Demand-controlled ventilation

Demand-controlled ventilation (DCV) is one approach to reduce energy consumption due to ventilation, that is gaining popularity in both industrial and nonindustrial applications. It is used in cases where ventilation requirements vary with time, regularly or irregularly. The control is based on a specified level of indoor air quality by means of continuous measurement of the parameters, that are expected to primarily determine the lAQ, such as the concentration of the main contaminant liberated from the production process. The principle is thus similar to the one in some better-known nonindustrial applications, e.g., CO2 levels in rooms with dense human occupancy (theaters, classrooms, etc.) or nicotine concentration in smoking rooms. See also Section 9.6. 

In thermal models, the ventilation airflow rates normally arc input parameters, to be defined by the user or to be calculated by the program on the basis of a nominal air exchange or flow rate) and some control parameters (demand-controlled ventilation, variable air volume flow ventilation systems), in airflow models, on the other hand, room air temperatures must be defined in the input (see Fig. 11.49). 

Demand-controlled ventilation (DCV) A ventilation system in which the room airflow rate is governed by an automatic control that depends on the level of a given pollutant within the space. A typical example is allowing the COt in a space to reach a certain level before the extract fans come into operation. However, in many industrial environments other pollutants control fan operation. 

Demand ventilation A system that is capable of supplying varying amounts of fresh air in response to either manual or automatic control. See Demand controlled ventilation (DCV). 

Fisk, B., and de Almeida, A. T. (1998). Sensor-Based Demand Controlled Ventilation. Energy and Buildings 29(l) 35-44. 

Fahl6n P., Andersson H. and Ruud S. (1992) Demand controlled ventilating systems Sensor tests. The Swedish National Testing and Research Institute, SP Report No. 1992 13, Boras, Sweden. 

Mechanisms of Respiratory Control. To meet the metabolic demands of the body and to maintain the acid-base balance, ventilation is regulated by various stimuli acting at several locations in the body. Although the mechanism by which each stimulus acts in amplifying or diminishing ventilation is not well known, these stimuli clearly inhibit and excite the central respiratory centers in the medulla, either directly or indirectly. The electrical impulses generated in these centers are responsible for the motor activities which produce the ventilatory response. 

The control of the activity of the respiratory muscles the process by which a pattern of activation (tidal volume, frequency of breathing) of the respiratory muscles is selected in which the average expenditure of energy is kept at a minimum for any level of ventilation demanded. 

The chemoreflex model provides a satisfactory explanation for the chemical regulation of ventilation as well as respiratory instabihty. However, it fails to explain a fundamental aspect of ventilatory control experienced by everyone in everyday fife the increase in ventilation during muscular exercise. Typically, Vp increases in direct proportion to the metaboHc demand (17002, VO2) such that the outputs of the chemical plant. Equation 11.1 and Equation 11.2, are well regulated at constant levels from rest to exercise. As a result, homeostasis of arterial blood chemistry is closely maintained over a wide range of work rates. The dilemma is if increases in metaboHc rate are not accompanied by corresponding increases in chemical feedback, then what causes exercise hyperpnea ... 

A potential drawback of such a hierarchical system is that it is nonrobust to perturbations. Changes in ventilatory load, for example, would disrupt the ventilatory command from the feedforward signal. This is at variance with the experimental observation of a load compensation response of the controller which protects ventilation against perturbations of the mechanical plant at rest and during exercise [Poon et al, 1987a, b Poon, 1989a, b]. Furthermore, if the prime objective of the controller were indeed to meet the metabolic demand (i.e., to maintain chemical homeostasis), then the hierarchical control system seems to perform quite poorly it is well known that arterial chemical homeostasis is readily disrupted environmental changes. 

Whether the LTMV patient is living at home or in an institution, family support is critical for coping with the demands of a ventilated patient. Provision of professional support to help address specific issues of concern is recommended to ease the burden for families (12,26). Interventions to enhance a family s advocacy skills, for patients, can also be helpful in increasing a family member s sense of control and self-efficacy. For palliative patients, providing family members with the opportunity to discuss their own feelings related to guilt and grief can be beneficial. 

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