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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

Chapter 3 Basic HVAC Calculations

- Applying Thermodynamics to HVAC Processes
- Looking at a simplified (but complete)

air-conditioning system - Terminology qsensible, mwater, qL, hw, solar

gains - Look at first law of thermodynamics (energy) and

conservation of mass - Air is removed from the room, returned to the

air-conditioning apparatus where it is

reconditioned, and then supplied again to the

room. - Many cases, it is mixed with outside air required

for ventilation - Outdoor air (o) is mixed with return air (r) from

the room and enters the apparatus at condition

(m) - Air flows through the conditioner and is supplied

to the space (s). - The air supplied to the space absorbs heat qs and

moisture mw, and the cycle continues.

Chapter 3 Basic HVAC Calculations

- Applying Thermodynamics to HVAC Processes

Chapter 3 Basic HVAC Calculations

- Applying Thermodynamics to HVAC Processes

Chapter 3 Basic HVAC Calculations

- Applying Thermodynamics to HVAC Processes

Chapter 3 Basic HVAC Calculations

- Absorption of Space Heat and Moisture Gains
- AC usually reduces to determining the quantity of

moist air that must supplied and the condition it

must have to remove given amounts of energy and

water from the space to be withdrawn at a

specified condition. - Sensible heat gain addition of energy only

not wrt water

Chapter 3 Basic HVAC Calculations

- Heating or Cooling of Air without moisture gain

or loss straight line on psychrometric chart

since humidity ratio is constant

Chapter 3 Basic HVAC Calculations

- Cooling and Dehumidifying Air
- Moist air brought down below its dew point

temperature some of the water will condense and

leaves the air stream - Assume condensed water is cooled to the final air

temperature before draining from the system

Chapter 3 Basic HVAC Calculations

- Cooling and Dehumidifying Air

Chapter 3 Basic HVAC Calculations

- Cooling and Dehumidifying Air
- Moist air brought down below its dew point

temperature some of the water will condense and

leaves the air stream - Assume condensed water is cooled to the final air

temperature before draining from the system - Cooling and dehumidifying process involves both

sensible heat transfer and latent heat transfer

where sensible heat transfer is associated with

the decrease in dry-bulb temperature and the

latent heat transfer is associated with the

decrease in humidity ratio.

Chapter 3 Basic HVAC Calculations

- Heating and Humidifying Air

Chapter 3 Basic HVAC Calculations

- Adiabatic Mixing of Two Streams of Air

Chapter 3 Basic HVAC Calculations

- Adiabatic Mixing of Moist Air with Injected Water

Chapter 3 Basic HVAC Calculations

- Moving Air

Chapter 3 Basic HVAC Calculations

- Approximate Equations Using Volume Flow Rates
- Since volumes of air change need to make

calculations with mass of dry air instead of

volume. But volumetric flow rates define

selection of fans, ducts, coils, etc. - Use volume while still considering mass by using

volume rates based on standard air conditions - Dry air at 20 oC and 101.325 kPa (68 oF and 14.7

psia) - Density is 1.204 kg/m3 (0.075 lb/ft3) dry air
- Specific volume is 0.83 m3/kg (13.3 ft3/lb) dry

air - Saturated air at 15 oC has about same density and

volume - Need to convert actual volumetric flow conditions

to standard - Say you need 1,000 cfm outside air rate at

standard conditions - Outside measured at 35 oC dry bulb and 23.8 oC

wet bulb corresponding to a specific volume of

14.3 ft3/lb. - Then, the actual flow rate would be 1,000

(14.3/13.3) 1,080 cfm - 1,000/13.3 1,080/14.3 mass rate (lb/min) of

moist air

Chapter 3 Basic HVAC Calculations

- Sensible heat gain corresponding to the change of

dry-bulb temperature for a given airflow (at

standard conditions)

Chapter 3 Basic HVAC Calculations

- Latent heat gain corresponding to the change of

humidity ratio W for a given airflow (at standard

conditions). - The latent heat gain in Watts (Btu/h) as a result

of a difference in humidity ratio DW between the

incoming and leaving air flowing at standard

conditions.

Chapter 3 Basic HVAC Calculations

- Total heat gain corresponding to the change of

dry-bulb temperature and humidity ratio W for a

given airflow (at standard conditions). - The total heat gain in Watts (Btu/h) as a result

of a difference in enthalpy Dh between the

incoming and leaving air flowing at standard

conditions.

Chapter 3 Basic HVAC Calculations

- Total heat gain corresponding to the change of

dry-bulb temperature and humidity ratio W for a

given airflow (at standard conditions).

Chapter 3 Basic HVAC Calculations

- Single-Path Systems
- Simplest form of all-air HVAC system serving a

single temperature control zone - Responds to one set of space conditions, where

conditions vary uniformly and the load is

stable. - Schematic of system return fan necessary under

certain conditions of Dp. - Need for reheat necessary to control humidity

independent of the temperature requirements. - Equations for single-path systems air supplied

must be adequate to take care of each rooms peak

load conditions. Peak loads may be governed by

sensible or latent room cooling loads, heating

loads, outdoor air requirements, air motion, and

exhaust. let us look at each of these loads and

what air volume is required to satisfy these

demands.

Chapter 3 Basic HVAC Calculations

- Single-Path Systems - schematic

Chapter 3 Basic HVAC Calculations

- Single-Path Systems equations for supply air

Chapter 3 Basic HVAC Calculations

- Single-Path Systems equations for supply air

Chapter 3 Basic HVAC Calculations

- Single-Path Systems supply air for ventilation
- Supply air for ventilation needed when the

amount of outside air is not adequate - Supply air not adequate for the amount of exhaust

makeup required no return air comes from the

room and entire volume of make-up ventilation air

becomes an outside air burden to system - Desired air exchange rate not satisfied supply

air is determined - Desired air movement not satisfied, based on area

index parameter, K. - Each of the above conditions are used at

different times Case 1 when outside air

governs, Cases 3 and 4 when air movement governs,

and Case 2 when exhaust governs.

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Example Problem 3-3

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Example Problem 3-3

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Example Problem 3-3

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Example Problem 3-3

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Cycle Diagram

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Cycle Diagram
- Each state point is identified both in summer and

winter - Change of Dt is result of sensible heat loss or

gain, qS - Change in DW is result of latent heat loss or

gain, gL - All return air is assumed to pass from the room

through a hung-ceiling return air plenum - Supply air CFMS at the fan discharge temperature

tsf (summer mode) absorbs the transmitted supply

duct heat qsd and supply air fan velocity

pressure energy qsf,vp thereby raising the

temperature to ts

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Cycle Diagram
- Room supply air absorbs room sensible and latent

heat qSR and qLR along the room sensible heat

factor (SHR) line s-R, reaching the desired room

state, tR and WR.

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Cycle Diagram
- Room (internal) sensible loads which determine

the CFMs consist of

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Cycle Diagram

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Psychrometric Representation

Chapter 3 Basic HVAC Calculations

- Single-Path Systems Psychrometric Representation

Chapter 3 Basic HVAC Calculations

- Single-Path System - Psychrometric Representation

Chapter 3 Basic HVAC Calculations

- Single-Path System Sensible Heat Factor (Ratio)

- Sensible heat factor (ratio), SHF or SHR, is the

ratio of sensible heat for a process to the total

of sensible and latent heat for the process. - The sensible and latent combined is referred to

as the total heat - On psychrometric chart, the protractor provides

this ratio and may be used to establish the

process line for changes in the conditions of the

air across the room or the conditioner on the

chart - The supply air to a conditioned space must have

the capabilty to offset both the rooms sensible

and latent heat loads. - Connecting the room and supply points with a

straight line provides the sensible heat factor

condition. The conditioner provides the

simultaneous cooling and dehumidifying that

occurs. - Horizontal line would be SHF 0.0 (only

sensible) - Line with SHF 0.5 would be half sensible and

half latent

Chapter 3 Basic HVAC Calculations

- Single-Path System Final Example

Chapter 3 Basic HVAC Calculations

- Single-Path System Final Example
- Sensible and latent loads given
- Room Conditions (75 oF and 55 RH) Supply at

58 oF - Outside Conditions 96 oF DB, 77 oF WB and 20

of total flow

Chapter 3 Basic HVAC Calculations

- Single-Path System Final Example

Chapter 3 Basic HVAC Calculations

- Single-Path System Final Example

Chapter 3 Basic HVAC Calculations

- Single-Path System Final Example

Chapter 3 Basic HVAC Calculations

- Single-Path System Final Example

Chapter 3 Basic HVAC Calculations

- Single-Path System Final Example