Because of this, the heating coil required to achieve the design load is relatively small. If a DN50 valve is fitted and the valve is fully open, the pressure drop is relatively small across the valve, and the steam supplied to the heat exchanger is at a fairly high pressure (and temperature).The valve is held fully open (see Figure 6.4.1). With saturated steam, temperature and pressure are strictly related therefore controlling the steam pressure easily regulates the temperature.Ĭonsider an application where steam at 10 bar g is supplied to a control valve, and a given mass flow of steam passes through the valve to a heat exchanger. To achieve this heat output, a certain saturated steam temperature will be required at the heat transfer surface (such as the inside of a heating coil in a shell and tube heat exchanger). Saturated steam flow through a control valveĪ heat exchanger manufacturer will design equipment to give a certain heat output. ΔTm is reduced, then the heat transfer from the steam to the secondary fluid is also reduced.

The overall heat transfer coefficient (U) does not change very much during the process, and the area (A) is fixed, so if the mean temperature difference This means that a smaller difference in temperature exists between the steam and the process, so the rate of heat transfer is reduced, in accordance with Equation 2.5.3. The steam pressure falls in the steam space and so too the steam temperature. Closing the valve reduces the mass flow.(The action of opening and closing the valve is often referred to as increasing or decreasing the ‘valve lift’ this is explained in more detail in Module 6.5, ‘Control Valve Characteristics’). If a modulating control system is used, as the temperature of the process approaches the controller set point, the controller will close the valve by a related amount, thereby reducing the steam flowrate to maintain the lower pressure required to sustain a lower heat load.Should, at any time, the flowrate of steam through the valve be less than the condensing rate (perhaps the valve is too small), the steam pressure and the heat transfer rate in the heat exchanger will fall below that which is required the heat exchanger will not be able to satisfy the heat load. The rate of steam flow into the equipment is governed by this pressure difference and the valve orifice size.The reduced pressure in the steam space means that a pressure difference exists across the control valve, and steam will flow from the high-pressure zone (upstream of the control valve) to the lower pressure zone (the steam space in the equipment) in some proportion to the pressure difference and, ideally, balancing the rate at which steam is condensing.This means that when steam condenses, the pressure in the steam space is reduced. The volume of condensate is very much less than steam.Steam condenses on the heat transfer surfaces, creating condensate.Steam passes through the control valve and into the steam space of the equipment where it comes into contact with the heat transfer surfaces.Steam is supplied at a specific pressure to the upstream side of the control valve through which it passes to a heat exchanger, also operating at a specific pressure.Before discussing the sizing of control valves for steam systems, it is useful to review the characteristics of steam in a heat transfer application.

Larger than light steam