Waste Heat Recovery and heat Pumps Flashcards
What are high temperature/medium and low devices
High: gas incinerators
Medium: Turbines,RE
Low: Air compressor, pumps, IC
High/Medium?low applications
med-high: Preheat combustion air
Low-medium: preheat boiler feedwater
Equation for heat transfer in HX
Q = UAdTm
What are recuperators?
Two concentric lengths of metal used for high T apps
What are heat wheels?
Porous disk from a material with a high heat capacity rotating between a hot and cold gas duct.
Application and limitations of heat wheels
Space heating
Limitations Mechanical difficulty from uneven expansion due to large differentials causing deformation of the wheel and difficulty maintaining seals. Also pollution from porous
What are air preheaters?
Applications
Alternative channels which puts the gas in close contact, separated by thin wall.
Heat/moisture recovery from moist rooms
Advantages and disadvantages of preheaters
+ no cross contamination and simple
- Spacious, expensive and difficult to maintain temperature control
How do heat pipes work?
Heat absorbed by exhaust gas evaporates entrained fluid, causing vapour to collect, latent heat is carried to cold end where it condensed, giving up latent heat.
Waste heat boilers
Water vaporized in the tubes and collected in a steam drum. Quality of steam depends on waste gas temperature.
Stages 1 and 2 of heat pump cycle
Isothermal evaporation of refrigerant.
Isotropic compression
Stages 3 and 4 of heat pump cycle
Constant pressure condensation process where refrigerant is cooled to saturation point, then condensed. Heat is rejected.
A throttling process such as orifice reduces the pressure to evaporation pressure. A portion of liquid flashes into vapour and enters evaporator. Constant enthalpy irreversible process.
What are air source heat pumps?
Heat is extracted from outside air by blowing through HX through fans. Even in winter there is energy due to large volume.
Ground source heat pumps
Extracts energy from ground by circulating a water antifreeze mixture through heat pumps
What is the COP of heat pumps?
Heat trasnferred in condenser/ compress work
COP= Th/(Th-Tl)
