To gradually adjust a flow rate on which thermal power, and therefore temperature, depends, as indicated above. It is then a question of choosing an “equal percentage” valve with a value of Kvs which ensures its authority. It is also necessary to consider its leakage rate Kv0 and its Kvr or its adjustment range.
Thus, to regulate the different globe valve systems, the main qualities expected from valves are not all the same.
Different models are offered by manufacturers to be adapted to different control applications, among the most common:
- Water temperature at the top of the heating or cooling distributions
- Air temperature downstream of the water-air heating or cooling coils
- Domestic hot water temperature produced by exchanger
- Energy supply by exchanger on the heating or cooling networks
- Ambient temperatures provided by the terminals: radiators, fan coils and other air conditioning terminal units
The valves are subject to several NF EN or ISO standards to define their characteristics and tests. But none deals specifically with characteristics suitable for energy applications in buildings and HVAC engineering.
Control valves are however built for these applications, their roles in the energy performance of buildings are of prime importance.
They should be the object of greater attention than the regulators or other automatisms with which they are associated.
The making
As we mentioned in the article The process of making an espresso , there are a number of controllable factors that we must take into account in order to make the perfect espresso. One of these is the pressure in the filter holder. In this article I will try to present the technical elements that control the delivery pressure and how it can influence the final product.
According to connoisseurs, for the preparation of a quality espresso, the ideal pressure with which the water must be pumped into the filter holder is about 9 bar, and the extraction time must be about 22-25 seconds. However, most commercial espresso machines, designed for home use, come factory-set at a delivery pressure of 12 bar. This pressure difference does not produce major differences in the brewed coffee. But these differences, for a connoisseur, are visible.
To understand the importance of delivery pressure, we need to know what its effects are on brewed coffee:
- Too low pressure: requires a grinder with a larger grain, which allows water to enter through the coffee puck in the filter holder. The result will be an under-extracted espresso, too weak, which is due to the fact that the coffee particles do not have sufficient exposure to water.
- Excessive pressure: imposes a grinder with a finer granulation, which does not allow the water to pass too easily through the coffee puck in the filter holder. The result will be an over-extracted espresso, too hard, which is due to the fact that the coffee particles have a much too much exposure to water.
From a technical point of view, the mode of operation of an espresso machine is as follows: the pump pushes the water towards and through the preparation group. To avoid excessive delivery pressures, most espresso machines are equipped with a safety valve, sometimes called a relief valve or overpressure valve (OPV). The construction of an overpressure valve differs from one espresso machine to another, but the functionality is the same: maintaining the maximum water pressure at which the espresso coffee is delivered. It is very important to note that this valve does not maintain the pressure at a preset level, but only controls the maximum pressure that can be reached.