Running a rooftop at lesser heat gain than designed for....

[Vicsidhu]

Hello Everybody!! This is an awesome sight and I have got good answers for my queries. Thanks. I have another beginners question. I have just begun in HVAC design.

I have a space which is being served by a single rooftop (heating and cooling - 10 ton) - an area in a single story building (3800 sq.ft). The client is moving into a part of the area (2800 sg.ft - requires only 2600 cfm) and wants to cap of ductwork to the neighboring space and not give free air to the neighbour (1000 sq.ft). Even if I dump all my air (minimum 3500 cfm) in this 2800 sq.ft I wont get the required heat load to run the unit efficiently.

My question is:
1. Is there a solution to this problem other than dumping by-pass air to the ceilng space. (Dumping air in the ceiling space might wont give me the required heatload anyways)

2. What will be the consequences of not sharing the unit on the unit - minimum 3500 cfm for a space where the requirement is 2600 cfm at the most. Damage to the unit ??

Thanks for taking time to read this.

 

[235zzzo]

Hi Vicsidhu!

One main point comes out of your question:

A big machine and a small necessity, turns always into an overcycling situation, with an unbalance control and an earlier damage possiblity for your equipment. In order to prevent this, there is a bill you have to pay!

Let's see:

1. Usually a common roof-top unit works in on/off mode, it is not a modulating device. It is a disvantage, as a matter of fact, because the compressor is an on/off machine (even it can work in step power mode).

2. However we can introduce some (cheap!) modulating effect at the ventilator in air circuit, by resistive means in the electrical circuit which serves that ventilator.
[Notice: I must call your attention, this is not a good measure in terms of energy management, that's the bill you have to pay]

3. In this way you can reduce (accept this as common tip), as much as 10 ~ 15 % of flow-rate, without to unbalance the roof-top. So we will have 2975 cfm (based on 3500 cfm), i think you can live with that!

4. Another way, without interfering with the unity, nor with the space, you can create an isolated by-pass circuit to the roof-top unit itself and driving there the excess air flow-rate, trimmed with manual duct air valves in accordance. In terms of energy-management, this measure is better, yet a bit more expensive. This is the other way to pay your bill. You have to choose. My advice, choose this last one!

5. Finally, i suggest you to take advantage on the spare thermal power you have in your unity, like for instance to provide more fresh air and less re-circulating (it improves the indoors quality air, unless there is outside strong air pollution), paying attention to the duct velocities of those air flows.

Have a Good 2002.
Kind Greetings
zzzo

 

[235zzzo]

Hello Again Vicsidhu!

I whould like to refer a few more aspects, related to my previous point, P.4 and P.5, so I will begin from P.6 in accordance. I'll try to be clear and simple as much as possible:

6. As I mentioned, the solution stated in P.4 asks for some attention related with the temperature control of your roof-top. It deals with the re-position of the temperature sensor/probe.

7. Two cases can happen:

7.1 If your roof-top is a parcial outdoor air flow unity, that means, there are involved four types of flows:
- Return Air Flow to the unity,
- Outdoor Air Flow,
- By-pass Air Flow.
- Supply Air Flow to the space, resulting from those three mixed flows minus the By-pass Air Flow, (or the Unity Total Supply Air Flow minus the By-pass Air Flow).

Thus, the controlling temperature probe must be located at the Return Air Duct, just after leaving the space.

7.2 If your roof-top turns to be an "almost 100%" outdoor air flow unity, that means: No existing Return Air Flow to the unity from the space.
The flows in presence are:
- By-pass Air Flow,
- Exhaust Air Flow promoted by an exhaust ventilator (= Unity Supply Air Flow - By-pass Air Flow),
- and you will have an Outdoor Air Flow which is fairly/quite the same/equal to the just above Exhaust Air Flow.
- The Supply Air Flow to the space is the Unity Total Supply Air Flow minus the By-pass Air Flow)

In this case, the controlling temperature probe must be located at the Exhaust Air Duct, just after leaving the space.

I hope this can be of some help.
Best Regards
zzzo

 

[heylj]

I am not sure you have a problem.

If you check with the vendor for the A/C unit you may find that you can turn the airflow down to levels that are close to what you need (resheave the unit and reduce the airflow).

Many a/c units today can operate from 300 to 500 cfm/ton (3000 cfm to 5000 cfm for a 10 ton). Check with your local vendor. They should know the capability of your equipment. Also, If you have much outside air load and with todays computer loads and other internal loads we experience the 1.25 cfm/sf you discuss (3500cfm/2800sf) is not necessarily too much air.

The concern over cycling the compressor is a legeitimate but you may be closer to the load than you think.

 

[235zzzo]

Hi, Heylj

Mayby you are right! But let´s make some discussion here!


1. Nevertheless what you are based on, are thumb-rules and those for certain specific cases can not work so well.

2. On other hand, considering our co-fellow Vicsidhu has come out corretly with that conclusion, something has to be done in order to fit the requirements for that new space!

3. Supposing indeed that 3500 cfm are to much, and what really needed, are 2600 cfm. Notice, the areas ratio gives a less 26.3% thermal load required (considering in proportion the same thermal load rate), and if you drive all that mass flow rate into the new smaller space, you can experience some air diffusion and acustic/noise problems from the terminal diffusion air devices, turning eventually in some disconfort conditions of the space.

Kind regards,
zzzo

 

[Vicsidhu]

Thanks for a very fruitful discussion. 235zzzo and heylj.

My question extends a little further.

Another Project:
My Heat Load Calculations give me requirement of 10-ton unit. (New building) Area: 5500 sq.ft., Office space, well shaded windows on east, other sides are a part of a garage with existing air circulation. Location is Toronto, Ontario.

But the person who is designing the system has a habit of over designing the system. He says – this is too small – and increases to 15-ton unit – without giving any logic. I know it is OK to over design heating by 50% - but I read somewhere that cooling system should not be over designed more than 20%. The unit will meet the same fate as discussed above due to lack of the required head load.

I would appreciate if someone can give me a reason for this and tell me if it is justified. Thanks again.