Heating and Air Conditioning I

Heating and Air Conditioning I Principles of Heating, Ventilating and Air Conditioning R.H. Howell, H.J. Sauer, and W.J. Coad ASHRAE, 2005 basic textbook/reference material For ME 421 John P. Renie Adjunct Professor – Spring 2009

Chapter 7 – Nonresidential Load Calculation • Heat Gain through Interior Surfaces • Whenever a conditioned space is next to a space at a different temperature – heat gain • If no heat sources – (tb – to) – 5 F • Floor in contact to ground or basement – no effect on cooling load estimate

Chapter 7 – Nonresidential Load Calculation • Heat Sources in Conditioned Spaces • People – See Table 7-14 • Lighting – often the major component of space load – therefore accuracy is needed. Different from electrical paper. • Convective portion immediate picked up by A/C system • Radiative is absorbed and re-released by wall, floors, furniture, etc, after a time lag – even when lights have been turned off. • Instantaneous rate of heat gain given by • Special allowance factor – Table 7-15

Chapter 7 – Nonresidential Load Calculation • Table 7-14

Chapter 7 – Nonresidential Load Calculation • Figure 7-15

Chapter 7 – Nonresidential Load Calculation • Light Heat Components • Heat to space load – light heat directly contributing to the space heat gain. • Heat to return load – light heat released into the above-ceiling cavity picked up by the return air • Return air plenum equations

Chapter 7 – Nonresidential Load Calculation • Typical Return Air Plenum – Heat balance equations

Chapter 7 – Nonresidential Load Calculation • Other components to heat gain • Power equipment – Table 7-16

Chapter 7 – Nonresidential Load Calculation • Other components to heat gain • Hooded and unhooded equipment – Table 7-18

Chapter 7 – Nonresidential Load Calculation • Other components to heat gain

Chapter 7 – Nonresidential Load Calculation • Other components to heat gain • Load diversity

Chapter 7 – Nonresidential Load Calculation • Other components to heat gain • Ventilation and infiltration air • Moisture transfer through permeable building materials • Miscellaneous sources of heat • Temperature rise in fan stream due to fan inefficiency • Temperature rise in fan stream as a consequence of air staic and velocity pressure • Temperature rise from heat generated by motor and drive inefficiencies • Air leakage into/ out of ducting

Chapter 7 – Nonresidential Load Calculation • Description of Radiant Time Series • Based on the assumption of steady-periodic conditions (the design day’s weather, occupancy, and heat gain coefficients are identical for those of preceding days – repeat on a 24 hour cyclical basis.) • Cooling loads must address two time-delay effects • Delay of conductive heat gain through opaque exterior surfaces • Delay of radiative heat gain conversion to cooling loads • Overview (see Figure 7-6) • Solar radiation – RTS method same as TFM and TETD/TA • Differences – computation of conductive heat gain, the splitting of all heat gains into radiant and convective portions, and the conversion of radiant heat gains into cooling loads. • Time series multiplication – to account for time delays – radiant time factors and conduction time factors derived from heat balance equations • Used for comparing the time delay impact of one construction to another

Chapter 7 – Nonresidential Load Calculation • Description of Radiant Time Series

Chapter 7 – Nonresidential Load Calculation • Description of Radiant Time Series • RTS Procedure • After summing to get the cooling load each hour, peak hour to select equipment – do this for each design month to find peak.

Chapter 7 – Nonresidential Load Calculation • Description of Radiant Time Series – part 2

Chapter 7 – Nonresidential Load Calculation • Conduction Time Series

Chapter 7 – Nonresidential Load Calculation • Conduction Time Series - continued

Chapter 7 – Nonresidential Load Calculation • Conduction Time Factors Table 7-28 (walls) and 7-29 (roofs)

Chapter 7 – Nonresidential Load Calculation • Thermal Properties Table 7-30 • Example – Wall 1 – made up of F01, F09, F04, I02, F04, G01, F02

Chapter 7 – Nonresidential Load Calculation • Cooling Load Calculations Using RTS • Instantaneous cooling load is the rate that heat is convected into the zone air at a given point in time. Complicated by radiative heat transfer and the time dependency (time lag or dampening) that is associated. Convective heat gain is immediately a cooling load. • Heat balance procedures in RTS rely on radiative-convective split to determine contribution of internal loads to the radiative exchange between surfaces (see Table 7-27) • The radiant time series method converts the radiant portion of hourly heat gains to hourly cooling loads using radiant time factors, the coefficients of the radiant time series. Current cooling loads based on the current and past heat gains. • Two different radiant time series are used • Solar – directly transmitted solar heat gains – floor and furnishing only • Nonsolar – all other types of heat gains – uniformly distributed on all internal surfaces – diffuse solar heat gain and direct solar heat gain from windows with inside shading

Chapter 7 – Nonresidential Load Calculation • Cooling Load Calculations Using RTS

Chapter 7 – Nonresidential Load Calculation • Cooling Load Calculations Using RTs

Chapter 7 – Nonresidential Load Calculation • Cooling Load Calculations Using RTS • Improved accuracy and less subjective inputs

Chapter 7 – Nonresidential Load Calculation • Heating Load Calculations • Same except ... • Credit for solar or internal heat gains not included • Thermal storage of building structure ignored • Worst case load … based upon • Design interior and exterior conditions • Infiltration and/or ventilation • No solar effect • No heat gain from lights, people and appliances • Account taken for night-time setback

Heating and Air Conditioning I