HVAC 2 - Study Guide

Water

Water is the ideal energy transportation medium with a high specific heat and high density. It is also inexpensive, nontoxic, and readily available. The freezing point can be easily reached at the chiller so pipes can freeze. We add ethylene glycol to lower the freezing point of water.

Water Systems

Different Water Loops

·      From a chiller condenser to a cooling tower

·      From a chiller evaporator to AHU cooling/dehumidifying coils

·      From a boiler to AHU heating coils

·      From a boiler to hot water baseboard-heating elements

Consists of :

·      Straight pipes

·      Horizontal runs

·      Vertical risers

·      Fittings/transitions/flow control devices

·      Bends: different fittings to move the water around turns.

·      Elbows

·      Inlets

·      Branches

·      Valves

·      Strainer

·      Expansion tank

·      Pumps: provide power to circulate fluids

Significant Pressure Losses are incurred

Major objective for hydronic system design: balance the piping system pressure drop and the pump pressure rise. (energy loss and pressure loss as water moves through pipes)

Fluid Flow Basics

Energy balance for adiabatic, steady flow of a fluid in a pipe or conduit (per mass unit)

Potential Energy – Comes with Elevation Change

Work – typically put in with use of a pump

Friction Loss- Losses in energy from traveling through the piping system

P= static pressure (psf, Pa)

ρ= mass density at a cross-section (lbm/ft^3, kg/m^3)

V= average velocity at cross-section (ft/s, m/s)

g= local gravity acceleration (32.17 ft/s^2, 9.81 m/s^2)

g_c= constant(1 kg-m/N-s^2 = 32.17 lbm-ft/lbf-s^2)

h= elevation (ft, m)

w= work (ft-lbf/lbm, J/kg) -> What you design pump for

l_f= head loss (ft or m)

Bernoulli’s in terms of head loss.

For straight Pipe (no branches)

For incompressible Fluid

Total Pressure = Static pressure + Velocity Pressure

Friction Losses in Straight Pipes

f= Moody friction factor (pg 302 of textbook Figure 10-1)

L= Length of Pipe (ft,m)

D= inside diameter of the pipe or duct (ft,m) (Table C-1 or C-2 back of HVAC book)

(C-2 ft^2 column should be in^2)

g= local gravity acceleration

V= average velocity (ft/s, m/s)

Friction Factors for Pipe Flow (Method 1)

Use Figure 10-1

Need relative roughness e/D

Need Reynolds Number=

D= inside diameter of pipe

µ= dynamic viscosity (lbm/ft-s or N-s/m^2 or Pa-s)

ν= kinematic viscosity (ft^2/s or m^2/s)

Reynolds Number

Used to determine status of flow

Re<2300 Laminar flow (turning faucet barely on and water stream is clear and slow)

2300<Re<4000 Transient flow

Re> 4000 Turbulent flow (when the water comes out fast and water stream is not clear)

Dynamic Viscosity

Figure 10-2b for aqueous ethylene glycol solutions

Centipoise (cp): 1cp-.1 Pa-s or 6.719E-4 lbm/ft-s

Friction Factors for Pipe Flow

Turbulent Flow:

Table 10-1 for e

For any turbulent flow, the friction factor is the same because horizontal line.

Colebrook’s natural roughness function

Good because it allows you to solve without using the chart.

)

From figure 10-21 (copper) or 10-20 (steel)

Friction loss per 100ft

Pipe Fittings & Valves

Dynamic loss in fittings caused by

·      Changes in flow direction

·      Changes in velocity

·      Flow around obstructions

Loss Coefficient

Resistance Coefficient Tables

Table 10-2 Figure 10-22a

Take K in 10-22a and multiply by f_t in 10-2

Equivalent Length Method

Figure 10-22b (only for schedule 40 steel)

Get Leq from 10-22b then use 10-20 to get head loss

Method 1 vs Method 2

Sometimes results not equal

Empirical methods derived under different conditions

Method 1 is simplified linear approximation

Happens a lot in real world.

Explain what method we chose and research.

Flow Coefficient Method

C_v

The Volumetric flow rate (C_v=Q_std) passing through the fitting required to produce a standard head loss (l_f,std)

l_f,std,IP=2.31 ft water, ΔP_std,IP= 1 (lbf/in^2)

l_f,std,SI=.102 m water, ΔP_std,SI= 1 kPa

Pumps

Purpose: overcome pressure losses to circulate water through piping system

Convert electrical energy to mechanical energy to total pressure rise. Electric motor, rotationg impeller, pump casing.

Axial: right before increase in elevation. Lower pressure head, higher flow rate

Radial: higher pressure head, lower flow rate

Pump Characteristics

Total dynamic Head (H_p) provide the pressure head needed for the fluid flow from one point to another

Efficiency: ratio of useful work to the fluid from the shaft

Pump Characteristic Curves

Usually pumps are designed for flow and pressure

Head curve: feet for head of a given flow rate

BEP- Best Efficiency Point: The flow at which point the pump operates at the highest or optimum efficiency for a given impeller diameter

Efficiency by flow rate

Brake Horsepower: power required from the motor to drive the pump at a given head to deliver the volumetric flow rate.

Cavitation & Net Positive Suction Head (NPSH)

Fluid static pressure<liquid vapor pressure= vapor bubbles and cavitation occurs damaging pump

NPSHA>NPSHR to avoid cavitation (NPSHR given by manufacturer)

System Curve

System curve gets steeper if you add a tightening valve.

Operating conditions where pump and system curve cross

Series Circuits

Tightening a valve is like adding resistance

Q is like the current if you double resistance the system curve changes

H_a+b=H_a+H_b

Parallel Circuits

Q_a+Q_b=Q_t at H_t

Balance pressure drops

Pumps in Parallel

Q_p=Q_p,1+Q_p,2

If you add a second identical pump in parallel flow rate will not double because the pump curve is not linear.

Pump in Series

H_p=H_p,1+H_p,2

Flow rate at pump 1 and 2 is the same system flow rate will differ.

Open Loop System

P=P_atm

Return pressure could be lower

Discharge pressure typically higher

Applications:

·      Cooling Towers

·      Water collet s in drain pan or sump

·      Water loss by evaporation

·      Pump should be placed below the sump elevation to avoid cavitation

Closed loop system

Entire system usually greater than P_atm

Requires expansion tank for thermal expansion of H2O

Pipe Sizing Guideline

<2” diameter V<4ft/s, 1.2m/s

>2” diameter l_f<4ft/100ft ,.4kPa/m

If noise is not an issue and pipes are relatively short guidelines can be relaxed.

Expansions Tank

Keep air from leaking into the system

Ensure enough NPSHA

Accommodate thermal expansion of water

Types of Expansion Tanks

Open tank: exposed to air

Close tank/ direct air water interface: closed to outside air, but air can mix with water

Diaphragm Tank: no mixing of air and water

Duct System Components

·      Straight duct

·      Fittings or transitions

Fan

Fan increases pressure

Helps maintain static and velocity pressure

Fan Performance

Total Pressure Rise

Static Pressure Rise

Power consumption

Total Efficiency

Static Efficiency

Fan Curve

If You select left or curve could have surge issues.

Surge

Happens when fan approaches blocked tight static pressure (BTSP) point

Fan deviates from balance point and is unable to return to the original point due to overshooting causing air surge back and forth through fan

Causes noise and vibration

Fan laws

Only work in a given duct system. Changes to system make them unusable.

Fans In Series

Same flow

Add pressures

Fan in Parallel

Same Pressure

Add flows

Possible instability problem

Unstable conditions cause fluctuating load and fan overload

Centrifugal Fans

Can handle higher pressures than axial fans often used in AHUs

Axial Fans

More Compact

Move large flow rates

Handles smaller static Pressure

Duct Friction Loss Method

Figure 12-21 Only applies to round duct

Try to size at whole values for availability reasons

Non-round Ducts

Convert using the equivalent diameter

Loss Coefficient Method

ΔP=C*P_v

_{Short Cut Equations for Velocity Pressure}

_IP:

_SI:

Equal Friction Method

Use an approximately uniform pressure loss to size all ducts

If possible stay within the dashed lines on friction loss chart because it has a good balance of operating/initial costs

Works best with symmetrical systems

Good for VAV system

Static Regain Method

For high flow rate systems with limited space

Equal friction method would result in abnormal high/low static pressures

Reduce velocity along duct

If flow rates do not change duct size should increase to reduce velocity

High velocity method

Move large amounts of air

Capacity Control Dampers

Change system characteristics by adding resistance

Modulate system characteristics based on the directions of the blades.

Simplest Method

Issues with surge noise and low efficiency

Capacity Control Inlet Vanes

Constant fan speed

Reduces air handling capacity, static pressure rise, hp consumption

Controlled by pressure sensor

Better efficiency than discharge dampers

Pressure sensor allows speed to be changed to more efficiently meet requirements

VAV System Operation

Originally balanced at point 1

Room load reduces or room temp drops cause damper to start to close

Static pressure sensor

Fan reduce te speed and shift the fan curve down to make point 2

Airflow rate drop room temp increase cause damper to open

System operates at point 3

Vapor Compression Refrigeration/Carnot Cycle

1-2 Compressor Work goes in Pressure increases Entropy is constant

2-3 Condensor rejects heat from system enthalpy decreases temperature and pressure remains constant

3-4 Turbine/Expansion Valve decrease pressure Enthalpy remains constant

4-1 Evaporator Enthalpy increases temperature and pressure remains constant

Carnot cycle not actually realistic. Heat transfer requires temperature difference

Compressor cannot handle a gas-liquid mixture will cause cavitation

Theoretical Single-stage Vapor-compression Cycle

One compressor

·      Saturated vapor

·      Isentropic compression process

No work is recovered

Work is degraded into heat

Irreversible process

Constant pressure heat rejection and heat absorption processes

Saturated liquid at the exit of the condenser

Refrigeration effect

Specific Work input

Power Consumption

Heat rejected

Actual Single-Stage Refrigeration Cycle

Superheat refrigerant at inlet of the compressor

·      Sufficient superheat to guarantee compressor safety at various operation conditions

·      Liquid slugging (cavitation to pump) can be prevented

Sub-cool refrigerant at the outlet of condenser

·      Refrigerant mass flow rate

·      Operation of expansion device increase refrigerating effect

Heat loss is compression

·      Negligible for dx units

·      Significant for chillers

Pressure Drop across heat exchangers

·      Is negligible for energy analysis