A generator set for cold store duty carries the refrigeration load through a defrost cycle and starts in a cold yard. Compare the rating, the block heater and the enclosure against the plant load list before you accept one. Tesla Power sizes the set on that list; send it to the technical desk and we confirm the frame.
- Ask for the load list with the defrost cycle shown, not just the running load.
- Check the block heater and the battery against the coldest night the site records.
- Look at the enclosure, because a cold store yard is wet as well as cold.
- Ask how long the set has to carry the plant before the mains returns.
What does a generator set for cold store duty have to do?
It has to carry the refrigeration plant, the lighting and the dock doors for as long as the mains is out, and it has to start on the coldest night of the year. A cold store loses stock in hours rather than days, so the set is a business continuity item rather than a convenience, and the stock is lost before the fault is found. The load is steady and the start is the hard part.
The set also has to hold a load that steps when a compressor starts and when a defrost heater switches in. Those two steps are different sizes and they arrive at different times. Tesla Power sizes the alternator for the larger of the two rather than for the total, because the total hides the step, because the plant does not draw it at once.
Which load matters at a cold store?
The running load of the compressors is the figure a buyer knows and it is not the figure that sizes the set. The defrost heaters and the condenser fans switch on together and they can add a third to the running load. A set sized on the running load trips on the first defrost after a power cut, and the alarm arrives after the compressors have already stopped.
A 500kVA set at 0.8 pf gives 400kW: 500kVA x 0.8 = 400kW, and a blast freezer with a defrost cycle can pull close to that on its own. Ask for the load list by circuit with the start and the run figures. Tesla Power Equipment checks the list against the compressor schedule before quoting a frame, because a compressor schedule and a circuit list often disagree.
| Item | Typical figure | Note |
|---|---|---|
| Running load | From the circuit list | Compressors and fans together |
| Defrost step | Up to a third again | Stagger the heaters if you can |
| Cold start | -25 C with a block heater | Battery heater as well |
| Hold time | 8 h at the hold load | The figure the insurer asks for |
Where is the set installed?
A set in a plant room is warm and quiet and it needs a fire rating and a louvre that pulls air from outside. A set in the yard is cold and wet and it needs an enclosure with a heater and a drain. The two positions need different sets from the same frame, so the position belongs in the enquiry, and it changes the canopy, the heater and the cable route.
A set placed against the cold store wall also takes the heat the condensers reject, and a set in that corner runs hotter than the ambient figure suggests. Keep the set away from the condenser discharge and off the loading apron. Ask for the position on the site plan rather than a description. Tesla Power marks the clearances on the general arrangement so the yard is laid out once, and a set that is moved later costs a cable run and a plinth.
What does the cold do to the set?
Cold air is dense and it helps the engine, and cold metal is hard to lubricate and hard to start. A block heater and a battery heater are the two items that decide a start at -25 C, and a set without them cranks slowly and floods. Ask for the heater rating and the thermostat setting on the same line, because a heater without a thermostat runs all summer.
Cold also condenses water inside a canopy when the set warms up, and that water lands on the alternator windings. A canopy heater on a thermostat and a drain at the low point keep the windings dry. Ask for the heater and the drain together. Yangzhou Tesla Power fits both on a yard installation and leaves the plant room set without them, because a plant room rarely drops below freezing.
How is the defrost load handled?
A defrost cycle is a resistive load that switches in steps, and each step is a load step the set has to absorb. A set with a small block dips in frequency at each step and the compressors see it. Sizing for the defrost total is the answer, and a load shedding relay is the cheaper one where the plant is already built.
A relay that holds the heaters off until the set is stable is worth more than a larger frame on a plant that is already built. On a new plant the defrost control can be set to stagger the heaters by a few seconds. Ask for the stagger in the control philosophy. A defrost that steps in one block is a defrost that tests the set every cycle, and the frequency dip is what the compressor contactors see.
What does the file need for a cold store?
A cold store file needs the load list, the start sequence, the fuel figure for the hold time and the maintenance schedule for a set that runs rarely. It also needs the ammonia or the refrigerant data where the plant room shares a space with the set, because a leak changes the ventilation requirement. Ask for the ventilation figure in the same pack, since the two systems share a room and a risk.
The hold time is the figure the insurer asks for, and it comes from the tank size and the burn at the hold load. A set with 4 h of fuel and a plant that needs 8 h is a set that fails the audit. Ask for the hold time in hours rather than the tank in litres, because the hours are what the site is judged on.
What does the offer need to state?
The offer should give the rating at the plant load, the step capability, the block heater, the battery, the enclosure class and the hold time. Six items on one line let a buyer check the set against the load list and the insurer without reading a drawing, and the hold time is the line an insurer reads first. Ask for the coldest start temperature the set is warranted for.
The wording on the offer matters too. Chinese generator set for cold suppliers often quote a rating with no step figure, and a generator set for cold store manufacturers page can show a heater the factory fits only on request. Ask for generator for a blast freeze, generator in a cold store and generator for the freezer as three separate cases, since the second of the three is the one that decides the defrost step. Tesla Power prints the six items on the plant room drawing.
What does the handover file record?
The file carries the rating at the plant load, the step capability, the heater settings, the hold time and the coldest start temperature. One page covers the set when those five items sit under the serial number, and the heater settings are the page the technician uses in winter. Add the conformity assessment for the store when the plant is a food premises and the set is part of the registered equipment list.
Hand the page to the refrigeration engineer rather than to the buyer who placed the order. the Yangzhou factory describes the bay where the heater is fitted, the commissioning notes lists the frames with a yard enclosure, the build range explains how the step capability is measured, the 500kVA silent set with ATS changeover shows the export build for this rating, the ISO 8528-1 duty definitions sets the rating basis the plant load comes from, and the NFPA 110 maintenance routine covers the food premises rule behind the file, and Tesla Power writes the hold time on the same sheet.
Frequently asked questions about cold store generator sets
Size for the running load plus the defrost step, and check the result against the largest compressor start. A set sized on the running load alone trips on the first defrost. Ask for the circuit list rather than a single figure.
It needs one wherever the yard drops near freezing, and a thermostat makes it cheap to run. A set that is hard to start in January is a set that fails the one test the site cares about. Ask for the heater and the thermostat setting.
It cannot, and the two must not share a room without ventilation, because a refrigerant leak displaces air at floor level. Keep the set in its own space or ventilate the shared one. Ask for the ventilation figure with the layout.
The stock warms at a rate that depends on the insulation and the door discipline, and the set buys hours rather than days. The hold time in fuel is what the insurer measures. Ask for the hold time in hours at the hold load.
The yard is cheaper and it needs a heated enclosure, and the plant room is warmer and it needs a fire rating and outside air. Neither is better without the site plan. Ask for the position and the clearance before the frame is chosen.
Monthly on load for at least 2 h, because a set that only idles does not prove the cooling or the fuel path. A cold store that tests at no load is a cold store that finds the fault during an outage. Tesla Power writes the test into the schedule with the load figure.