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Air Cooled vs. Water Cooled condenser tonnage...are they equal???

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Air Cooled vs. Water Cooled condenser tonnage...are they equal???

Air Cooled vs. Water Cooled condenser tonnage...are they equal???

seetwo

(Specifier/Regulator)

(OP)

30 Jun 03 12:01

Hello,

I am in the process of trying to decide if our company should switch from: a 60 ton water cooled condenser to: a 60 ton air cooled condenser.

My problem is that I have been unable to get a firm answer on whether a 60 ton air cooled condenser will give the same cooling power as our current 60 ton water cooled????

This if for an apartment complex in the state of Kentucky, USA. Temps typically peak in the mid 90's, occasionally 100+ degrees F. Average yearly humidity is 58% @ 1:00 p.m. to 81% @ 7:00 a.m.

We have been able to maintain a 42 degree chiller water temp pretty consistently. If we could get into the 38 or 40 degree range though, that would be a nice bonus, but that is not critical.

The existing water cooled tower is corroding and must be replaced soon.

It seems that in the long run, an air cooled condenser would be cost beneficial as there would be no water consumption and treatment costs. That seems to offset the increased electrical usage of the air cooled type.

We are using a chiller with 2 reciprocating compressors located indoors and the condenser nearby outdoors.

Any advise on the cooling power of the same size water cooled vs. air cooled condensers would be greatly appreciated.

Thanks. SeeTwo

tonnage

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

MintJulep

(Mechanical)

30 Jun 03 15:01

The answer is "it depends".

Condenser ratings are provide at specific conditions, and these conditions should be clearly stated by the manufacturer.  The conditions include things like mass flow rates for the primary and secondary fluids, the entering and leaving temperatures for the primary and secondary fluids, pressure drop through the primary and secondary sides, etc.

Since you are considering going from water cooled to air cooled you cannot really make a direct comparison.  You need to figure out what your actual capacity needs to be, and select a new condenser that will be able to provide that capacity at your location under whatever "design" conditions you elect to use.

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

Cooky

(Mechanical)

30 Jun 03 19:37

SeeToo,

The flow and return condenser water temperatures from the cooling tower will always be lower than the ambient dry bulb temperature (I would estimate by 10 deg. F).

If you replaced the cooling tower with a dry cooler of equivalent duty, the flow and return condenser water temperatures would be approximately 10 deg. F higher than the ambient dry bulb temperature.

Consequently, the chiller will reduce in capacity by at least 15% (probably more), it will also be more prone to trip-out on high pressure during the hottest days.

Assuming your company does not wish to replace the existing chiller, I can suggest three options for your problem.

1. Replace the existing cooling tower with a new cooling tower - there will be no adverse affects on the chiller.

2. Replace the existing cooling tower with a dry cooler - the chiller will be derated and may trip-out more frequently on the hottest days.

3. Replace the existing cooling tower with an adiabatic cooler, this is a dry cooler with water spray fitted - It will give performance very similar to the cooling tower, but without the water treatment costs.

I hope this helps.

Cooky
 

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

rbrunt

(Specifier/Regulator)

1 Jul 03 10:41

I don't think you will be happy with the performance of an air cooled condenser unless you have capacity to spare.  Water temps will be a good bit higher.  But frankly considering you mention chill water temps below 40 and a reciprocating chiller I really think you need to consider a new chiller and some AHU coil replacement.  There is no need to run that low of chill water temps.  A new chiller would pay for itself in a very short time.  It also sounds like you need a system that requires little maintenance.  Tower Tech in OK. makes a virtually maintenance free tower that is all FRP so it won't rust.  Nalco chemical sells a computer controlled water treatment system that can be remotely check via a modem.  Often one can hook up with an energy management company who will finance the whole thing with money saved in power consumption and there will be no out of pocket costs.

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

2

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lilliput1

(Mechanical)

1 Jul 03 17:55

Air cooled units are noisy and need more space for condensers. They also require more power. Air cooled receip & screw = 1.20 to 1.40 Kw/ton (incl compressor, cond. fan & CHW pump). Water cooled would be about 1.05 to 1.1. The air cooled unit will require less maintenance. If you can provide the additional space for condensers  locate them so noise is not a problem, then go air cooled. Note that in the 200 ton & larger size the water cooled system using centrifugal chillers can go down to about 0.8 kw/ton (includes compressor, cooling tower fan, condenser pump & chilled water pump.
CHW pump = 0.05 kw/Ton
Cond Pump = 0.10 kw/Ton
Cooling Tower Fan = 0.05 kw/Ton
Condenser Fan = 0.10 to 0.15 kw/Ton
Recip air Cooled Chiller Compressor = 1.05 Kw/Ton
Air cooled screw chiller 1.2 Kw/Ton
Rotary water cooled chiller compressor = .8 to .9 kw/ton
water cooled centril compressor (200 Tons 7 up) = 0.6 kw/ton

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

with

(Mechanical)

5 Jul 03 01:38

You will get a temperature of ambient plus 8-10 deg C in an air cooled condenser, and with time and conditions this may further go up by 2 deg C. This may reduce the overall tonnage of the chilling plant.
krk

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

r22

(Electrical)

6 Jul 03 16:36

First of all is the existing an evaporative condenser with refrigerant flowing through it or a water loop that cools water and pumps it inside to be used to condense refrigerant. If it is the latter I would not do it. I would replace the tower with a new one.  I can tell you that with small condensing units a tube in tube water condenser can reject much more heat than a refigerant air condenser when changed out like for like(tonnage rating). If the air condenser is undersized you're beat, you can't control variables like water temperature and flow rates to squeeze more capacity out.

With an air condenser you have dirty coils to deal with and refrigerant leaks. Make sure the piping is done corectly to account for compressor capacity control or you could suddenly start losing compressors when you never did before.

Best would be a cooling tower to keep the refrigerant charge localized near the compressors. Don't trade short term savings for long term problems.

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

seetwo

(Specifier/Regulator)

(OP)

10 Jul 03 15:10

To: All,

Many thanks for all the advise from all the repliers!

I will monitor the water temps we are now experiencing and update this thread after I have enough data.

It sounds like going to the same size air cooled condenser on a water cooled system that is currently adequate, but barely so (not adequate on 100+ days*) would not be advisable because of the loss of cooling capacity.

* However, this spring we did some maintenance work on the coils in the apartments that were not cooling adequately in years past. We replaced isolating valves and flushed coils. So far, there has been much improvement.

So, chiller/tower capacity size is possibly okay. We need more time to test the system in the hottest weather.

To: Cooky,

You are correct that we do not want to replace the reciprocating chiller at this time. We recently re-built both compressors. However, at some point in the future we would consider upgrading to a modern scroll type chiller in our indoor boiler room and continue to have a outdoor condenser (either water or air cooled). I've heard they are less noisy and more efficient with fewer moving parts.

Re: your suggestion #3, wouldn't the evaporating water sprayed into an adiabatic cooler leave mineral deposits behind (and degrade performance in time)? That is why our current water cooled tower is set to bleed off water onto the ground isn't it (to drain off the minerals left by evaporating water)?


To: r22,

The condenser set up we use is the 2nd one you mentioned: A water cooled condenser tower with a water loop that is pumped inside to the chiller.

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

vanspall

(Mechanical)

31 Jul 03 12:41

Suggest that you replace cooling tower with an Adiabatic Dry air cooler.
there are such coolers on the world market which can acheive a recooled liquid temp of 7 deg C above wet bulb temp. Spray water is once through and is only operating when ambient temps are high rest of year operates as standard dry cooler. Also sprays are not directed at coil but into incoming air stream thus saturating air .

RE: Air Cooled vs. Water Cooled condenser tonnage...are they equal???

PSucher

(Mechanical)

3 Aug 03 12:13

Depending on where the "Design" loading of your compressors are, you may be able to switch.  You probably know that the evap coolers lower the condensing temperature of the refrigerant.  If you use straight air-cooled HX (no evap), then you lessen the "tonnage" (you have reduced the delta-enthalphy).  To get the same "tonnage" of refrigeration (which is the basic goal), then you need to move more refrigerant (i.e. mas flow).

Another effect you'll see by going with Air Cooling is that you need to compress to a higher pressure.  SO you may have a motor problem on your compressor.

The best thing to do is to go to your compressor manufacturer or local system salesmen and ask him/her to look into this issue...

For the most part, you will not likely be able to do what you are asking to do without major changes (i.e. costs).

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News

One of the recommendations we frequently make to our clients is to move from air-cooled to water-cooled compressors. Air-cooled is the most common type of cooling equipment out there, serving spaces from individual residences, apartment buildings, and large commercial buildings.

In the buildings we work on, we see it most commonly in the form of a Rooftop Unit (RTU). We seem to see RTUs everywhere, but, notably, not on more efficient buildings. Those will almost always have a chiller and cooling tower – the bones of the centralized water-cooled system.

We’ll explain the details of “how” below, but the basic reason that water-cooled equipment is so much more efficient (up to 215% more!) is that it takes advantage of evaporating water, just like the human body does through sweat. If it is so much more efficient, why would anyone install an air-cooled system?

First Cost: RTUs often win (or at least they used to…)

Too often, the decision on which type of cooling system to install is based on the first cost of the unit – but not the lifetime cost of owning and operating the equipment. RTUs are selected because they “cost less” than installing a water-cooled system. RTUs, or “packaged units,” contain all the needed equipment in one module. Thus, they require little engineering to install. Additionally, they have basic controls (thermostats) and do not require skilled in-house staff for operation. Since they are modular, they can be added piecemeal as spaces are built out without needing to consider system-wide impacts. However, “first cost” only takes into consideration the upfront cost of a unit and does not factor in the higher ongoing maintenance costs and increased energy consumption.  Additionally, most RTUs only last about 15 years before they need to be replaced. Water-cooled systems typically last 25 – 30 years or more with good maintenance. This often means that one water cooled system can last two lifetimes of air-cooled condensing units.

Full Cost of Ownership

If the total life cycle costs of the equipment options are considered, then the water-cooled system has a significantly lower cost of ownership. The figure here shows the installed, annual electrical costs, and life cycle replacement costs for both types of systems designed for a 200-ton load on a 120,000 SF building. The first costs are: the air-cooled is $250,000; and the water-cooled system is $340,000. With a price tag that is roughly 1/3 more expensive than the air-cooled system, it is easy to imagine why an owner without good information on life-cycle costs might choose air-cooled systems. However, the water-cooled system will quickly pay for itself. In the St. Louis climate, for example, a typical water-cooled system will have more than made up for the additional cost by the middle of year four. Based on a summer electrical rate of $0.10/kWh, the annual operating budget is reduced by $20,000. The $90,000 premium first-cost thus has a 23% ROI. Over a period of 30 years, that initial premium is only 4.8% of the total cost of ownership of the air-cooled system, and the life-cycle cost of the water-cooled system is only 58% of the air-cooled system. Every dollar invested in energy efficiency (in first costs) is repaid more than eight times over the life of the equipment. Depending on the run-time of your cooling systems, and the climate conditions, the savings can be fifteen times the investment.

These days, with energy efficiency financing available through PACE or other third-party lenders, that extra price tag for the more efficient system should not be an obstacle. The lending costs are overwhelmed by the savings generated, and you’ll have lower costs from day one.

System Differences, the Maintenance Perspective

All types of conventional HVAC equipment will have parts that will require regular inspection and maintenance.  In the two-hundred-ton case study being used, it is likely that the air-cooled system is comprised of five, forty-ton RTUs, each serving 24,000 SF. This means five compressors, five furnaces or heating coils, five sets of condenser fans, five condensing coils, five evaporator coils, etc.  The central plant will have one chiller, one cooling tower, two chilled water pumps, and two condensing water pumps that provide the chilled water to five, forty-ton air handling units that deliver the cooling to 24,000 SF each.  As a general rule, with more pieces of equipment, more maintenance will need to be performed, so the RTU system is likely to have significantly higher annual maintenance requirements.  If maintenance is not conducted frequently enough, then the performance of the units may be significantly impacted.

There are also specific cooling related maintenance requirements: air-cooled condensing units have coils that, ideally, require cleaning multiple times throughout the cooling season. Water cooled compressors typically reject their heat to cooling towers that evaporate water in this process.  This evaporation process requires makeup water that will require on-going water treatment. Biocides are typically added to prevent fouling; this is typically done through automatic feeders and is usually handled by an outside vender. The other water quality issue is scaling from hard water.  As water evaporates the total dissolved solids (TDS) build up in the water loop and require periodic “blow down,” which is the dumping the water with a higher TDS concentration and replacing it with “clean” water. 

These two processes need oversight but are often automated and only require a few hours monthly.  Water cooled condensers should have their tubes cleaned annually. This work is often scheduled during slow times in the off season. If the building owner does not want to employ maintenance staff for this work, it can also be contracted out.  Some cooling towers have basin heaters to prevent the water from freezing during the winter months. However, for most buildings the chillers can be shut down and the towers are drained during the winter months. Any required cooling during the winter months is better provided through outdoor air economizers – free cooling using cold outdoor air.

There are also different space requirements. Often chillers are placed indoors, either in basement or penthouse mechanical rooms. RTUs are placed closest to their loads, which means scattered across rooftops. In multistory applications, this requires allocating chases to provide for supply and return ductwork to reach lower floors from the roof.  On balance, there is less maintenance for cooling towers and chillers than for RTUs, though the maintenance of chilled water systems requires a broader familiarity with mechanical systems.

A Cooling Tower undergoing preventative maintenanceA Central Plant Chiller in action

How is water cooling more efficient?

There are two primary reasons why water-cooled systems are more efficient than air-cooled systems.  The first reason comes down to the basic physics of the two heat transfer media: air and water. The specific heat of water is 1.0 BTU/lb.-°F, and the specific heat of air is 0.24 BTU/lb.-°F.  Therefore, the system needs to move four pounds of air compared to one pound of water to transfer the same amount of heat that is being rejected. As a result, this means more fan power to move the air than pump power is needed to move water.  The second reason is that compressors on air-cooled systems must create higher head pressure. Air-cooled condensing units and RTUs are commonly placed on rooftops. When units are located on the roof of a building they are in direct sunlight. This raises the sensible temperature around the unit by 10 – 15°F. On the other hand, a cooling tower, using water, depends on the wet-bulb temperature, which is often about 10° cooler than the dry-bulb (sensible) air temperature.  This means that the effective air temperature between these two systems can differ by more than 20° under the exact same outdoor weather conditions.  At the higher temperature, the compressor on the air-cooled system will have to create higher head pressure to effectively reject heat. 

For example, consider equipment using R134a refrigerant in St. Louis. The average high temperature for July is 90° with 70% relative humidity.  Under these conditions, the wet bulb temperature will be about 82° whereas the ambient temperature for the air-cooled unit may be 105°. This means that the compressor on the water-cooled system will need to produce about 90 PSIG and the air-cooled compressor will need to produce 135 PSIG to reject the heat in the system.  A typical evaporator pressure is around 37 PSIG. This means the compressor on the water-cooled system needs to produce a 53 PSI pressure differential and the air-cooled system needs to create a 98 PSI pressure differential. Thus, the air-cooled system needs to produce nearly twice the pressure differential which requires nearly twice as much energy. 

Energy efficiency for a cooling system is often measured in kilowatts per ton (kW/ton).  That is, how many kW of electrical power are needed to produce the equivalent cooling of one ton (2000 pounds) of ice (the measure of the ton goes back to the days when cooling was accomplished through ice delivery). Air-cooled condensing units typically have an efficiency rating ranging from 1.13 – 1.25 kW/ton.  A conventional water-cooled unit with a cooling tower has an energy efficiency rating ranging from 0.58 – 0.79 kW/ton. Therefore, water-cooled systems can produce the same amount of cooling as an air-cooled system while only consuming about half the energy.

Do Air-Cooled Chillers and RTUs have a place?

Yes, they do! First, air-cooled systems are a good option when hydronic cooling is needed through the winter. This happens in certain primarily industrial uses. Ideally, they are put in as winter-only use when they can even be slightly more efficient than water-cooled systems. Air-cooled systems can also be the best choice when they are put in as a backup of more efficient systems. In this case they have few run-time hours and can provide system resilience at lower cost.

They can also make sense when your system is not very large. It is difficult to find a water-cooled system smaller than 80 tons. This means that chilled water systems are not often found on buildings less than 50,000 square feet (while small systems can use the ground for heat exchange, this can be impossible given site conditions, and is a subject for another day). This is a function of the way building complexity and maintenance increase with scale. If the system is small, it takes longer for the energy savings to justify the initial first costs of the additional engineering and the number of systems. The real maintenance benefits of chilled water systems also grow as the number of air-cooled systems being replaced increases.

RTUs may also make sense if the system is temporary, or where modularity or mobility is more important.  However, installing a temporary system can be a slippery slope; once one RTU is installed they tend to stick around longer than intended and multiply. We recommend that if an RTU is temporarily installed that a plan also be put in place for when and how it will be integrated into the centralized cooling system.  In such circumstances, RTUs can readily be replaced with air handling units with chilled and hot water coils (or gas fired furnace modules) that “plug” into the existing piping infrastructure.

It should also be mentioned that the air-cooled vs water-cooled argument varies by region.  Factors such as water cost, full-load cooling hours, humidity, and energy costs will impact the efficiency and economical operation of a cooling system.

Want more information on air cooled condensing units manufacturer? Feel free to contact us.