The plant is built once; the site is fixed
A plant for an export project is assembled in a workshop and then travels to a site whose conditions were decided long before the order. The container dimensions and the delivery route can be adjusted in the commercial conversation, but the frequency of the supply and the climate cannot be adjusted at all. That asymmetry is why site conditions have to be settled early.
The site data that matters is short: the supply voltage, frequency and phase count; whether the supply is a grid or a generator and how stable it is; the range of ambient temperature and humidity; the coldest-month water temperature; the available footprint and access; and the discharge point. Each changes the equipment that can be offered, and each is easier to answer before the offer than after it.
Frequency cannot be changed by a transformer
Grid frequency is set by the market the site sits in, and it belongs to either the 50 Hz family or the 60 Hz family. A transformer changes voltage, and only voltage: it cannot change the frequency of the supply that passes through it. A plant built for one frequency does not become a plant for the other by adding a transformer.
The equipment that cares is any drive whose speed is tied to the supply. An induction motor turns faster when the frequency rises, and because the load it drives does not follow in proportion, it draws more current and heats up. A pump built to turn at one speed, moving water against a resistance fixed by the pipework, does not simply run faster in a useful way.
The real answers are electronic rather than magnetic. A variable frequency drive takes the incoming supply at whatever frequency the site provides and creates its own output frequency, so the motor sees the frequency it was built for. A rectifier followed by a direct-current bus does the same for equipment that runs on direct current. Both should be decided before the offer rather than after delivery.
50 Hz vs 60 Hz: what moves, and in which direction
The two supply families differ by a fifth, and the interesting part is that the same fifth does not arrive at the same size on every piece of equipment. The relations below are the conventional ones and are given to show direction rather than to be used as conversions; the value that applies to a particular machine comes from its own curve.
- A motor: the speed of an induction motor follows the supply frequency and the number of poles in the winding. The same motor on a 60 Hz supply turns faster than on 50 Hz, and the direction of that change is fixed — 60 Hz is the faster of the two for a given winding.
- A centrifugal pump: flow moves roughly in proportion to speed, head roughly with the square of it, and shaft power roughly with the cube. Running a pump faster therefore buys flow in a straight line but asks for power disproportionately, and it also moves the duty point along the pump curve, away from the point the pipework was designed around.
- A fan or blower: air volume moves roughly with speed, pressure roughly with the square and power roughly with the cube, as with the pump. A blower sped up to deliver more air costs appreciably more energy than the extra air alone would suggest, and it can move the operating point away from the range the diffusers were selected for.
- The reverse direction: a motor wound for 60 Hz on a 50 Hz supply turns more slowly, so a pump delivers less flow and a blower less air, and a plant sized on the nameplate figures will under-perform on aeration before it fails on anything else.
- Voltage per hertz: a motor keeps its designed magnetic working point only if the supply voltage and frequency stay in the ratio it was wound for. A frequency change without the corresponding voltage is a design change to the motor, not an adjustment of it.
- Mechanical consequences: higher speed also raises bearing loads, vibration and the noise of the driven machine, which is part of why the driven equipment is checked as well as the motor.
A variable frequency drive removes the constraint, because the motor then sees the frequency the drive produces rather than the frequency of the site supply. Where a drive is fitted, the site frequency stops being a limit on the machine; where a drive is not fitted, it decides how the machine has to be wound. The two cases should be settled per drive, not for the plant as a whole, because a plant usually carries both.
The practical consequence is a short list to settle before the offer is fixed rather than after the plant lands, and it belongs in the same enquiry as the voltage and the phase count.
- The site frequency written down as a number, with the country it belongs to.
- Which drives are on a variable frequency drive and which run direct on line.
- The frequency each motor is wound for, and whether the nameplate is dual-rated.
- The pump duty and the blower air volume at the frequency the site actually supplies, rather than at the frequency the machine was tested on.
- Whether the control cabinet, the instruments and any timer or clock in the panel behave the same at the site frequency.
- Whether the site also runs on a generator, and whether that generator supplies the same frequency as the grid.
Voltage, phase and the supply the equipment is built for
Voltage is the part of the supply a transformer can deal with, and the phase count has to be fixed before the equipment is chosen. A three-phase motor and a single-phase motor are different machines, and the control cabinet is built around whichever the site provides.
The published operating voltage of the incinerator range here is 380 V or 220 V, quoted on the basis of the supply the equipment is built for. A wastewater plant carries its own electrical specification, fixed during sizing from the site data, which is why the checklist asks for the power supply as voltage, frequency and phase together rather than as a voltage alone.
The practical answer is to state all three in the enquiry. A site described only as 380 V leaves the frequency and the phase count open, and an offer built on an assumption may have to be revised later. The three values together let the drives, the control cabinet and any frequency conversion be chosen once.
Generators, stability and continuous loads
A plant that runs on a generator is a different electrical project from one on a grid, and the difference is the shape of the load rather than its size. The aeration blowers and, on a membrane plant, the permeate and cleaning pumps run continuously: they are the base load whenever the plant is working. A generator has to carry that base load plus the starting current of the largest motor, and a supply that is adequate for average demand may still fail when a blower starts.
Stability matters as much as capacity. On a site that runs on a generator or a grid that sags, a weak supply is a design input rather than a fault to be resolved later: the control and instrumentation have to tolerate the variation, and the process has to be protected against interruption. The guidance for remote sites puts it plainly: the power available on site decides the drives, the control mode and any standby arrangement.
Climate: temperature, humidity and coastal air
Ambient conditions reach the equipment and the biology at the same time. High ambient temperature changes the working point of motors and electrical equipment, and it changes the oxygen demand of the biology, because warm water holds less dissolved oxygen than cold. Coastal air adds salt to the same enclosure, and humidity adds condensation inside control cabinets. The container envelope and the materials are specified against the ambient range of the site for that reason.
Winter water temperature deserves its own line in the site data. Nitrification slows in cold water, so the aerobic volume needed to hold an ammonia limit grows as the water cools; and the ammonia limit itself depends on temperature, because the grade carries a bracketed value that applies at or below 12 °C. A design compared against the summer value alone can be compliant in the warm season and short in winter.
Spares and remote support
Distance changes the maintenance plan. Where a technician is a flight away and a spare part is a shipping cycle away, the plant is chosen and stocked differently. A membrane plant has modules that are cleaned periodically and replaced eventually, and its spare strategy follows that cycle. Extended aeration has no membrane to replace, and it can be the better fit where the site is far from specialist support.
The other half of remote support is the documentation that travels with the plant. An operation and maintenance manual, a control philosophy and a spare parts list let a site operator keep the plant running between visits, and they belong in the delivery rather than in a later request. The plants here are tested before shipment, which shortens commissioning on site but does not remove the need for that site file.
What to write into an export enquiry
An enquiry that answers the following list can be priced without a round of clarification.
- The supply voltage, frequency and phase count at the site.
- Whether the supply is a grid or a generator, and how stable it is.
- The range of ambient temperature and humidity through the year, and the coldest-month water temperature.
- The available footprint, and the access for a container delivery with the lifting available.
- The discharge point and the standard the plant has to meet.
- The location of the site, so the delivery route can be planned.
- The spare parts and service strategy the site can support.
The list is short because the site data does most of the work. A plant matched to the conditions it will work in arrives ready to connect; a plant matched to conditions the site does not have arrives with a list of changes.