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The “Tank-in-Tank" concept

Tank-in-Tank: what is it?

The Tank-in-Tank is a Domestic Hot Water storage exchanger (DHW) completely immersed in a steel outer tank that contains primary water. At ACV, the inner “tank" is always made of stainless steel. The wall of the inner “tank" itself is a heat exchanger between the primary circuit (connected to the boiler) and the DHW storage tank (Fig.1).

Tank-in-Tank: how does it work?

The boiler is connected to the primary circuit of the water heater: the hotwater storage tank thermostat switches on the pump between the boiler and the water heater when the temperature falls below the selected set-point. The heat generated by the boiler is transferred to the storage tank via the heat exchanger (the wall of the inner "tank") and heats the DHW to the desired temperature.(Fig.2)

The benefits of the “Tank-in-Tank" system

  • All the hot water you need: the secret to highly stabile hot water supply

    The large surface area of the heat exchanger (often twice that of a coil) enables the use of all of the boiler’s output to heat DHW very rapidly. That avoids the necessity of stopping the boiler whilst the tank is being refilled, which blocks the heating system and causes the boiler to start and stop repeatedly, thereby impairing fuel economy and increasing the rate of harmful emissions. (Fig.3)

  • Minimum storage and reduced losses

    Thanks to the exceptional heat transfer characteristics of the Tank-in-Tank system, the volume of hot water stored can be reduced, resulting in a more compact water heater design and reducing static heat losses via the exterior walls: that cuts initial investment and operating expenses.

  • Sanitary and maintenance-free

    At ACV, the inner tank is always made of stainless steel: stainless steel is sanitary (it is generally used in hospitals and in the pharmaceutical and food and beverage industries), requires no maintenance whatsoever (it does not require anode protection), and enables the use of high storage temperatures.

  • Self-descaling: high efficiency and long service life

    Each DHW draw-off creates a brief underpressure in the tank (at the moment that the draw-off tap is opened), followed by a slight overpressure (upon closure of the draw-off tap). As the inner tank is free to move, its walls expand and contract very slightly under the influence of these pressure changes and prevent the formation of lime scale on the exchanger surface: thorough testing and the testimony of numerous users in regions with calcium-laden water confirm that ACV Tank-in-Tank water heaters are truly effective in preventing the formation of lime scale deposits as opposed to traditional coil systems that necessitate frequent descaling.

    Thanks to this self-descaling feature, the heat transfer efficiency of our water heaters and their hot water output remain high throughout the service life of the installation. (Fig.4)

  • Anti-legionellae

    Legionellae bacteria are micro-organisms that can cause serious diseases of the respiratory tract. Legionellae remain in a latent state in cold water (below 20°C) and can proliferate in warm water (from 20°C up to 45°C). They can be eliminated (in a few hours), however, at temperatures higher than 50°C – or in a few minutes above 60°C. It is recommended never to store the hot water thereafter at a temperature below 60°C.

    The “Tank-in-Tank" system prevents the formation of warm pockets because even the base of the inner tank is a heat exchanger that remains at the temperature of the boiler (typically from 60 to 80°C): thus, any “Tank-in-Tank" water heater operating at 60°C ensures that the hot water remains free of legionellae and safe for the consumer. (Fig.5)

  • Installation in any position

    Tank-in-Tank systems can be installed in any orientation because the temperature is uniform throughout the tank. Note that when electrical heating element booster systems are fitted to Tank-in-Tank water heaters, they are always located in the primary circuit: this keeps the hot water perfectly clean and prevents the formation of lime scale deposits on the heating element. Yet another example of our focus on sanitation and long, maintenance-free service life!


Combined boilers and water heaters

 

The “Tank-in-Tank" type combined boiler and water heater: what is it?


A combined boiler and water heater is a boiler that integrates the heating function together with domestic hot water generation (DHW).

In a “Tank-in-Tank" boiler, the tank is ring-shaped and the flue pipes runthrough its centre. The tank is made of stainless steel and holds alimited volume of water to meet short term demand. The tank wallbecomes an indirect heat exchanger for generating DHW.

As with all ACV stainless steel Tank-in-Tank systems, the boiler operates at high domestic hot water temperatures(above 90°C if necessary), and is available in versions that arespecially designed for very corrosive water such as sea water. Ofcourse, the boiler has a primary circuit and provides heat during theheating season. (Fig.1)



The “Tank-in-Tank" type combined boiler and water heater: how does it work?


Theboiler operates in heating mode just like any traditional boiler: theboiler thermostat controls the burner and lights it when the boilertemperature drops below the set-point.

In hot water draw-offmode, a domestic water thermostat senses the introduction of cold waterto the tank, cutting off the heating pump in order to save all theboiler’s output for heating. Its large heat exchange surface areaallows the tank to absorb all of the burner’s output to gradually heatthe water as it enters the tank: hot water supply is exceptionallystabile and the periods of interruption in heating mode are limited toa strict minimum. (Fig.2)



The benefits of the “Tank-in-Tank" system


  • All the hot water you need: the secret to ensuring highly stabile hot water supply

    The tank’s large surface area and the burner’s high output guarantee an unparalleled stability of hot water supply, very rapid refilling of the tank and minimal interruption of heating mode.

  • Minimum storage and reduced losses

    Thanks to the exceptional heat transfer characteristics of the Tank-in-Tank system, the volume of hot water stored can be reduced, resulting in a compact boiler design and reducing static heat losses via the exterior walls: that cuts initial investment and operating expenses.

  • Sanitary and maintenance-free

    At ACV, the inner tank is always made of stainless steel: stainless steel is sanitary (it is generally used in hospitals and in the pharmaceutical and food and beverage industries), requires no maintenance whatsoever (it does not require anode protection), and enables the use of high storage temperatures.

  • Self-descaling: high efficiency and long service life

    Each DHW draw-off creates a brief underpressure in the tank (at the moment that the draw-off tap is opened), followed by a slight overpressure (upon closure of the draw-off tap). As the inner tank is free to move, its walls expand and contract very slightly under the influence of these pressure changes and inhibit the formation of lime scale on the exchanger surface. Thanks to this self-descaling feature, the boiler’s efficiency and hot water output remain high throughout the service life of the installation.

  • Anti-legionellae

    The tank prevents the formation of warm pockets because even the base of the inner tank is a heat exchanger that remains at the temperature of the boiler (typically from 60 to 80°C): thus, any Tank-in-Tank operating at 60°C ensures DHW that is free of legionellae and safe for the consumer.

  • Fuel oil, gas and electricity

    ACV Tank-in-Tank boilers can be operated with fuel oil or gas and there is even a range of electric Tank-in-Tank boilers! That enables users to adapt to changes in the market (which can change significantly over the course of an ACV boiler’s long service life) and to convert their boilers to a different fuel than the one selected originally.


    It comes as no surprise that ACV combined boilers and water heaters are selected by our customers in more than 40 countries situated at all latitudes, from Siberia (outside temperatures as low as – 65°C) to Mexico and the Persian Gulf (outside temperatures over 50°C). Do you know of any other integrated system that provides heating in the winter and hot water 365 days per year, all the while delivering exceptional performance and stability throughout the service life of the installation?


 

 

 

 

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