Air Conditioning Systems and Water Lines

With any closed-circuit distribution system, the challenge is to keep the end-of-loop users supplied with sufficient utility fluid while ensuring that not too much utility fluid is supplied to the users at the beginning of the loop, "short-circuiting" the end users. So some sort of flow regulation will therefore be required wherever utility fluid is passed through a user from the supply pipe to the return pipe. Balancing the system between supply and demand is the challenge.

For analogy, it's the difference between a "1-pipe" and a "2-pipe" system in a domestic central heating circuit where these days, ideally, each radiator has a thermostatic flow control valve, and each radiator takes only the flow that it needs.

While piping network calculations can be done manually they are iterative and tedious, and many designers resort to software packages that can take the drudge out of the task.

PWSlack is generally right on. The EASIEST way to build up a distribution system that meets ALL criteria of a chilled water system- flow through the chiller and flow to all users- is to use a primary-secondary system. This has fallen out of favor a bit lately, but it is definitely the safest system and easiest to control.

Essentially, one set of constant speed and volume pumps (one or more) feeds water through the chiller ONLY. Another set of pumps (variable speed, variable volume) are connected to a second pipe loop that feeds the end users. This inlet and outlet of this secondary pipe loop are connected to the main loop within a few feet of each other- to minimize any pressure differential between them that might induce flow in the secondary loop that is not required of wanted.

Depending on your preference for system design, you can use different ranges of temperature differential at each system- perhaps a 12F differential in the main and maybe 16F differential in the secondary loop (to reduce total flow and corresponding pump power usage). Essentially, the main flow of, say 200 GPM from 42F to 54F will support a secondary flow of 150 GPM from 42F to 58F.

At the junction with the inlet to the secondary, 200 GPM enters the tee, 150 GPM is drawn out to the secondary and the remaining 50 GPM goes on down to the second tee (secondary return). The 150 GPM of up to 58F water (at full cooling load) mixes with the 50 GPM of 42F water to make 200 GPM of 54F water to the chiller return.

The secondary piping is connected to each load with a balancing valve (usually on the discharge side of the coil) and a 2-way valve on the coil inlet side. The 2-way valve is controlled by the space temperature sensor/thermostat to provide the proper temperature of cool air to satisfy the zone's cooling load. Unneeded water will bypass the coil and go on to the next.

The return piping begins with the outlet of the first coil and follows the supply main (increasing is size as potential water from each down stream coil is available). This assures that the overall pressure drop of the system will be essentially even at all coil connections. The supply main reduces as fast as the return increases, and will be sized to support ONLY the last coil at the end of the line. The secondary return will be full size at the end of the line after the final coil.

A pressure sensor located about 2/3 of the overall secondary pipe supply main length will control the speed of the supply pump to maintain a fixed pressure differential at that point.

By the way- by using a cooling coil that is about 200% of the required area (about 225-250 FPM vs 450-500 FPM face velocity) you can actually use a chilled water temperature difference of 30F (42F to 72F return) with full functional operation of the HVAC cooling system. Such a large differential will cut pump size and HP in half, plus generally reduce the size of the piping by at least i, and usually 2 pipe sizes- this combination will provide for a very cost effective system installation and operation since water flow will be about 50% or their normal volumes resulting in piping that is installed at about 2/3 of the size that is normally required with all the cost savings that this entails.

This will supply all the chilled water that the coils need with the least amount of power and operating hassle.

This same system can be used for the building heating hot water loop as well, with the same level of controlled flow at each coil and minimized pumping energy although Hot Water systems usually use a primary temperature difference of about 20F (180F to 200F) and secondary temp difference of 40F to as high as 100F (200F supply and 100F return). As long as the return water temperature is higher than the entering air temp and the entering water temperature is higher than the desired supply air temperature, any HW temperature or temperature differential will work.

I'm designing a chilled water cooling system for a manufacturing plant with the following phisical conditions:

• Building area : 93,600 SQ.Feet.
• Dry Bulb average: 97 F
• Relative humidity :57%

Desire Conditions :

• 77 F to 80 F
• 55% to 60% RH

Actually the are 9 supply fans introducing air direct from outside to 30 CFM each .

I'm trying to build air handlers using the actual fans to work on Draw Through direction , using customized performance water coils.

What do you recomend about ?????? :

• Chiller( capacity , type , quantity, etc)
• Pumps ( capacity , types , locations, etc )
• Coils ( Type , capacity , Performance , pipe size , etc )
• Hydronic pipe ( size , material , distributions , etc )
• Control elements .

Im trying to make a low energy consumption project .

Please gige your general recommendations.

Thanks in advance

I will gladly give you general answers to SOME of your questions, but actually fully designing the HVAC system is a "Paid" response.

1- I think the CFM you list for the fans is WAY too low. You need to check this out.

2- If the intake for the fans truly is 100% outdoor air, is there any reason (high exhaust, many contaminants, etc.) that would require this level of ventilation? 100% outdoor air will increase cooling loads by over 100% (near 150% for your part of the world) and- unless the ventilation is required, you want to minimize the amount of fresh air.

3- What kind of internal loads do you have in this building? High heat production, high moisture production? Both? Lots of bodies doing basic handwork? Medium density occupancy with heavy effort? Each of these affects the overall design load.

Regarding the chillers- How reliable (and "clean") is your electrical power? Is your load essentially 10-hours a day, or 24/7? What is the availability of relatively fresh water- does not have to be potable-grade- for cooling tower make-up? What type of waste water system do you have?

There is a thermal storage system available that supplies its chillers as well. It makes 29F water under pressure that is turned to ice crystals for after-hours storage that will reduce peak load capacity and allow using lower capacity chillers- saving money and daily operating energy and expense. The CHW is 32-34F, so even lower water flow is possible.

Regarding piping- Do you have a reasonably competent water-treatment contractor available or are you on your own? Will you be installing this system with your own people or will you hire competent contractors? Can the piping mains be supported by the exterior walls and structural members? I lean toward Sch 40 PVC for the mains and Sch 80 PVC for the branches based on chemical resistance, minimization of any corrosion (adding future pressure drop), resistance to scale build-up (pressure drop), smooth walls (pressure drop), relative ease of installation, and partial self-insulation allowing lower overall thicknesses.

Pumps- Base mounted using standard efficiency, 1750 RPM open drip-proof motors (unless there is some pollutant risk) and standard center axis inlet, tangential outlet cast iron volute with brass or bronze trim design pumps. Use ceramic-stainless seals for the chilled water pumps and teflon packing for the cooling tower pumps.

By the way- you WILL want to use cooling towers because there is a tower upgrade addition that is now available that reduces water usage by 20% and electrical usage by 30% on peak load days when running as a tower (further reductions in cooler weather) and allows easy, on-the-fly replacement of the chiller below about 55-60 ambient.

You talked about a draw-through air handler upgrade. I think that a blow-through would be a better choice for two reasons- First, the blow-through removes all of the fan heat as well as system heat. Second- the blow-through will allow easier installation of the over-sized coils that I spoke about earlier- allowing for a 30F CHW temp differential and much smaller pumps and piping an lower energy costs. With the thermal storage, the DeltaT can be increased to 40F for even lower pump and piping sizes and reduced energy usage.

Your coils should be standard copper tube, aluminum fin (unless you are in an area with high salt water concentrations- like near an ocean or other large salt-water body. If that is the case, you need copper/copper to eliminate electrolysis.

Your controls can be very simple- To give reasonable control range, use 2 electrically actuated valves on each coil controlled by a wall mounted (or column mounted) 2-stage thermostat. Easy to install, easy to trouble shoot, and very reliable and cheap without compromising any end-user comfort.

Well said, and good rules of thumb.

A related topic in primary/secondary systems is "decoupler headers," on which there are several CR4 threads.

I made a mistake at the writen specs .

It's 30,000 CFM

That is more reasonable, but very likely totally out of what you need for a cooling system. That is nearly 3 CFM per Sq Ft- which is somewhere between 150% to 200% of what you likely need.

Without doing any actual calculations, the 9 X 30,000 CFM outdoor air would have a peak A/C load of about 1500 to 1800 tons JUST for conditioning the air.

Put another way, an office building will have a typical cooling load of about 300 Sq Ft per ton, or about 300 tons for your site.

A hospital will have a cooling load of about 150-175 Sq Ft per ton, because of their high vent load and large amount of internal loads, or about 530 to 620 tons.

SO- unless you absolutely MUST have that much outdoor air entering you site, you should be able to operate with somewhere between 300 tons and 600 tons.

With a worst case 600 tons, and using the thermal storage plus 40F DeltaT CHW, you can make cooling air at 50F. Based on your plan for 80F internal- which will be no problem- you could support the 600 tons with about 180,000 CFM of air, which will be about 20% outdoor air- or 36,000 CFM (with a dedicated cooling load of 200 Tons).

Since you are planning to use the existing fans, and there will be SOME added pressure drop from higher efficiency filters and the cooling coils, you will likely have to re-sheave your air handlers to supply a total of 180,000 CFM of combined recirculation air.