In short. Dairy continuity is a chilling problem before it is a kilowatt problem. Raw milk must be brought to 4 °C and held there, pasteurisers divert flow the moment temperature or power is lost, and ammonia refrigeration carries the whole plant. The generator is sized from the chilling load, the compressor restart sequence and the pumps that keep product moving, and it is specified for the long running hours that rural bulk milk coolers and chilling centres accumulate.
The chilling chain and where power matters
Milk arrives warm at the village bulk milk cooler, is chilled, is collected and chilled again at the chilling centre, and is then processed, packed and stored cold at the dairy. Every stage depends on refrigeration, and most stages depend on pumps to move the milk. In Telangana and Andhra Pradesh, where the cooperative and private dairies draw from thousands of village collection points with unreliable rural feeders, generators run for hours every day at the cooler and chilling centre level, while at the main dairy they protect a much larger and more complex process. Reddy Generators supplies and maintains sets at both ends of that chain, and each deserves its own brief.
| Stage | Key electrical loads | Consequence of outage | Backup priority |
|---|---|---|---|
| Bulk milk cooler (village) | 1 to 5 kW compressor, agitator, lighting | Milk warms, bacterial count rises, batch rejected | Small DG in near-prime duty, or solar-battery hybrid |
| Chilling centre | Chillers 20 to 100 kW, transfer pumps, ice bank | Collection delayed, milk spoiled in tankers | DG with AMF, often many hours per day |
| Reception and raw milk silos | Pumps, agitators, chilled water | Raw milk out of temperature | High |
| Pasteurisation and processing | HTST unit, homogeniser, separator, hot water set | Flow diversion, product recycled or dumped, re-sterilisation needed | High for controlled stop; continuity if hours justify |
| Refrigeration plant | Ammonia screw or reciprocating compressors, condensers, glycol pumps | Cold store and process cooling lost | Highest; staged restart |
| Packing and cold store | Fillers, conveyors, evaporator fans | Packed product warms, dispatch stops | Cold store high, packing medium |
| CIP, boiler auxiliaries, water, ETP | Pumps, dosing, burner controls | Hygiene cycle aborted, boiler trips | High |
Chilling load sets the baseline
The refrigeration demand follows the milk, and the milk arrives in two peaks a day. The generator at a chilling centre has to carry the chillers at full output during the reception peak, when the plant is pulling warm milk down, and a much smaller load between peaks. The thermal duty is easy to estimate and worth doing, because it anchors the compressor size the generator must start.
Q = m × c_p × ΔT / t; P_elec ≈ Q / COP
Q is the cooling duty in kW, m the mass of milk chilled in kilograms, c_p its specific heat of about 3.9 kJ per kg per kelvin, ΔT the temperature drop, t the time in seconds, and COP the refrigeration coefficient of performance at the operating conditions. A worked example (illustrative): chilling 10,000 litres of milk from 35 °C to 4 °C in three hours needs 10,000 × 3.9 × 31 / 10,800, about 112 kW of cooling. At a COP of 2.5 on a hot day the compressors draw roughly 45 kW electrical, with condenser fans, pumps and agitators adding more. The generator must carry that plus the starting step of the largest compressor; a chilling centre of this size commonly lands on a 125 to 160 kVA set once the pump and lighting loads are included. Ice bank systems shift part of this duty to off-peak hours and reduce the step the generator sees. See generator sizing.
Pasteurisation, homogenisers and controlled stops
An HTST pasteuriser has a flow diversion valve that returns milk to the balance tank the moment holding temperature is lost, which happens as soon as the hot water set and pumps stop. On restoration the unit must re-establish temperature, and depending on the plant’s procedure the system may have to be re-sterilised before product flows again. The homogeniser is the largest single motor on most processing lines, often 75 to 160 kW, and starting it on a generator mid-process is the step that trips undersized sets. The separator is a high-inertia machine that takes minutes to accelerate and should never be restarted until it has spun down or the supplier’s procedure allows. The plant’s quality system decides whether interrupted product is reprocessed or rejected; the generator’s task is to make the stop and restart orderly. Controls, flow diversion logic and recorders belong on a UPS so the record of the event survives. See motor starting.
Refrigeration restart and compressed air
Ammonia plants restart in a fixed order: glycol or chilled water pumps, condenser fans and pumps, then compressors one at a time with slide valves unloaded or capacity control at minimum. The refrigeration PLC should hold this sequence automatically on generator supply, with timers long enough for the engine to recover frequency between steps. Compressed air is the quiet dependency; fillers, valves and CIP all stop without it, and the air compressor must restart unloaded early in the sequence. Dairy plants with several sets should parallel them with load-dependent start and stop so that the between-peak load does not leave a single large engine at 20 per cent for hours. See load management and paralleling.
Duty, fuel and the monsoon
At coolers and chilling centres on weak feeders, the generator is not standby equipment. Sets that run 1,500 to 3,000 hours a year must be rated for prime duty, serviced by hours rather than by calendar, and supplied with fuel on a schedule that survives the monsoon, when village roads and tanker access are at their worst. A level sensor and a remote hour meter on each set let the dairy plan deliveries and services from the office instead of discovering a dry tank at the morning reception. New sets up to 800 kW are CPCB IV+ compliant and need clean fuel and correct oil for their aftertreatment, which matters for sets refuelled from barrels in the field. The service structure we use for distributed dairy fleets is described in annual maintenance contracts.
Common mistakes
- Sizing the chilling centre set from the connected load rather than the reception peak and compressor start. The set is too large for most of the day and still trips at the peak.
- Restarting the homogeniser or all compressors immediately after transfer. The step exceeds the set’s acceptance and the plant loses power a second time with milk in the lines.
- Buying standby-rated sets for coolers that run several hours every day. Engines wear out in a few years and the warranty position is weak.
- Leaving the pasteuriser controls and recorders off the UPS. The flow diversion record is lost and the batch cannot be released.
- Planning fuel delivery for dry-season roads. The monsoon cuts access, tanks run dry, and milk is rejected at the point of collection.
What to send us
Send the daily milk intake pattern with the reception peaks, the refrigeration plant schedule with compressor types and ratings, the processing line motor list with the homogeniser and separator details, the number and location of coolers and chilling centres, and the hours each existing set runs per month. Photographs of the generator nameplates, controllers and fuel storage at a sample of sites tell us a great deal. Send your application details or ask for a fleet review of your collection network.
Questions engineers ask
What size generator does a bulk milk cooler need?
Most village coolers of 1,000 to 5,000 litres fall in the 7.5 to 15 kVA range once the compressor start and agitator are considered, and a solar-battery hybrid can cut the diesel hours substantially. The compressor’s starting current and the local voltage conditions decide the final figure.
How long can milk sit in a silo without chilling?
That is a quality decision for the dairy, based on the milk’s temperature and the ambient conditions, and it is short. The generator and refrigeration restart should be designed to restore chilled water well within the limit the quality team sets, and the limit should be tested, not assumed.
Can the dairy run full processing on diesel?
Yes, if the sets are sized for the homogeniser start and the refrigeration restart and rated for the hours involved. Many dairies instead protect refrigeration, reception, CIP and cold storage and treat processing as a controlled stop, because the hours lost are cheaper than the capacity needed.
Why do our chilling centre generators fail early?
Almost always because they are standby-rated machines running prime duty, serviced by calendar instead of hours, and fuelled from unfiltered barrels. Re-rating the fleet and moving to hours-based service with fuel filtration addresses all three causes.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.