Heat recovery ventilation

Heat Recovery Ventilator - A device that captures the heat from the exhaust air from a building and transfers it to the supply/fresh fresh air entering the building to preheat the air and increase overall heating efficiency. 

ASHRAE 62, Ventilation for Acceptable Air Quality This standard can assist professionals in the proper design of building ventilation systems. Important aspects of the standard include the definition of acceptable air quality and information about ventilation effectiveness. It makes a recommendation about using source control through isolation and local exhaust. The standard also contains information on the use of heat recovery ventilation and provides a guideline for allowable carbon dioxide levels. 

Heat recovery ventilator (HRV) is the most efficient method to bring in fresh ontdoor air all year-round and which is much more efficient and controllable than just opening windows. HRV incorporate one of several designs of heat exchanger cores. 

The influence of airflows from ventilating systems must also be considered. Processes using mediums of different physical qualities when mixed will have separation into different layers. Transmission of energy between molecules in flowing mediums takes place in the direction of the velocity. This strengthens the separation into parallel layers. The level of fluid in containers and tanks is due to stratification of horizontal temperature layers, while airflow after batteries, heat-recovery systems, and humidifiers or dehumidifiers will separate into parallel layers. 

Example A ventilation system (Fig. 9.64) handling 20 mVs of air needs to heat the supply air from 10 °C to 20 C. Doubling the number of heat exchangers from one to two increases the heat-recovery efficiency from 50% to 75% and introduces an extra pressure drop of 300 Pa. As we can see from Table 9.19, this is probably a cost-efficient measure. 

Design of innovative natural ventilation systems such as systems with solar towers, glazed double facades, facades with passive shaft ventilation or with some means of heat recovery... 

Considering the energy and maintenance costs during the life cycle, the cheapest investment is not always the best. It may, for instance, be profitable to buy a ventilation unit with a heat recovery system, which may increase the unit investment by 50%. The return on the investment in such a case may be in excess of 20%. 

LCC calculations frequently provide energy-efficient solutions. This gives reduced energy consumption and a reduction in environmental pollution. For example, the installation of a heat recovery system in a ventilation system may reduce the energy consumption and emissions by 50 to 80%. Figure 16.1 compares life cycle cost and life cycle assessment calculations. 

When the present value of energy costs is 50% of the total costs, a discount of purehase price by 10% equals a 20% change in used energy (e.g., ventilation unit with heat recovery, interest rate 6%). This means that in this case a 10% more expensive unit will use 20% less energy than a less expensive unit. 

There are several measure sets with immediate payback SE, SF, SH, SI. Measures SE and SF are radiant panel systems with displacement ventilation. These systems have a similar cost to the base case, but they offer energy savings. Furthermore, significant sizing reductions, mainly in the cooling tower and chiller sizes, offset the incremental cost of the envelope and heat recovery measures. Because the elevator efficiency measures offer a net savings in capital cost, the capital cost of the other measures is further offset. 

Fresh air supply ventilation systems (heat recovery or nonheat recovery). 

Some of the remedial measures tested in this study were not regarded as likely to form part of a long-term control stategy. For example, the installation of a mechanical ventilation system, with a heat recovery unit, would not be used in a dwelling of this type, because of the very high installation cost. Nevertheless, the availability of the dwelling enabled devices to be tested under real housing conditions, rather than in the laboratory. 

The most efficient utihzation of the motor losses is apphcable in locations with smnmer/winter climate where the hoist room needs heating during a substantial part of the yeru. The outlet ur from the motor is directly vented into the hoist room. A controlled damper instcJled in the wall outlet is opened when the ambient temperature is high. Another controlled damper is installed between the hoist room and the motor ventilation intake air plenum chamber. In this way the inlet air is preheated in the cold season. This reduces or eliminates the need for separate preheating equipment which would otherwise be required, which in turn would increase the total energy consumption of the hoisting system. A summary of different heat recovery equipment is shown in Table 10. 

The heat recovered can be used to heat a variety of streams. The recovered heat is transferred to e.g. the circulation water heating the machine room ventilation air, to process water and to the heating of supply air to the dryer section. This can be seen from Fig. 1 where the main streams are depicted. In the figure no decisions about the matches in the heat recovery section are yet indicated. The moist exhaust air from the hood is removed at three difierent locations. 

FIGURE 35.8 Function of a heat-recovery unit with closed vapor hood (1, vapor hood 2, exhaust fan 3, supply air heat exchanger 4, supply air booster heater 5, fan for supply air 6, air distributor 7, pocket ventilation 8, room air heat exchanger 9, room air booster heater 10, temperature-control flaps 11, room-supply air fan 12, ceiling air distributor 13, air outlet louvers 14, warm-water unit 15, warm-water discharge). (Courtesy of J.M. Voith GmbH.)... 

The whole dryer section is enclosed in a drying hood with doors which can be opened e. g. for inspection. It allows controlled flow of the hot and dry make-up air as well as of the vapor laden exhaust air. The pressure inside the hood should be balanced in such a way that a minimum of air is blown from the hood into the machine hall or sucked from the machine hall into the hood. For effective pocket ventilation the hot air enters via blow boxes or blow rolls and flows to both sides of the machine where it is sucked off To prevent condensation the hood walls are insulated and make up air is supplied along both hood sides from underneath. These measures allow a low amount of make-up air, a high air dew point of the exhaust air and effective heat recovery. 

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