Bulk tank

In dairy farming a bulk milk cooling tank is a large storage tank for cooling and holding milk at a cold temperature until it can be picked up by a milk hauler. The bulk milk cooling tank is an important milk farm equipment. It is usually made of stainless steel and used every day to store the raw milk on the farm in good condition. It must be cleaned after each milk collection. The milk cooling tank can be the property of the farmer or being rented to the farmer by the dairy plant.

Bulk tank types

Raw milk producers have a choice of either open (from 150 to 3000 litres) or closed (from 1000 to 10000 litres) tanks with direct expansion or with ice builder. The cost can vary considerably, depending on manufacturing norms and whether a new or second hand tank is purchased.

The tank capacity and type will depend on herd size, calving pattern, frequency of milk collection, required milk quality, energy and water availability and future plans for development.

In direct expansion tanks, milk is cooled by refrigerant plates, which are in direct contact with the outer surfaces of the tank walls.

In ice bank tanks, milk is cooled when chilled water, from an ice bank, is sprayed against the outer surfaces of the tank walls.

Milk silos (10,000 litres and plus) are suitable for the very large producer. These are designed to be installed outside and adjacent to the dairy, all controls and the milk outlet pipe being situated in the dairy.

Bulk tanks manufacturing norms

Norms define among other criteria : insulation, milk agitation, cooling power required, variations in milk quantity measurement, calibration, ... Some are more demanding than others.

ISO ISO standard 5708 published en 1983. Northern American standard 3A 13-10 updated in 2003. European Union standard EN 13732 published in 2003

Milk cooling tank description

A milk cooling tank, also known as a bulk tank or milk cooler, consists of an inner and an outer tank, both made of high quality stainless steel. For a direct expansion tank, attached to the inner tank is a system of plates and pipes through which the refrigerant fluid/gas flows. The refrigerant withdraws heat from the tanks content (milk in this case). Direct expension milk cooling tank come with a generator set with a condensing unit which circulates the refrigerant and conveys the withdrawn heat to the air.

The space between the outer tank and the inner tank is isolated with polyurethane foam. In case of a power failure with an outside temperature of 30° C, the content of the tank will warm up only 1° C in 24 hours.

To facilitate an adequate and rapid cooling of the entire content of a tank, every tank is equipped with at least one agitator. Stirring the milk ensures that all milk inside the tank is of the same temperature and that the milk stays homogeneous.

On top of every closed milk cooling tank is a manhole of about 40 centimetres diameter. This enables thorough cleaning and inspection of the inner tank if necessary. The manhole is covered by a lid and sealed watertight with a rubber ring. Also on top are 2 or 3 small inlets. One is covered with an air-vent, the other(s) can be used to pump milk into the tank.

A milk cooling tank usually stands on 4, 6, or 8 adjustable legs. The built-in tilt of the inner tank ensures that even the last drop of milk will eventually flow to the outlet.

At the bottom, every milk cooling tank has a threaded outlet, usually including a valve.

All tanks have a thermometer, allowing for immediate inspection of the inner temperature.

Most tanks include an automatic cleaning system. Using hot and cold water, an acid and/or alkaline cleaning fluid, a pump and a spray lance will clean the inner tank, ensuring an hygienic inner environment each time the tank is emptied.

Almost every tank has a control box. It manages the cooling process by use of a thermostat. The user can turn the system on and off, allow for extra and immediate stirring, start the cleaning routine, and reset the entire system in case of a failure.

New and bigger milk cooling tanks are now being equipped with monitoring and alarm systems. These systems guard temperature of the milk inside the tank, check the functioning of the agitator, the cooling unit and temperature of the cleaning water. In case of malfunctioning of any of these functions, the alarm will activate. The monitoring system will also keep a record of the temperature and of all malfunctions for a given period.

Milk pre-cooling

For energy savings and quality reasons it is advisable to pre-cool with the milk before it enters the tank using a plate or a tube cooler (shell and tube heat exchanger) supplied with chilled water from the well water, the ice builder or the condensing unit. The quicker milk is cooled after leaving the cow the better. This system achieves most of the cooling before the milk enters the tank, so that chilled milk, rather than warm milk, is being added to the already cooled milk in the tank.

Cooling temperature

Generic temperature for milk storage is 3 to 4°C. For raw milk cheese manufacturing, it would be advisable to keep the milk at 12°C, as milk characteristics will be kept in a better state.

The milk cooling tank is usually not completely filled at once. A 2 milking tank is designed to cool 50% of its capacity at once. A 4 milking tank is designed to cool 25% of its capacity at once, and a 6 milking tank is designed to cool 16.7% of its capacity at once.

The cooling performance depends on the number of milking it takes to completely fill the tank, the ambient temperature and the cooling time.

Milk cooling tank cleaning system

Automatic bulk tank cleaners are available for closed tanks and are normally activated by the milk collection truck driver after each milk collection. Hot wash involves the following stages : - pre-rinsing with cold water, - pre-rinsing with hot water to heat up the surfaces of the tank, - circulating a suitably formulated detergent steriliser solution at 50°C for ten minutes, - rinsing with cold water (in some cases chlorinated), - final rinsing with cold drinkable water. Tanks washed with hot detergent sterilisers should be treated with milkstone remover once per month or more often if required, in accordance with manufacturer’s instructions. Care should always be taken when handling chemicals. The correct health and safety procedures need to be followed.

Operating costs

Substantial reductions in running costs can be made when an ice builder is used in conjunction with off-peak electricity. Pre-cooling milk using a plate or a tube cooler supplied with mains or well water can also reduce costs and add to the cooling capacity of the tank.

Bulk tank condenser units, which are not an integral part of the tank, should be fitted in an adjacent, suitable and well ventilated place.

If at all possible, condenser units should not be fitted on a south facing wall. They should be installed in a way which allows them to draw in and discharge adequate quantities of air for efficient operation.

Bulk tank should be easily accessible by large bulk collection tankers and positioned so that the tanker approaches can be kept clean and free from cow traffic at all times.

Although tanks have been calibrated when first installed, bulk tank miscalibration is not uncommon and in some cases it can result in significant loss of income. Milk tanks calibrated on the low side, can cheat raw milk producers by up to 22 litres on each shipment. It is therefore advisable to re-calibrate a bulk tank.

Bulk tank outlet standards

Swedish outlet (SMS 1145), German outlet (DIN 11851), English RJT (BS 4825), IDF (ISO 2853), tri-clamp (ISO 2852), Danish outlet (DS 722), .... can be found, not to mention different diameters. They vary from country to country. Non standard outlets are often painful for the milk collection process as the operator needs to adapt to each different standard/diameter.

Bulk milk cooling tank maintenance suggestions

Check the agitator motor for grease leaks, noisy operation, worn shaft shields and bearings. Replace leaking agitator motor seals. Tighten bolts holding motor mounting brackets. Replace worn shaft shields and bearings.

Check the timer to be sure that it will start the agitation process and advance to the "off' position. Replace timer if not ok.

Check thermometer accuracy, should read 0°C when in ice water. Be sure it is not sticking. Replace thermometer if faulty.

Check milk tank outlet valve for leaks and cleanness. Replace valve "O" ring, if leaking.

Check the running time of the cooler. It should cool to 10°C within one hour of first milking and cool to 4°C and hold that temperature after the second hour. The blend temperature of the second, third and fourth milking should remain under 10°C. If running time is too long check and clean condenser coil. Check refrigerant.

Check for foam, churned milk, and frozen milk on the milk surface. The presence of foam indicates air indicates air leaks in the milking system or excessive agitation of the milk; churned milk (clumps of fat floating on the surface) usually is caused by excessive agitation and slow cooling of the milk. Check refrigerant; and frozen milk on the surface or as layers of ice on the bottom of the tank. The freezing of milk can be avoided by turning on the refrigeration when the milk level reaches the level of the agitator blades and setting the tank thermostat so that the milk is cooled to 4°C.

Check condenser coil for dirt or dust. Air must flow freely through the coil and exit into the atmosphere. If condensor coil is dirty turn disconnect switch to "off" position, brush wash with milk detergent solution, rinse with tap water from the fan side out and allow to drain for three hours before restarting.

Check refrigeration unit and tubing for signs of leaks (grease spots). Call to the attention of refrigeration service man, any air or refrigeration leaks.

Check refrigerant sight glass after unit has been operating 15 minutes refrigerant should be clear without any indication of foaming. If refrigerant is foaming, have refrigerant added by serviceman.

Check condensor fan motors. Call to the attention of refrigeration service man any malfunction of the condensor fan motor

Other usage of milk cooling tanks

Milk cooling tanks are also used to heat or cool a fluid or simply to keep it isolated and warm/cold. Because of the hygienical finishing of the inner and outer side of the tanks, almost any fluid can be stored : water, fruit juices, honey, wine, beer, ink, paint, cosmetics, aromatic food-additives, bacterial cultures, cleansers, oil, blood, ...