Chiller basics
May 1, 2015 | By DAVE DEMMA
A review of applications and systems in the field.
CHilr/noun
noun: chiller; plural noun: chillers
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?
Let’s 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 chiller’s 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 chiller’s 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 o
verview of chillers and their use. And the next time you hear mention of “Frankenstein” just smile. <>
