Humidifier/De-Humidifier

If your application is strictly for confort, you´ve done your heat balance right and selected the right cooling unit you should not need a dehumidifier. The cooling coils will easily give you 60-65 % RH which is very confortable for the average majority of people.

Coffeebean is correct. Unless you suspect you have an unusual situation, these items ( especially humidifiers ) can also be a maintenance nuisance.

I've designed and installed systems for medical facilities requiring humidity ranges. Dehumidification is easily accomplished by controlling reheat but adding humidity is done via many options for humidity injection.

I like steam humidifiers personally. But limiting a humidifier's output is critical lest we saturate a duct and foster growth of the "M" word.....sh-h-h-h-h. You REALLY don't want accusations of mold!

I'm also a fan of CO-2 monitors to control fresh air, bring in only what we need unless it's a special application. Treating fresh air is costly and often fresh air dampers get "defeated" to save $$.

There are a myriad of reasons for comfort complaints. Drafts, temperature drifts etc. And studies show a correlation between worker productivity and comfort too!

Coffeebean's "heat balance" is also critical, meaning dividing the system's output properly, lest we have annoying temperature lag in some areas.

If you had specific concerns such as drafts ( or lack therof ) and NOISE, these concerns should be voiced to the installing contractor so that design criteria addresses them, otherwise, an "acceptable" installation for the best possible price is the driving criteria for materials and workmanship.

Remember...the distraction from an ugly wall paint or cheesy furnishings are soon forgotten but the distraction from noisy, drafty grilles or poor temperature regulation haunts you forever !

This question is not very clear, you have to get answers of these questions and result will Tell you that you need, like humidifier/dehumidifier- static air cleaners?

1- What is your cooling load?

If equipment selection is done accordingly than you do not need dehumidifier.

2- What are ambient psychometric conditions, specifically in winter?

If winter temp. goes down beyond Zero Celsius, then you need humidifier. This thing has an exception with, No. of people working in the conditioned space and percent of fresh make up ambient air.

3- How much (at what level) clean air you need?

If there are lot of odors, and lot of people working in the conditioned space, you may need extra activated charcoal filters too, along with normal filters.

If you need extra clean air (like for labs) than you need ionizing static air cleaner too.

Abdul Rahim.

AES Lal Pir (Pvt) Thermal Power Plant.

1 - I spent a good deal of time and feel very confident that my cooling load has been done correctly. I believe that I have a well designed system.

2 - The application is in Canada where winter temperatures can go as low as -30 degrees celsius. I don't believe the number of people working in the conditioned space will have much effect on this.

3 - I don't believe this is relevant in this application.

As an engineer you must understand that you will get much better answers if you give even a minimal amount of data ... An eight ton cooling unit implies that you either live somewhere like Singapore, have a very large office, or a very badly insulated one. Why do you have so many different zones? A cooling unit can both cool and dehumidify. Mechanical dehumidifiers are actually just refrigeration systems where you cool the air to condense out the moisture and then return the heat to the system. Since you already have a monster condensing unit, which I hope has been sized for the load rather than purchased on the "bigger is better" principal, all you need to dehumidify the system any more than can be achieved simply by proper sizing, is a method of returning heat to the system when it is already holding your set temperature. This will keep the condenser running so that the evaporator can collect moisture and drop it down the drain. (Condenser sizing close to load is needed for this sort of thing so that the running cycle is long enough to allow condensate to drop off the coil). If you want to increase the humidity, keep the condensate in the system by either short cycling (by oversizing the condenser) or using a water feature like a waterfall or fountain. In Northern California where the weather is ideal as far as I am concerned, 70ºF and about 70% RH is just right.

I will try to give a little more data here:

3200 sq ft office space

6 different zones that each have there own by-pass terminal unit that is controlled by it's own thermostat

Packaged a/c unit is 8 ton, 3500 cfm, 25% fresh air

Any additional information that you think may be of help?

Maybe in Canada you can get be by with 25% outdoor air with your air conditioning system, but in more temperate climates, you would probably have excess humidity problems in the summer. At 8 tons the air conditioner is probably 2 stage DX, if it is a packaged unit. When the load in the space is satisfied the unit will be bringing in humid outside air without removing the moisture, which only occurs when a compressor is running. One solution is to provide reheat and have a dehumidification cycle where the compressor runs and the air is reheated for comfort until the humidity in the space drops and then the control goes back to normal.

If you have fully modulating cooling. the problem will be worse, unless it is a variable air volume system that supplies constant 55 deg F air in the cooling mode. A DX system can also be designed for VAV if you employ a bypass duct to maintain the needed minimum airflow through the air handler. DX systems need a minimum airflow or else the heat exchanger will ice up. In case you did not know, DX is short for direct expansion -- and is the most common type of smaller AC cooling coil where refrigerant is expanded within the cooling coil heat exchanger.

In the winter, you have to determine the minimum humidity you want to provide for on a design day. For an 8 ton system, you may have to provide about 20 pounds of moisture per hour to achieve 30% RH on a design day. 40% may be more comfortable, but becomes impractical when temperatures are sub-zero.

Packaged units typically are not supplied with humidifiers, so that would be something installed in the duct downstream of the air handler. Various manufacturers provide systems for using gas or electricity to vaporize potable water and insert it into the air stream. The control would be a humidistat in the space and a high limit control in the supply duct. An electric powered humidifier would draw about 7 kW at maximum load to deliver 20 lb/hr steam to the air stream. There are atomizing humidifiers, but any minerals in the water will become airborne and create a fine dust. Also the atomizing humidifiers cool the air stream, so additional heating is needed for the supply air.

The net sum of my comments is that there are many consideration to take to designing a good AC system and we have not even talked about exhaust air.

What part of the country are you in and how close do want the conditions to be in you space?

Miketheboilerguy@aol.com

As SNICCUS so aptly pointed out, noise is considered to be as offensive as drafts. Spending a little extra time sizing the ducts to produce the required volume at the lowest homogenizing velocity will reduce the noise to its intrinsic level.