Water Pipe Design Lecture2.pdf

7/29/2019 Water Pipe Design Lecture2.pdf

1/20

Pipe Design Lecture No.(2) By Badran M. Salem

Chilled Water system

Introduction

For large installations the Condenser, Evaporator, Compressor and

Expansion device can be purchased as a package unit, known as a

Chiller . The usual package consists of electrically driven

compressor(s) mounted on top of two shell and tube heat exchangers,one for the evaporator and the other for the condenser.

The cooling coil(s) are piped up to the chiller in the conventionalmanner as shown below.

In some countries the Cooling tower is the preferred method of

removing heat from the system.

Cooling towers that are open to atmosphere are not often used since

the water may become contaminated.

A closed cycle cooling tower or evaporative cooler can be used to

reduce contamination risk.


2/20


2

Chiller Schematic Diagram with Air Cooled Condenser

Cooling Coil(s)

Condenser

Evaporator

Compressor

Heat Rejection

Heat Input

Expansion

valve

1

23

4

Refrigerant Gas

Refrigerant Liquid

+ Vapour

-

Pump

Control

Valve

Chilled

Water

flow

Chilled

Water

return

Fans

Chiller


3/20


3

Cooling Coil(s)

Condenser

Evaporator

Compressor

Heat Rejection

Heat Input

Expansion

valve

1

23

4

Refrigerant Gas

Refrigerant Liquid

+ Vapour

-

Pump

Control

Valve

Chilled

Water

flow

Chilled

Water

return

Chiller

Cooling Tower

Condenser Water

Chiller Schematic Diagram with Cooling Tower


4/20


4

Alternative arrangements are shown for smaller installations where the

condenser may be mounted on the roof or external wall of a building and

cooled by outside air.

The evaporator may be installed directly into the ductwork or air handling unit

(AHU) for smaller installations.

This is known as a Direct Expansion (or DX) coil.

A typical Direct Expansion (or DX) system is shown below.

This avoids using condenser water and chilled water in the system and

installing the accompanying plant.

Direct Expansion Cooling Coil

also known as DX coil in Air

Handling Unit (AHU)

Condenser

Compresso

Heat Rejection

Heat Input

Expansion

valve

1

23

4

Refrigerant

Gas

Refrigerant Liquid +

Vapour

-

Refrigerant

Flow

Refrigerant

Return

FansAir Cooled

Condenserincorporating

Compressor(s)

Direct Expansion (DX) System


5/20


5

Chilled-water System

In larger buildings and particularly in multi-story buildings, the split-system

approach begins to run into problems.

Either running the pipe between the condenser and the air handler exceedsdistance limitations (runs that are too long start to cause lubrication difficultiesin the compressor), or the amount of duct work and the length of ducts becomes

unmanageable.At this point, it is time to think about a chilled-water system.

In a chilled-water system, the entire air conditioner is situated on the roof orbehind the building.

It cools water to between 4.0oC and 8.0

oC.

This chilled water is then piped throughout the building and connected to the

cooling coils in air handlers as needed.There is no practical limit to the length of a chilled-water pipe if it is well-insulated.


6/20


6

Chilled Water Temperatures

Typically chilled water flow and return temperatures to cooling coils is generallybetween 7

oC and 12

oC, depending upon the dew point to be maintained.

When this water is pumped through the evaporator section of the chiller this water

temperature will be lowered by about 4oC to 6oC.In order that the necessary heat transfer may take place, the refrigerant must be atsome temperature below that of the leaving water but, at the same time, it must

generally be slightly above freezing point.

In a typical case, the following water temperatures may be used:

Apparatus dew point 12CCooling coil outlet 10C

Cooling coil inlet 6C

Water at evaporator outlet 5.5C

The refrigerant in the evaporator would in this case be maintained at about 1C

giving a differential for4.5C for heat transfer.As will be appreciated, this small temperature potential means that the coolingsurface of a simple tubular type would need to be very extensive: a variety of

devices has been developed to augment the transfer rate.

Ethylene/glycol solutions may be used in cooling coils in order to allow lower airtemperatures to be obtained.

The temperatures of the fluid circulating may be -7C from the evaporator and -3Creturning to it, or lower as required.

In instances where cooling for an air-conditioning system is provided from arefrigeration machine by direct expansion, the refrigerant is piped directly tocooling coils in the air stream which thus become the evaporator.

The surface temperature of the coils is a function of the leaving air temperaturerequired, the form of the coil surface and the velocity of the air flow.Refrigerant temperatures much below freezing point are inadmissible owing to the

risk of build-up ofice on the coil surface when dehumidification is taking place.An apparatus dew point of3C is normally considered as the practical minimum for

such coils if frosting is to be avoided.


7/20


7


8/20


8


9/20


9


10/20


1


11/20



12/20


2


13/20


3


14/20


4


15/20


5


16/20


6


17/20


7


18/20


8


19/20


9


20/20


21

Documents

Water Pipe Design Lecture2.pdf