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Abstract
An untreated sewage source heat pump system directly makes use of the urban raw sewage instead of that treated by a sewage plant. At present In China, most systems adopt the indirect mode for avoiding the harmful effect of the sewage on the heat pump unit. In this article a direct-mode untreated sewage source heat pump system, which shows less theoretical analysis of the mathematical model, is theoretically designed and analyzed to simulate the system dynamic characteristics in the heating mode. The results show that the system COP changes from 4.1 to 3.4 and the heating capacity from 9.5 to 15.3 kW when the sewage inlet temperature is 12°C and the frequency increases from 18 to 32 HZ. The condenser heat-transfer coefficient increases with the frequency reducing while the change trend of evaporator heat-transfer coefficient is the opposite. The highest values of them are 303 and 1617 W.m−2•K−1, respectively. The frequency control simulation supplies the operation adjustment with theoretical instructions and some reference values.
Keywords:
Nonmenclature
Abbreviation
Symbols
| = | coefficient of capacity | |
| = | pressure [Pa] | |
| = | efficiency | |
| = | mass flow rate [kg · s-1] | |
| = | frequency [HZ] | |
| = | actual displacement [m3 · h-1] | |
| = | the specific volume of compressor [m3 · kg-1] | |
| = | time [s] | |
| = | charge [kg] | |
| = | enthalpy [kJ · kg-1] | |
| = | mean enthalpy [kJ · kg-1] | |
| = | dryness factor | |
| = | specific heat [kJ · kg-1 · k-1] | |
| = | temperature [°C] | |
| = | condensation temperature [°C] | |
| = | evaporation temperature [°C] | |
| Q | = | heat convection [W] |
| M | = | mass [kg] |
| k | = | total heat-transfer coefficient [W · m-2 · k-1] |
| A | = | heat-transfer area [m2] |
| = | heat-transfer coefficient of medium [W · m-2 · k-1] | |
| = | thermal conductivity [W · m-1 · k-1] | |
| = | density [kg · m-3] | |
| g | = | acceleration of gravity [m · s-2] |
latent heat of vaporization [kJ · kg-1]
| = | kinetic viscosity [N · s · m-2] | |
| Re | = | Reynolds number |
| Pr | = | Prandtl number |
| = | molecular weight | |
| vm | = | mass flow velocity [kg · m-2 · s-1] |
| tb | = | normal boiling point [°C] |
| d | = | diameter of heat exchange tube [m] |
| = | thermal resistance [m2 · K · W-1] | |
| = | thickness [m] | |
| Nin | = | consumed power [w] |
COP coefficient of performance
Subscript
| i | = | ith time step |
| el | = | electric |
| r | = | refrigerant |
| 1 | = | evaporator inlet |
| 2 | = | condenser inlet |
| 3 | = | condenser outlet |
| 4 | = | evaporator inlet |
| c | = | condenser side |
| e | = | evaporator side |
| si | = | sewage inlet |
| so | = | sewage outlet |
| wi | = | water inlet |
| wo | = | water outlet |
| v | = | saturated gas |
| l | = | saturated liquid |
| i | = | tube interior surface |
| o | = | tube exterior surface |
| f | = | equivalent |
heat-transfer tubes in the heat exchanger