CHILLER BASICS

A review of applications and systems in the field.

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By: DAVE DEMMA

2015-05-01

CHilr/noun

noun: chiller; plural noun: chillers

1.

1. a machine for cooling something, especially a cold cabinet or refrigerator for keeping stored food

a few degrees above freezing.

noun

1. a person or thing that chills.

2. Informal. a frightening or suspenseful story or film; melodrama.

noun

1. The definition of a chiller is something that produces cold, or a scary story or movie involving

violent evil or the supernatural.

1. An example of a chiller is a freezer.

2. An example of a chiller is Frankenstein.

There are many online definitions for the word chiller. And they are all painfully inadequate, although

in some building somewhere there is probably a poorly operating chiller that could be referred to as a

Frankenstein chiller. So, what is a chiller?

Lets start with the basic refrigeration cycle, consisting of:

1. Compressor, whose function is to take a low-pressure vapour and convert it to high-pressure

vapour.

2. Condenser, whose function is to transfer heat from the high-pressure vapour, allowing it to change

states into a high pressure liquid.

3. Expansion device, whose function is to cause the high pressure liquid to experience a pressure

drop. The corresponding saturation temperature to the low pressure is at a level that makes it useful

to transfer heat from the refrigerated space or load.

4. Evaporator, whose function is to transfer heat from the refrigerated space or load to the low

pressure, low temperature saturated liquid refrigerant.

The load on a typical evaporator in a refrigeration/air conditioning system is the air in the refrigerated

space. The typical evaporator is a finned tube fan coil design. Heat from the air flowing through the

finned tube evaporator is transferred to the refrigerant, resulting in a lower air temperature in the

refrigerated space.

A chiller is quite different, in that it involves two stages of heat transfer. The heat load on the chillers

evaporator is a fluid. This fluid is brought down to the design temperature for the particular

application and is pumped to secondary heat exchangers in the refrigerated space. The heat load

from the actual refrigeration load is transferred to the pumped fluid, which is then transferred to the

refrigerant in the chillers evaporator.

While there are many different types of chiller applications, utilizing varying types of evaporators, the

two main types are shell in tube heat exchangers and brazed plate heat exchangers. With shell in

tube heat exchangers there is a tube bundle inside the heat exchanger, with refrigerant flowing

through the copper tubes in the bundle. The secondary fluid flows through the heat exchanger,

providing contact between the secondary fluid and the refrigerant circuit. Baffles are used inside the

heat exchanger to ensure full and thorough contact between the secondary fluid and the tube

bundle. Heat is transferred from the refrigerant to the fluid flowing through the barrel tubes.

Brazed plate heat exchangers employ a series of metal plates to transfer heat between the circulating

fluid and the refrigerant. The major advantage of this design is that the greater amount of available

heat transfer surface, increases the rate of temperature change and reduces the size of the

evaporator.

COMFORT COOLING

A typical chiller application would be to provide the comfort cooling in an office building or hotel. Each

zone (or hotel room) has its own hydronic finned tube coil heat exchanger as a means to transfer

heat from the space. The flow of secondary fluid through the heat exchanger is controlled by a

solenoid valve, which is energized/de-energized by a thermostat in the space.

One advantage to this design is the elimination of large refrigerant piping runs to each zone and the

tremendous amounts of refrigerant that would involve. Instead, the refrigerant circuit is rather small,

being contained within the piping circuit of the chiller only. This reduces the charge from potentially

several thousand pounds to perhaps a hundred or so.

The secondary fluid for a chiller providing comfort cooling does not require any measureable amount

of glycol because the application does not have a significant chance of operating at refrigerant

saturation temperatures below 32F. As such, the relative cost for the secondary fluid is very

inexpensive compared to refrigerant. While there is a desire to eliminate the potentially

environmentally damaging effects of refrigerant leaks, there is relatively little negative effect of a

secondary fluid leak on a chiller.

SUPERMARKET APPLICATIONS

In addition to comfort cooling, there are many commercial and industrial uses for chillers. The list is

almost endless: bakeries, beverage bottling, breweries, dairies, food processing, fruit/vegetable

washing, wineries, cement mixing, chemical plant processes, dry cleaning facilities, jacket cooling, oil

cooling, MRI testing, photo processing, plating processes, plastic injection molding, welding machines

and so on.

Given the potentially large amount of refrigerant charge required, supermarkets today face significant

cost and environmental challenges. One of the ways to eliminate that challenge is to employ a

secondary refrigerant system.

Given the potentially large amount of refrigerant charge required, supermarkets today

face significant cost and environmental challenges.

Rather than pumping refrigerant to direct expansion evaporators to each system of display cases and

walk in boxes, a chiller will provide a supply of secondary fluid at the appropriate temperature to flow

through hydronic coils in each of the above mentioned fixtures.

Medium temperature applications will utilize a blend of propylene glycol and water to allow the fluid

to operate at temperatures below 32F without freezing. Some supermarkets are utilizing liquid CO2 as

the secondary fluid for low temperature applications. Both of these applications achieve the goal of

reducing potentially large/expensive amounts of refrigerant necessary to provide the refrigerating

capacity required in the typical supermarket, and eliminate a large percentage of the potential

environmental issues resulting from large refrigerant leaks.

While the subject of chillers could easily take up several volumes of technical information, this gives a

brief overview of chillers and their use. And the next time you hear mention of Frankenstein just

smile.

Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration

technician before moving into the manufacturing sector where he regularly trains contractor and

engineering groups. He can be reached at ddemma@uri.com.

Consider the total picture

Canadian ERV wheel retrofit and specification must consider RER, frosting and

maintenance.

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By: LEN KOBYLUS

2015-05-01

Canadian HVAC contractors will replace tens of thousands of enthalpy wheels within the next five

years. The replacement candidates are rotary air-to-air heat exchangers that have surpassed their

useful lifecycles. They are found in packaged rooftop and supplemental dedicated energy recovery

ventilators, which were introduced to the Canadian market in the last 30 years.

If the HVAC service contractor selects a wheel replacement based on price rather than total

performance, commercial building clients may not get the best overall value. Enthalpy wheels are

decidedly different from each other in static pressure efficiency, maintenance and condensation

frosting prevention. The latter is an extremely critical challenge in Canadas cold climate.

Proper selection is even more important due to the recent development of drop-in replacement

unitary wheel cassettes. Replacement wheels no longer need to be the same OEM (original

equipment manufacturer) model that came with the HVAC system. The cassettes are made in several

matching replacement wheel diameters and come pre-packaged with an inverter duty motor and

industrial-type drive belt that simplifies and speeds up installation.

A vast majority of existing unitary equipment wheels specified for the Canadian new construction

market in the last 20 to 30 years have an OEM plastic or polymer film-type of wheel. Polymer-based

wheels were preferred decades ago because of their lower cost. Recently, wheels with aluminum and

other substrate materials have become more competitively priced, according to Ed Carney, partner, at

Mississauga, ON-based, Kilmer Environmental.

Carney believes service contractors and consulting engineers should consider higher performance

metal wheels as an alternative to a plastic wheel replacement. Furthermore, on new construction

specification, metal wheels should be considered and compared for total performance when ERV

manufacturers offer different OEM wheel options.

Technological advancements in enthalpy wheels in the last decade offer energy-efficiency (via

reduced fan power), improved frosting prevention and maintenance savings that can equate to tens

of thousands of dollars over its lifecycle.

RER EQUALS ROI

The larger flute openings seen in aluminum wheels with molecular sieve desiccant create a non-

turbulent laminar flow that allows particles to pass through without accumulation within the wheel

media. The aluminum media can also have an anti-stick surface coating applied to both sides of the

wheel to further prevent particles from accumulating and plugging the wheel in highly polluted air

streams.

Other options may collect more dust and debris, resulting in reduced air flow and higher maintenance

costs to clear static pressure-robbing air flow blockages.

A wheel replacement selection should be calculated for recovery efficiency ratio (RER), according to

the Air Conditioning, Heating and Refrigeration Institutes (AHRI) Guideline V Calculating the

Efficiency of Energy Recovery Ventilators and Its Effect on Efficiency and Sizing of Building HVAC

Systems. The RER takes into consideration the efficiency and the static pressure of a desiccant wheel

replacement.

Intended for service contractors, engineers and building owners, Guideline V provides a means for

calculating the

impact of applied energy recovery equipment on the energy efficiency of the HVAC system at a single

selected operating condition.

More simply, the calculations comprehensively take a host of factors into consideration, such as

geographic climate, fan/motor efficiency, exhaust air transfer ratio (EATR), pressure drop, energy

recovery methodology, and many other parameters. Guideline V also allows service contractors or

their manufacturer representatives to calculate ERV wheel comparisons and arrive at a

comprehensive savings for heating and cooling.

Contractors who calculate BTUs transferred divided by the number of watts used will be surprised at

the RER difference between wheels. For example, energy recovery wheels of equal diameter at an air

flow of 5000-cfm may offer the same total effectiveness but one wheel may have a lower static

pressure of 0.28-inches w.g. This lower static pressure drop results in a higher RER for both the

cooling and heating seasons. Although both wheels have identical filters, some wheels require higher

efficiency filters to prevent the wheels from plugging due to airstream particulates. This will result in

a higher static pressure loss and an even greater RER difference between wheel types.

Generally, the annual savings when choosing a wheel that delivers less static pressure could be up to

$100 per 1000-cfm. That sum can rise significantly to the $20000 range, for example, when

considering a 200000-cfm building ventilation system, depending on the geographic location, climate

and other variables.

It is not just efficiency, but both efficiency and static pressure combined that will provide the greatest

return on investment (ROI).

FROSTING IS A MAJOR CHALLENGE

Since total enthalpy wheels adsorb moisture from the airstream, frost accumulation on a wheel may

damage it. In addition, airflow may be blocked resulting in an increase in static pressure and reduced

efficiency. Furthermore, many wheel models (both new and retrofit) have a higher sensible than

latent efficiency, which presents a higher dew point saturation temperature on the psychrometric

chart and greater frosting potential.

Aluminum wheels are designed to handle temperatures as low as -26C (-15F) without frosting

because exhausted water vapours will be transferred back into the supply airstream. This has two

benefits: firstly, the exhaust air becomes drier with a lower dew point saturation temperature; and

secondly, wintertime humidity for the space is reintroduced. This minimizes the need for

humidification from an additional HVAC component, which can be expensive to operate.

Solutions to frosting are wheel-speed modification, electric pre-heat, or a total bypassing of the

energy recovery process by exhausting and using electric heat during extreme low temperatures. The

latter defeats the whole concept of wintertime energy recovery.

Preheating can be an energy-efficient approach to avoid wintertime frost formation when moisture

conditions are above 30 per cent relative humidity (RH) and temperature is frequently below -26C (-

15F). The total energy wheel can operate at full recovery on very cold days, thereby providing for

maximum humidification recovery. Reducing the wheel speed to cut temperature recovery also

reduces the latent recovery.

The required preheat temperature is determined by:

1. Locating the return air condition on a psychrometric chart;

2. Drawing a line tangent to the saturation curve;

3. Connecting to the heating design outdoor air humidity content;

4. Reading the dry bulb intercept value, as shown in Figure 1, line RA-OA1 (return air-outdoor air);

and

5. Preheating the outdoor air to the dry bulb intercept value, as shown by Figure 1 line OA2-OA1.

Controlling wheel speed to avoid frost formation is a better choice when: heating season

humidification is not provided, conditions are below 30 per cent RH during cold days and the total

number of hours below -26C (-15F) are infrequent.

The dew point control set point is determined by:

1. Locating the RA condition that exists when the winter outdoor air design condition is reached;

2. Plotting the RA point on the psychrometric chart and draw a line between it and the winter design

point;

3. Determining the higher dry bulb temperature at which this line intercepts the saturation curve

(exhaust air (EA), on Figure 1);

4. Adding 1.1C (2F) to this temperature and this becomes the control (winter) set point;

Besides frosting, condensation formation is also detrimental to IAQ because exhausted water-soluble

contaminants will be transferred onto the wheels media and re-introduced to the space.

Todays technological advancements in rotary wheel air-to-air heat exchange offer vastly different

options than 10 years ago. Contractors and consulting engineers faced with wheel replacement or a

wheel OEM option in new unitary equipment can provide a better service to their building owner

clients by considering RER, better frost prevention and reduced maintenance when selecting a total

enthalpy wheel.

Len Kobylus, who is currently the OEM sales manager at Semco, has 28 years experience in HVAC/R

with companies such as Trane Co. and York International, as well as fan and ERV companies. He can

be reached at len.kobylus@flaktwoods.com.