat to ambient air. A radiator may consist of finned panels, welded channels, tubes, headers or a related heat-exchanger structure.
The radiator normally includes:
Oil inlet and outlet connections.
Radiator panels, tubes or finned heat-transfer surfaces.
Top and bottom headers or connecting manifolds.
Isolation valves where the design requires them.
Drain and vent points.
Fan brackets or fan guards in forced-air systems.
Temperature, flow or pressure monitoring points.
Structural supports and weather protection.
The radiator’s job is to transfer heat. It does not generate cooling by itself. The result depends on oil circulation, radiator surface, ambient air, airflow, transformer load, fouling and installation clearance.
CSTHEATEXCHANGER’s published transformer air-cooled oil-cooler information describes a cooler made mainly from a cooling fan, heat sink or radiator. The radiator receives hot transformer oil, while air moving over the surface carries heat away. The cooled oil then returns to the transformer.
The cooling cycle starts inside the transformer. Electrical losses in the core and windings produce heat. The insulating oil around those components absorbs the heat and becomes warmer. Because warmer oil is less dense, it tends to rise toward the upper part of the tank in a natural-circulation system.
The hot oil then reaches the radiator through the upper connection. Inside the radiator, the oil spreads through the panels or tubes. The large external surface exposes the oil to ambient air. Heat moves from the oil through the metal wall and into the air.
As the oil cools, its density increases. In a natural-oil system, the cooler oil moves downward through the radiator and returns to the lower part of the transformer tank. This creates a continuous circulation loop:
Core and windings generate heat.
Transformer oil absorbs the heat.
Hot oil rises or is pumped to the radiator.
Heat passes through the radiator wall.
Natural or forced air carries heat away.
Cooled oil returns to the transformer tank.
The same cycle can use fans, pumps or water when the transformer’s heat duty requires more controlled heat rejection. CSTHEATEXCHANGER’s published transformer-oil cooler information describes water and air as common cooling media and identifies oil-to-medium heat exchange as the basic operating principle.
ONAN means Oil Natural, Air Natural. The transformer oil circulates without a forced-oil pump, and air moves over the radiator through natural convection. There are no cooling fans in the normal ONAN path.
When the transformer is operating, hot oil rises from the active parts toward the radiator. The radiator surface warms the surrounding air. Warm air rises away from the radiator, drawing cooler ambient air across the surface. The cooled oil returns to the transformer tank and the cycle continues.
ONAN has a simple flow path and relatively few moving parts. Its cooling performance depends on radiator size, ambient air temperature, oil circulation, transformer load and clear airflow around the radiator. If the radiator is blocked by a wall, debris or nearby equipment, natural air movement can be reduced.
ONAN cooling is not defined by one universal transformer size or temperature. The transformer manufacturer determines the approved cooling designation and operating limits. We use the nameplate, thermal design and site conditions when reviewing a radiator for a new or replacement application.
ONAF means Oil Natural, Air Forced. The oil still moves through the transformer and radiator by natural circulation, but fans force air across the radiator surface. The fans increase air velocity and improve heat rejection compared with natural air movement.
A typical ONAF sequence is:
The transformer oil absorbs heat from the core and windings.
Natural oil circulation carries hot oil to the radiator.
Fans start according to the transformer control sequence.
Forced air crosses the radiator tubes or panels.
Heat moves from the oil into the air.
Cooled oil returns naturally to the tank.
The fans may operate in stages or according to oil temperature, winding temperature, load or another approved control signal. When the transformer load is lower, the fans may stop and allow the system to return to a natural-air stage, if the transformer design permits it.
Fan failure can reduce cooling performance even though oil continues to circulate. For this reason, fan status, motor protection, airflow and temperature alarms should be checked during commissioning and maintenance.
OFAF means Oil Forced, Air Forced. An oil pump drives the transformer oil through an external radiator or heat exchanger, while fans force air across the cooling surface.
CSTHEATEXCHANGER’s published OFAF article describes the process in clear steps. A pump draws hot oil from the transformer tank and pushes it through a finned-tube cooler. Fans blow air across the tubes and remove heat. The cooled oil then returns to the tank to absorb more heat.
The forced circulation path provides more control over oil flow than natural circulation, but it adds pumps, motors, seals, valves, sensors and control requirements. The oil pump must provide the required flow without creating excessive pressure drop, vibration, leakage or oil stress.
An OFAF system needs coordinated pump and fan operation. If the pump stops, the transformer may need to reduce load, start another cooling stage or trip according to the approved protection scheme. If the fans stop, oil may continue to circulate but the radiator may not reject enough heat.
OFWF means Oil Forced, Water Forced. The transformer oil is pumped through a water-cooled heat exchanger, while a separate water circuit is forced through the other side. The oil and water remain separated by the heat-transfer surface.
A typical OFWF system includes an oil pump, water pump or controlled water supply, oil-to-water heat exchanger, isolation valves, strainers, temperature sensors, flow switches and leak protection. Water removes heat from the transformer oil and carries it to a cooling tower, plant-water system or another approved heat sink.
Water-cooled systems can provide a compact installation, but they need closer attention to water quality, scaling, corrosion, pressure balance and leak detection. A water leak into transformer oil can affect insulation and safe operation. The final heat exchanger and water circuit must follow the transformer manufacturer’s design and the project’s water-quality requirements.
CSTHEATEXCHANGER’s transformer-cooling product references include air, oil and water heat-exchanger applications. We select the cooling medium from the transformer duty, site conditions and maintenance capability rather than treating air and water systems as direct substitutes.
A radiator provides the heat-transfer surface, while fans and pumps determine how quickly heat reaches and leaves that surface.
A fan increases air velocity across the radiator. More air movement can increase heat rejection, but the fan also adds electrical load, noise, vibration and a maintenance requirement. Fan blades, guards, motors and controls need clearances and protection from weather and debris.
An oil pump increases the flow through the radiator. Higher oil flow can reduce the oil-side temperature rise and distribute heat more quickly, but the pump must match oil viscosity, pressure, seals and transformer connections. A pump that is too large can create unnecessary pressure drop or stress the oil circuit.
We review fan and pump operation together with the transformer control sequence. Starting a fan without sufficient oil circulation, or starting a pump without correct valve position, may not provide the intended cooling. The control philosophy should define start conditions, alarm conditions, standby equipment and the response to a failure.
A complete radiator system usually includes more than the radiator panels. Each component contributes to flow, heat transfer or safe maintenance.
The radiator body provides the heat-transfer area. Headers distribute oil into the panels or tubes and collect it at the outlet. The internal passages must support the required oil flow without excessive resistance.
Upper and lower connections link the radiator to the transformer tank. Their size and location influence natural circulation, draining, isolation and maintenance. Connection design must follow the transformer drawings.
Fans force air across the radiator in ONAF and OFAF systems. Guards protect operators and reduce the risk of damage from foreign objects.
The oil pump forces circulation in OFAF and OFWF systems. Valves allow isolation, balancing, draining and service where the design permits.
Temperature, oil flow, water flow, fan status, pump status and pressure signals can be used for control and alarms. The exact sensors and settings depend on the transformer protection design.
CSTHEATEXCHANGER’s industrial heat-exchanger information lists material options such as copper, aluminum, stainless steel, Cu-Ni, brass, titanium and carbon steel for suitable services. Material choice for a transformer radiator depends on oil, air or water conditions, outdoor exposure, corrosion, pressure, temperature and the approved project specification.
We begin with the transformer’s nameplate and cooling designation. A radiator cannot be selected reliably from appearance or tank size alone.
The selection review normally includes:
Transformer rated power and cooling mode.
Oil type, oil viscosity and required oil flow.
Heat duty at minimum, normal and maximum load.
Oil inlet and outlet temperature conditions.
Ambient temperature, altitude and solar exposure.
Natural or forced airflow requirements.
Fan quantity, airflow, motor data and redundancy.
Oil pump flow, head, seals and motor data.
Water flow and quality for water-cooled systems.
Allowable pressure drop.
Radiator dimensions, connections and mounting.
Corrosion exposure and material requirements.
Drain, vent, inspection and cleaning access.
Alarm, control and protection interfaces.
For a replacement radiator, we also request drawings, photographs, nameplate data, flange dimensions, connection locations, radiator bank arrangement and maintenance history. A new radiator must fit the transformer hydraulically and mechanically, not only match its external outline.
Installation begins with transformer isolation, oil-handling controls and a review of lifting, supports, piping and electrical work. We check that the radiator is correctly supported, connected, vented and protected from accidental damage.
During commissioning, we verify:
Radiator and transformer-tank connections.
Oil level, oil cleanliness and valve positions.
Fan direction, airflow and motor protection.
Oil-pump direction, flow and alarm signals.
Water flow and leak protection for OFWF systems.
Temperature sensors and control signals.
Drain and vent operation.
Cooling-stage changeover and fault response.
Transformer response under approved operating conditions.
Maintenance includes cleaning radiator surfaces, checking fans, inspecting oil pipes and valves, verifying oil-pump operation, checking water-side fouling, reviewing temperature trends and inspecting for leaks. The surrounding area should remain clear so air can enter and leave the radiator.
If oil temperature rises, we check transformer load, ambient conditions, fan operation, oil flow, radiator cleanliness, valve position, pump status and sensor accuracy before assuming the radiator has failed. Any oil draining, electrical work or transformer entry should follow the transformer manufacturer’s procedures.
When we review a transformer radiator request, we separate the transformer’s thermal duty from the radiator’s construction. We ask for the transformer nameplate, cooling designation, oil data, ambient condition, required heat rejection, radiator dimensions, connections, fan or pump data and project documentation.
CSTHEATEXCHANGER’s Transformer Air-Cooled Oil Cooler information explains natural oil circulation, radiator heat transfer and forced-air cooling. Its OFAF oil-pump explanation describes the forced-oil path from the transformer tank through a finned-tube cooler and back again. The Transformer Oil Cooler article describes air and water cooling media, while the Generator Cooler category includes related power-equipment cooling products.
We support new and replacement heat-exchanger projects, design data and production drawings. Contact CSTHEATEXCHANGER to discuss a transformer radiator, air-cooled oil cooler or forced-oil heat exchanger.
A transformer radiator works by circulating hot insulating oil from the tank through an external heat-transfer surface and returning the cooled oil. Natural convection may be assisted by fans or replaced by forced oil circulation and water cooling. ONAN, ONAF, OFAF and OFWF describe different combinations of oil movement and cooling-medium movement. The correct radiator, fan, pump and control system must be selected from the transformer’s approved thermal design and site conditions.
