Heating and Air Conditioning I

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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
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Chapter 6 Energy Estimating Methods

• General Considerations.
• Energy Resources and Sustainability
• Because energy used in buildings and facilities
  comprises a significant amount of the total
  energy used for all purposes affecting energy
  resources
• ASHRAE recognizes the effect of its technology
  on the environment and natural resources to
  protect the welfare of posterity
• Regulation of energy conservation through
  building permits
• Energy sources on-site energy in the form that
  it arrives at or occurs in a site (electricity,
  gas, oil, coal).
• Energy resource is the raw energy that (1) is
  extracted, (2) is used to generate the energy
  sources delivered to the building (coal used to
  generate electricity), or (3) occurs naturally
  and is available at a site (solar, wind,
  geothermal)
• This chapter takes an introductory look at the
  methods for estimating energy use.
• Primary objective is economic which option has
  the lowest total (lifetime) cost.
• Compliance with energy performance codes

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Chapter 8 Energy Estimating Methods

• General Considerations.
• Energy Estimating Techniques
• Share three elements based on calculation of
  (1) space load, (2) secondary equipment load, and
  (3) primary equipment energy requirements.
• Primary central plant equipment that converts
  fuel or electric energy for heating and cooling
• Secondary equipment used to distribute the
  heating, cooling, or ventilating medium to the
  conditioned space.
• Space load amount of energy that must be added
  or extracted from a space to maintain thermal
  comfort simple method would function of outdoor
  dry-bulb temperature only more complex involves
  solar effects, internal gains, heat storage, etc.
  most sophisticated involves hour-by-hour
  analysis.
• Translation into secondary equipment load
• The translation into the fuel and electricity
  required by the primary equipment considering
  efficiencies and part-load characteristics
• Economic analysis cost effectiveness of energy
  conservation, capital equipment, time of energy
  use, maximum demand, etc.

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Chapter 8 Energy Estimating Methods

• General Considerations.
• Energy Estimating Techniques
• Sophistication of calculation procedures
  function of number of ambient variable and/or
  time increments used.
• Simpliest method one measure such as
  degree-days single-method measures
• Bin methods using more information such as the
  number of hours under an anticipated condition
  simplified multiple-measure method.
• Detailed simulation methods require hourly
  weather data, as well as hourly estimates of
  internal loads such as lighting or occupants.
• Calculations are nonlinear, dynamic, and very
  complex need for computer modeling. See US DOE
  for list of software.

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Chapter 8 Energy Estimating Methods

• Component Modeling and Loads.
• Loads
• After determining peak load, select equipment to
  offset load. Since most of time will be at
  partial loading, this aspect of sizing is also
  important
• Calculating instantaneous space load is key to
  simulation
• Heat balance method
• Weighting factor method
• Both use conduction transfer functions to
  calculate heat gain or loss differences arise
  in the subsequent internal heat transfers to the
  room
• Secondary System Components
• Everything between the overall building energy
  system between a central heating and cooling
  plant and the building zones
• Air handler equipment, air fans, ductwork,
  dampers, humidifying equipment, etc.
• Divided into distribution components and heat and
  mass transfer components.
• All methods approximate the effect of the
  interactions with part-load performance curves
  shape of curve depends on the effect of flow
  control on the pressure and fan efficiency
  detailed analysis

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Chapter 8 Energy Estimating Methods

• Component Modeling and Loads.
• Primary System Components
• Consumes energy and deliver heating and cooling
  to a building includes chillers, boilers,
  cooling towers, cogeneration equipment, and
  plant-level thermal storage equipment the major
  energy-consuming equipment important to
  accurately model
• Energy consumption based on design, load
  conditions, environmental conditions, and
  equipment control strategies
• Usually the energy consumption characteristics of
  primary equipment is modeled using regression
  analysis on manufacturers published design data
  based on full-load with correction for partial
  load.
• Many forms of data curves sometimes the use of
  data interpolation from tables in employed.

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Chapter 8 Energy Estimating Methods

• Overall Modeling Strategies.
• In developing a simulation model two basic
  issues must be considered
• Modeling of the components or subsystems
  (equations)
• The overall modeling strategy (sequence and
  procedures to solve the equations)
• Building energy programs load models are
  executed for each space for every hour.
• This is followed by running models for every
  secondary system, one at a time, for every hour
  of the simulation
• Finally the plant simulation model is executed
  again for the entire period. Each sequential
  execution processes the fixed output of the
  preceding step (load, systems, plant interation
  can cause unmet conditions only reported, not
  corrected)
• Alternative approach is to solve all calculation
  simultaneously superior but costly
  computationally. More accurate???

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Chapter 8 Energy Estimating Methods

• Overall Modeling Strategies
• See Figure 8-1 Overall Modeling Strategy
  flowchart
• Figure 8-1 represents an economic model to
  calculate energy costs based on the estimated
  required input energy maintaining running sums
  yields monthly or yearly energy usage or costs
• These methods only compare design alternatives
  uncontrolled number of variables usually rule out
  these methods for accurate prediction of utility
  bills.
• Most energy analysis programs include a set of
  preprogrammed models that represent various
  systems equations are arranged so that they can
  be solved sequentially. Or a smaller number of
  equations are solved simultaneously
• Inflexibility in this approach
• Solution series of components may be organized
  in a component library and individually selected
  by the program resolution of the specifications
  between components is simultaneously solved.

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Chapter 8 Energy Estimating Methods

• Integration of System Models.
• Energy calculations for secondary systems involve
  construction of the complete system from the set
  of HVAC component.
• Example of a VAV system which is a single path
  system that controls zone temperature by
  modulating the airflow while maintaining a
  constant supply air temperature.

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Chapter 8 Energy Estimating Methods

• Integration of System Models.
• VAV system simulation consists of a central
  air-handling unit and a VAV terminal unit with
  reheat coil located at each zone.
• Central air-handling provides the air at a
  controlled setpoint
• VAV unit at each zone varies the airflow to meet
  the cooling load
• As the zone cooling load decreases, the VAV
  terminal decreases the zone airflow until the
  unit reaches it minimum position then reheat
  coil is used to meet the zone load.
• Variable-speed fan/drive is used to control the
  supply fan
• Algorithm for performing the calculations is
  given in Figure 8-3
• The algorithm directs sequential calculations of
  system performance. Calculations proceed from
  the zones forward along the return air path to
  the cooling coil inlet and back through the
  supply air path to the cooling coil discharge.
• Subsequent modifications to the basic algorithm
  heat balance and weighted factor approaches
  zone temperature variation and readjustment,
  limits, enhancements, etc.

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Chapter 8 Energy Estimating Methods

• Integration of System Models.
• VAV algorithm

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Chapter 8 Energy Estimating Methods

• Integration of System Models.
• Forward modeling
• Description of building system or component of
  interest and defines the building being modeled
  according to its physical description.
• Based on sound engineering principles and
  widespread acceptance
• Order of analysis is presented in Figure 8-4 that
  is typically performed by a building energy
  simulation program
• Inverse modeling
• Based on empirical behavior of the building as it
  relates to one or more driving forces. This
  approach is referred to as system identification,
  parameter identification, or inverse modeling.
• A structure or system is assumed first and then
  important parameters are identified by a
  statistical analysis.

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Chapter 8 Energy Estimating Methods

• Integration of System Models.
• Forward modeling

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Introduction
• Simplest methods for energy analysis and are
  appropriate if the building use and the
  efficiency of the HVAC equipment are constant
• Where efficiency or conditions of use vary with
  outdoor temperature, the energy consumption can
  be calculated for different values and multiplied
  by the corresponding number of hours bin
  methods
• When indoor temperature is allowed to vary as
  well as interior gains, these simple models cant
  be used.
• Cooling methods less established than heating
  method smaller temperature differences and more
  dependent on solar and interior gains.
• However, similar cooling degree-day methods have
  been established and used.
• Accurate for seasonal calculations (long term
  versus short term) when indoor temperatures and
  internal gains are constant
• Valid for envelope dominated heating and cooling
  and loads based on temperature difference only.

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Balance-Point Temperature and Degree-Days
• Balance-point temperature is the average outdoor
  temperature at which the building requires
  neither heating or cooling for the HVAC system.
• Degree-day procedures recognize that heating
  equipment need to meet only the heating not
  covered by internal sources and solar gain
• Energy requirements of the space is proportional
  to the difference between the balance-point
  temperature and the outside temperature

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Balance-Point Temperature and Degree-Days
• Balance temperature, tbal, found when setting qH
  0 and solving for to
• Heating only required when temperature drops
  below tbal.
• Determination of heating-degree day summed over
  month, season, or entire year
• Cooling degree-days (note balance point could be
  different)

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Seasonal Efficiency, h
• Depends on factors such as steady-state
  efficiency, sizing, cycling effects, and energy
  conservation devices installed.
• Can be lower or nearly equal to steady-state
  efficiency
• Neglecting ducting loss (from NIST)
• CFpl is a trait of the part-load efficiency of
  the heating equipment

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Heating Degree-Day Method
• Assumption is that in the long term, solar and
  internal gains offset heat loss when the mean
  daily outdoor temperature is equal to the
  balance-point temperature.
• Assumption that fuel consumption is proportional
  to the difference between the daily mean and the
  balance-point temperature
• Heat loss per degree difference being constant
• Theoretical heating requirement is given by

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Heating Degree-Day Method
• General form of the degree-day equation for fuel
  consumption

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Heating Degree-Day Method
• Typical heating values h found in Table 19-5
• Heating degrees-days for balance point of 65 F
  have been widely tabulated in past
• Today, it may overestimate due to improved
  building construction error adjust due to CD
  factor in equation
• Recommend using variable-base degree-day approach

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Heating Degree-Day Method Table 8-2

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Heating Degree-Day Method Example 8-1

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Variable-Base Heating Degree-Day (VBDD)
• Variable-base degree-day method count degree-days
  based on actual balance-point temperature rather
  than 65 F.
• Can give good results for the annual heating
  energy of single-zone buildings dominated by
  gains through the walls and roof and/or
  ventilation
• Table 8-2 provides multiple base values for
  cooling and heating degree-days.

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Cooling Degree-Day Method

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Example 8-2

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Example 8-3

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Example 8-3

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Example 8-3

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Chapter 8 Energy Estimating Methods

• Degree-Day Methods.
• Example 8-3

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Chapter 8 Energy Estimating Methods

• Bin Method (Heating and Cooling).
• Introduction
• Sometimes the degree-day methods shouldnt be
  employed because the heat loss coefficient K, the
  equipment efficiency, and the balance-point
  temperature may not be constant.
• Annual consumption can be determined if different
  temperature intervals and time periods are
  evaluated separately
• Energy consumption bin, Ebin, determined at
  several outdoor temperatures and multiplied by
  umber of hours, Nbin

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Chapter 8 Energy Estimating Methods

• Bin Method (Heating and Cooling).
• Table 8-3 Hourly Temperature Occurrences

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Chapter 8 Energy Estimating Methods

• Bin Method (Heating and Cooling).
• Modified Bin Method
• Refinements such as seasonal variation on solar
  gains
• Use of a diversified (part-load) rather than a
  peak-load value to establish the load as a
  function of outdoor temperature
• Effect of primary and secondary equipment
  included
• Effect of reheat and recovery included
• Characterization of time-dependent diversified
  loads
• Transient effects of building mass
• Degree-Day from Bin Data

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Chapter 8 Energy Estimating Methods

• Bin Method (Heating and Cooling).
• Degree-Day from Bin Data
• First determine the balance point temperature

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Chapter 8 Energy Estimating Methods

• Bin Method (Heating and Cooling).
• Bin-method Data form (Table 8-4)

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Chapter 8 Energy Estimating Methods

• Bin Method (Heating and Cooling).
• Bin-method Example 8-4

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Chapter 8 Energy Estimating Methods

• Bin Method (Heating and Cooling).
• Bin-method Example 8-4

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Chapter 8 Energy Estimating Methods

• Bin Method (Heating and Cooling).
• Heat pump capacity and building load

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Chapter 8 Energy Estimating Methods

• Bin Method (Heating and Cooling).
• Bin-method Example 8-4